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                 *            Introduction to PPM              *                
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           Contents                                                             
           ========                                                             
            1. Scope of PPM                                            1        
            2. Construction of PPM                                     2        
            3. Properties of PPM                                       6        
            4. Practical Usage of PPM                                  9        
            5. List of Critical Comments on Individual Stars          10        
            6. List of Remarks on Individual Stars                    12        
            7. Short Explanation of the PPM                           21        
            8. Short Explanation of the Printed Catalogue             24        
               References                                             26        
               Acknowledgments                                        27        
                                                                                
                                                                                
                                                                                
           Preface                                                              
           =======                                                              
           The idea of this Introduction to PPM is to deliver a short           
           explanation together with the catalogue itself. It should not be     
           understood as a detailed description of the work performed in the    
           construction of PPM. This will be given in a forthcoming paper       
           (Bastian and Roeser 1990).                                           
                                                                                
                                             S. Roeser and U. Bastian           
                                             Astronomisches Rechen-Institut     
                                             Heidelberg, June 1989              
                                                                                
                                                                                
                                                                                
           This machine-readable version of the PPM has 2 files, the introduc-  
           tion file and the data file. It was received on tape as a five file  
           set. A preliminary version of the PPM from 1988 was included. Two    
           different versions of the southern part of the SAO catalogue were    
           also included to give the user a southern extension for the two data 
           files.                                                               
                                                                                
                                                                                
           1. Scope of PPM                                                      
           ===============                                                      
           PPM gives positions and proper motions of 181731 stars north of      
           -2.5 degrees declination. Its main purpose is to provide a           
           convenient, dense and accurate net of astrometric reference stars    
           on the northern celestial hemisphere. This net is designed to        
           represent as closely as possible the new IAU (1976) coordinate       
           system on the sky, as defined by the FK5 star catalogue (Fricke      
           et al., 1988).  In other words, it is a representation of the FK5    
           system at higher star densities and fainter magnitudes.              
           Two older catalogues of similar character have served the same       
           purpose in the past decades: AGK3 and the SAO Catalog.  There        
           are three major reasons to replace these now:                        
                                                                                
           1) SAOC and AGK3 are representations of the now obsolete FK4         
           system of positions and proper motions.  Astronomers should have     
           direct access to the FK5 system.                                     
                                                                                
           2) The accuracy of positions and proper motions in AGK3 and SAOC     
           is no longer satisfactory. Astronomers should have a more            
           accurate tool.  Over more than a century astrometrists have          
           accumulated a vast treasure of measured star positions. The fact     
           that much of this treasure is now available in machine-readable      
           form and the power of present-day computers make it easy to          
           analyse and combine this quantity of data.                           
                                                                                
           3) Proper motions in AGK3 and SAOC were derived from only two        
           separate source positions per star. This lack of redundancy leads    
           to a large number of coarse errors in those catalogues.  With        
           more than two measurements per star such errors can be largely       
           avoided.  In this way astronomers now get a more reliable            
           astrometric tool.                                                    
                                                                                
           For purely practical reasons PPM presently covers only the           
           northern celestial hemisphere plus the strip between the equator     
           and -2.5 degrees declination. An extension of PPM to the south pole  
           is obviously necessary. It has to be provided in the near future.    
                                                                                
           This machine-readable version of the PPM has 2 files, the introduc-  
           tion file and the data file. It was received on tape at the          
           Astronomical Data Center (NASA/Goddard Space Flight Center) as a five
           file set. A preliminary version of the PPM from 1988 was included.   
           Two different versions of the southern part of the SAO catalogue were
           also included to give the user a southern extension for the two data 
           files.                                                               
                                                                                
                                                                                
           2. Construction of PPM                                               
           ======================                                               
           In principle, the construction of PPM proceeded in the same way      
           as for other astrometric compilation catalogues in the past: For     
           each star, a mean position and a proper motion are derived from a    
           number of measured positions (found in published source              
           catalogues) by a least-squares adjustment.  In detail, however,      
           quite a number of new methods and concepts were used which           
           greatly reduced the effort for this undertaking.  PPM was            
           constructed in several steps, each of which will now be explained    
           very briefly.                                                        
                                                                                
           Step 1: Compilation of the star list.                                
           The star list for PPM was constructed starting from the magnetic     
           tape version of AGK3, which contains 181581 stars.  All FK5 stars    
           (Basic FK5 plus Bright Extension) north of -2.5 degrees              
           declination were added, if not already included.  In the course      
           of the work six AGK3 stars had to be deleted (because they are       
           non-existent) and five close companions to AGK3 stars were added.    
           This gave the final list of 181731 stars.                            
                                                                                
           Step 2: Identification of stars in the source catalogues             
           The complete list of source catalogues is given in Chapter 3,        
           below, along with an explanation of their abbreviations, with        
           bibliographic references and some statistics.                        
           The stars in each of the source catalogues were identified with      
           the objects of the star list.  This was done in a fully automatic    
           process.  Problem cases recognized in this process were              
           investigated manually to detect typing errors in the source          
           catalogues, to correctly identify double star components, to         
           avoid the effects of grossly incorrect positions in the star list    
           etc.                                                                 
           Prior to the identification of Astrographic Catalogue data (given    
           as x,y plate coordinates originally) a rough plate reduction was     
           performed, using published approximate plate constants.              
                                                                                
           Step 3: Construction of a preliminary system of positions and        
                   proper motions.                                              
           A preliminary system of positions and proper motions is necessary    
           to eliminate systematic deviations between the individual source     
           catalogues. For this purpose we used AGK3, but with its positions    
           and proper motions corrected to the system of NIRS (Corbin,          
           1978), the latter being regarded as a representation of the FK4      
           system. This correction was done using the systematic differences    
           between AGK3 and NIRS given by Schwan (1985).                        
                                                                                
           Step 4: Reduction of source catalogues to the preliminary system     
           Zonal systematic deviations as well as magnitude- and colour-de-     
           pendent systematic deviations between each of the source             
           catalogues and the preliminary system were determined and            
           eliminated.                                                          
           For Astrographic Catalogue zones we performed second-order plate     
           reductions, using the preliminary system as reference. The           
           resulting star positions were further corrected for magnitude-       
           and colour-dependent deviations and for effects of higher-order      
           distortion ("coma").                                                 
                                                                                
           Step 5: Determination of weights for the source positions            
           In a least-squares adjustment weights have to be assigned to the     
           individual measurements, according to their mean errors.             
           Magnitude-dependent mean errors of the positions were determined     
           for each source catalogue. For Astrographic Catalogue zones this     
           was achieved by investigating the differences between positions      
           of the same star measured on different (overlapping) plates.  For    
           Yale and AGK1 zone catalogues the differences between the source     
           positions and a preliminary version of PPM (Roeser and Bastian,      
           1988) were investigated.  For the Carlsberg Meridian Circle          
           catalogues (CMC 1985-1988) the differences from FK4 were             
           investigated by R. Bien (1988).  For AGK3R the mean errors given     
           by Eichhorn (1974) were used.                                        
                                                                                
           Step 6: Least-squares adjustment                                     
           A standard weighted least-squares adjustment of the mean             
           positions and proper motions was performed for each star -           
           separately for right ascensions and declinations. Automatic tests    
           for unduly large scatter among the measurements (based on the        
           chi-square sum) and automatic elimination of obvious outliers        
           (based on appropriately normalized individual residues) were         
           implemented. All stars having bad chi-square sums beyond a           
           certain significance limit, but still not showing obvious            
           outliers, were marked as "problem cases" (see flag 'P' in column     
           19.1 of the printed catalogue). This occurred in about 950 cases.    
                                                                                
           Step 7: Manual treatment of difficult cases                          
           The automatic outlier elimination deleted about 5500 measured        
           coordinates (out of more than 2 million).  Tight restrictions,       
           however, were imposed on this automatic process in order to avoid    
           spuriously "good" results. This created about 800 "difficult         
           cases" that were treated manually. A majority of these turned out    
           to be coarse errors in AGK3, the rest were caused by errors in       
           other source catalogues, mixed-up double star components etc.        
           For about 180 objects no satisfactory solution could be found.       
           These were added to the "problem cases" of step 6, giving a total    
           of 1120 PPM stars with flag 'P'.                                     
           Another group of stars was subject to manual checks: Those AGK3      
           stars not found in any source catalogue except AGK2 and AGK3         
           (about 60 cases). We checked these for coarse errors of AGK3 by      
           locating the stars on Palomar Sky Survey prints - with a             
           precision of a few arcsec. In most cases they were found at the      
           predicted positions. Nine cases were indeed caused by errors in      
           both AGK2 and AGK3. We kept the data for these nine objects, but     
           marked them as severe problem cases (see flag 'C' in column 19.1     
           of the printed catalogue).                                           
                                                                                
           Step 8: Transformation from the preliminary to the final system      
           The positions and proper motions derived thus far were transformed   
           from the preliminary system (described above) to the new IAU         
           (1976) coordinate system, as defined by FK5. This step had two       
           major parts: First, a transformation from the preliminary system     
           to a presumably better representation of the FK4 system was          
           performed.  Then a standard transformation from the FK4 system       
           (B1950) to the FK5 system (J2000).                                   
           The first part became necessary because of the discovery of two      
           sorts of systematic deviations between the preliminary system and    
           the FK4 system:                                                      
            - Uncorrected atmospheric dispersion in the original AGK2 and       
           AGK3 positions had caused a strong colour-dependent and              
           declination-dependent systematic error.  This was corrected.  No     
           other colour or magnitude equations were applied to the              
           preliminary system. In other words: The colour and magnitude         
           system of PPM is identical to that of AGK3 (except for the           
           dispersion effect, of course).                                       
            - In the course of the work we found indications for zonal          
           errors of our preliminary proper-motion system. Therefore we did     
           not keep this for the final system of PPM. Instead we constructed    
           a new system of positions and proper motions from the systems        
           of AGK3R (unchanged) and the CMC catalogues (after transformation    
           to the FK4 system).                                                  
           The transition from FK4 system to FK5 system followed exactly the    
           same procedure as was used in the construction of FK5 (Fricke, et    
           al. 1988).                                                           
           More details on the construction and properties of the system of     
           PPM will be given by Roeser (1989).                                  
                                                                                
           Step 9: Introduction of FK5 data for FK5 stars                       
           For the stars contained in FK5 Part I (the Basic Fundamental         
           Stars) and in FK5 Part II (the Bright Extension Stars), PPM gives    
           the original FK5 data. These were introduced by simply copying       
           them from the machine-readable version of FK5.                       
                                                                                
           Step 10: Provision of cross-references and auxiliary data            
           In addition to the astrometric data PPM gives cross-references to    
           other star catalogues, magnitudes, spectral types and a number of    
           flags for each star.                                                 
           Much manual labour went into the provision of cross-references.      
           They were determined by different methods, ranging from direct       
           cross-identification (based on positions only) to simply copying     
           historic work done by others (in the cases of catalogues having      
           low positional accuracy such as DM and HD).  Cross-references are    
           given explicitly for four catalogues: Bonner Durchmusterung          
           (abbreviated DM), SAO Catalog, Henry Draper Catalogue (HD) and       
           AGK3. Three more cross-references are given implicitly by flags      
           (see columns 19 of the printed catalogue):                           
           Flag 'F' in column 19.4 denotes the stars contained in FK5 (Parts    
           I and II).                                                           
           Flag 'H' in column 19.3 indicates stars that are contained in        
           AGK3R and/or one of the CMC catalogues.                              
           The double star flag 'D' (column 19.2) was created by                
           cross-referencing PPM with the Catalogue of Components of Double     
           and Multiple Stars (Dommanget 1988).                                 
           The magnitudes and spectral types were copied either from AGK3 or    
           from FK5 (see Chapter 8 for details).                                
           Various sorts of auxiliary information are given in the List of      
           Critical Comments and in the List of Remarks (Chapters 5 and 6),     
           and indicated by the 'C' and 'R' flags.                              
                                                                                
           3. Properties of PPM                                                 
           ====================                                                 
                                                                                
           Random Errors                                                        
           _____________                                                        
           The table below shows a summary of the error budget of PPM.  Each    
           line of the table gives the following data for the particular set    
           of stars indicated:  The number of stars in the set, the average     
           number of source positions per star, the average of the mean         
           epochs (for right ascension and declination), the average of the     
           mean errors of proper motion (for right ascension and                
           declination) and the average of the mean errors of position at       
           epoch 1990 (again for right ascension and declination). Units are    
           seconds of arc and seconds of arc per century, respectively.  At     
           the bottom of the table the corresponding values for AGK3 and        
           SAOC are given for comparison (a more complete comparison of PPM     
           with SAOC, AGK3 and the - future - HIPPARCOS catalogue is given      
           by Roeser and Bastian 1989, Fig. 3).                                 
                                                                                
                                                                                
                         Table 1: Error budget of PPM                           
           set of stars   No.     No.   mean epochs   mean err.   mean err.     
                          stars   obs.                prop. mot.  pos. 1990     
           PPM, all stars 181731  6.2  1931.5 1930.7  0.43 0.42   0.27 0.27     
           PPM, HPS stars  31841  7.8  1950.3 1948.0  0.24 0.25   0.12 0.12     
           PPM, FK5 stars   1365  ---  1954.2 1945.2  0.08 0.10   0.04 0.05     
           AGK3           181581  2.0  1945   1945    0.95 0.95   0.45 0.45     
           SAOC, north    133000  2.0  1930   1930    1.5  1.5    0.9  0.9      
                                                                                
           Three subsets of PPM are shown in the table: The first line          
           refers to the entire catalogue, the last line to the 1365 FK5        
           stars and the second line to the High-Precision Subset (HPS) of      
           PPM. This subset will be briefly discussed now.                      
           PPM essentially was constructed by combining AGK3 with               
           early-epoch catalogues. So the mean epochs of PPM are quite early    
           and the present-day accuracy is dominated by the accuracy of the     
           proper motions. Therefore it would be highly desirable to have a     
           repetition of AGK3 at present epoch.  This not being available,      
           we decided to introduce the Carlsberg Meridian Circle (CMC)          
           catalogues, nos. 1 to 4 (CMC 1985 -1988).  In addition, we           
           introduced AGK3R because of its higher accuracy compared to AGK3.    
           The High-Precision Subset of PPM is defined as the set of PPM        
           stars for which either CMC or AGK3R observations are available.      
           Its superior precision in present-day positions is mainly due to     
           the fine work done at the Carlsberg Meridian Circle.                 
           Note that (on average) more than 6 measured positions are            
           available per star.  This redundancy allowed us to discover (and     
           avoid) a large number of coarse errors in the source catalogues.     
           It ensures that PPM contains very few coarse errors. For 1064 PPM    
           stars the redundancy was not sufficient to resolve discrepancies     
           between the source positions. They carry the 'P' flag mentioned      
           above.                                                               
                                                                                
           Source Catalogues                                                    
           _________________                                                    
           More than 1.1 million source positions (i.e. right ascensions and    
           declinations) were used for the construction of PPM.  The            
           following table lists the various groups of source catalogues        
           used and the total number of positions derived from each group.      
           More information on the catalogues as well as bibliographic          
           references are given in the book by Eichhorn (1974), except for      
           the CMC catalogues.                                                  
                                                                                
                                                                                
                    Table 2: Source Catalogues                                  
                Astrographic Catalogue zones   466 278                          
                AGK3                           181 581                          
                AGK2                           181 581                          
                AGK1                           141 146                          
                Yale zone catalogues            80 574                          
                Gyllenberg, 1926                11 498                          
                Prager, 1923                     8 602                          
                CMC 1, 2, 3, 4                  25 656                          
                AGK3R                           20 581                          
                Total                        1 117 497                          
                                                                                
           Systematic Errors                                                    
           _________________                                                    
           We did our very best to ensure that PPM is on the system of FK5.     
           Such a goal, however, can be reached to a certain accuracy only.     
           We estimate that the residual systematic deviations between PPM      
           and the FK5 system have a typical size of 0.05 arcsec for the        
           positions at mean epoch and of 0.2 arcsec per century for the        
           proper motions.                                                      
                                                                                
           These numbers do not hold, however, for a few special groups of      
           objects: The very blue stars (spectral types B2 and earlier), the    
           very faint stars (fainter than about 12 in photographic              
           magnitude) and the very bright stars (brighter than about 7, but     
           not in HPS or FK5). Because these groups are very small by number    
           it was neither possible to check our preliminary system to the       
           desired accuracy, nor could we correct the individual source         
           catalogues to this system with the desired accuracy. So we just      
           do not know anything about possible systematic errors for these      
           small groups.                                                        
                                                                                
           Another possible systematic error stands out clearly from our        
           data, but still cannot be removed. It is a magnitude equation in     
           right ascension, affecting stars fainter than about 10.5 mag         
           (photographic). It shows up as a systematic difference between       
           PPM and the CMC catalogues (remember that, by definition, the        
           magnitude dependence of the proper-motion system of PPM is that      
           of AGK3. The CMC catalogues were transformed to that system          
           before inclusion into PPM). It has a mean value of about 0.08        
           arcsec for stars between 10.6 and 11.0 mag, and 0.15 arcsec for      
           stars fainter than 11.0 mag. The cause of this systematic            
           difference cannot be identified with existing astrometric data.      
           Therefore, it cannot be removed in a reasonable way.  More details   
           will be given by Roeser (1989).                                      
                                                                                
           Some Statistics                                                      
           _______________                                                      
           PPM contains 131502 SAOC stars, which corresponds to almost 99       
           percent of all SAOC stars in the celestial region covered by PPM.    
           It contains 85357 HD stars, corresponding to about 70 percent of     
           all HD stars in the region.  There are 16031 double star flags       
           ('D'), 1120 problem cases ('P' flags), 22 critical comments ('C'     
           flags) and 228 less important remarks ('R' flags). The 'R' flags     
           refer to peculiar DM designations and variable-star designations     
           mostly.                                                              
                                                                                
           4. Practical Usage of PPM                                            
           =========================                                            
           PPM gives positions for a standard epoch (J2000) and a standard      
           coordinate system (mean equator and equinox for J2000 in the new     
           IAU (1976) system of astronomical constants).                        
                                                                                
           To apply PPM as astrometric reference at a different                 
           observational epoch, but still in the J2000 coordinate system,       
           proper motions must be taken into account.  For most purposes a      
           simple linear formula (position difference equals epoch              
           difference times proper motion) is sufficient. But note that this    
           procedure will give inaccurate results close to the pole.  Better    
           formulae are given, e.g., by Murray (1983) and in the introduction   
           to FK5.                                                              
                                                                                
           To apply PPM as astrometric reference for a different equator and    
           equinox, but still in the IAU (1976) system of astronomical          
           constants, precession has to be taken into account. The relevant     
           formulae and numerical values are given, e.g., by Lieske (1979) and  
           in the introduction to FK5.                                          
                                                                                
           Note that simply precessing from J2000 to B1950 will still not       
           give results in the old (FK4) astronomical coordinate system.        
           This differs from the new IAU (1976) system by a correction to       
           the precessional constant, a correction to the position of the       
           vernal equinox, by the treatment of elliptical aberration and by     
           complicated zonal corrections.  All this adds up to several          
           tenths of an arcsec in position and a few tenths of an arcsec per    
           century in proper motion.                                            
                                                                                
           Application of proper motion and precession still gives mean         
           positions. In order to derive apparent positions for PPM objects     
           one also has to apply nutation, aberration, stellar parallax, and    
           relativistic light-bending (see e.g. Murray, 1983).                  
           For most practical uses of PPM these effects can be ignored. But     
           PPM contains a small number of stars with fairly large paral-        
           laxes. They are not indicated in the catalogue.                      
                                                                                
           For each star PPM gives mean errors for right ascension and          
           declination at their respective mean epochs T. Standard error        
           propagation calculus can be used to calculate the mean error of a    
           PPM position for an epoch t different from the mean epoch:           
           Denoting the mean error of one coordinate at mean epoch by E(T),     
           the mean error of proper motion by E(PM), the mean error of          
           position at epoch t is given by                                      
                                                                                
                      2            2            2            2                  
                  E(t)    =    E(T)    +   E(PM)    *   (T-t)                   
                                                                                
           Using more PPM stars in an application will usually give better      
           results because of the partial cancellation of the purely            
           accidental errors of the PPM data.  However, this improvement        
           will never get beyond the systematic errors of PPM, as discussed     
           in the previous chapter.                                             
                                                                                
           Applications demanding utmost accuracy rather than high star         
           density should use HPS stars only.                                   
                                                                                
           5. List of Critical Comments on Individual Stars                     
           ================================================                     
           The following list gives, for each star included: The PPM number,    
           the AGK3 number (if any) and a critical comment.  Any positions      
           given in the comment refer to equinox J2000.0.                       
                                                                                
           All stars included in this list carry the 'C' flag.                  
            20679 +66 0812 :                                                    
            Planetary nebula NGC 6543                                           
                                                                                
            42555 +45 0003 :                                                    
            An orbital double star. There is also +45 0002 in                   
            AGK2, but not in AGK3. A difficult problem, which                   
            cannot be solved at the moment.                                     
                                                                                
            43225 +40 0059 :                                                    
            BD +40 147. Centre of M32 according to SIMBAD(CDS).                 
                                                                                
            43228 +40 0060 :                                                    
            BD +40 148. Centre of M31 according to SIMBAD(CDS).                 
                                                                                
            50954 +41 0889 :                                                    
            FK5 339. This is an orbital double star. FK5 data                   
            are given referring to centre of gravity.                           
                                                                                
            56062 +43 1399 :                                                    
            Globular cluster NGC 6341 = M92                                     
                                                                                
                                                                                
            72938 +31 0789 :                                                    
            FK5 287. This is an orbital double star. FK5 data                   
            are given referring to centre of gravity.                           
                                                                                
            76814 +38 1224 :                                                    
            Companion to FK5 485. AGK2 and AGK3 are wrong. In our               
            opinion GC gives the correct position and proper                    
            motion. Difficult case in photographic astrometry.                  
            PPM position and proper motion is derived from the                  
            the GC mean position and one CMC observation.                       
                                                                                
            86045 +38 2138 :                                                    
            see 86047 and 86049.                                                
                                                                                
            86047          :                                                    
            FK5 793. The centre of gravity of 61 Cyg. Data copied from          
            FK5. See also 86049.                                                
                                                                                
            86049 +38 2136 :                                                    
            86045 and 86049 are the components of 61 Cyg.                       
            see also 86047. Both stars are of poor quality in                   
            PPM, because positions and proper motions are derived               
            neglecting the large parallax of these stars.                       
                                                                                
            112552 +21 2332 :                                                   
            Inspection on POSS shows: This star does not exist.                 
            The companion AGK3 +21 2333 is a double star with                   
            approximate separation 15 arcsec.                                   
            AGK3 data are copied. Do not use them.                              
                                                                                
            150880 +5 0838  :                                                   
            Inspection on POSS shows: Both AGK2 and AGK3 are wrong.             
            Positions and proper motions are derived from two AC                
            measurements. There is another star at position:                    
             6 37 17.19 +5 7 57.6, Epoch 1915.0.                                
                                                                                
            150973 +08 0809 :                                                   
            This is NGC 2261                                                    
                                                                                
            151210 +2 0793  :                                                   
            Inspection on POSS shows: Both AGK2 and AGK3 are wrong.             
            There is a star in the AC at position:                              
             6 46 27.50 +2 9 42.8, Epoch 1910.0                                 
            AGK3 data are copied. Do not use them.                              
                                                                                
            151371 +3 0869  :                                                   
            Inspection on POSS shows: Both AGK2 and AGK3 are wrong.             
            This star is n o t  BD +3 1431. The position of                     
            BD +3 1431 is  6 50 54.36 +3 27 23.9, Epoch 1910.2.                 
            AGK3 data are copied. Do not use them.                              
                                                                                
            152571 +8  973  :                                                   
            Inspection on POSS shows: Both AGK2 and AGK3 are wrong.             
            Positions and proper motions are derived from two AC                
            measurements.                                                       
            AGK3 claims that AGK3 +8 973 is BD +8 1757, but according           
            to the coordinates given in the Bonner Durchmusterung the           
            the BD star is about 4 arcmin south and at the same R.A. as         
            given in AGK2/3. This star is in AC at position:                    
             7 24 20.78 +8 19 15.9, Epoch 1920.12. This is the BD star.         
            The declination difference suggests an error in the AGK2/3          
            measurements of 2 mm.                                               
                                                                                
            153068 +05 1072 :                                                   
            FK5 291. This is an orbital double star. FK5 data                   
            are given referring to centre of gravity. This                      
            star is omitted in the printed version of AGK3.                     
                                                                                
            159607 +2 1655  :                                                   
            Inspection on POSS shows: Both AGK2 and AGK3 are wrong.             
            There is a star in AC at position:                                  
            13 15 56.82 +2 40 39.4, Epoch 1910.4.                               
            AGK3 data are copied. Do not use them.                              
                                                                                
            165440 +9 2185  :                                                   
            Inspection on POSS shows: Both AGK2 and AGK3 are wrong.             
            Positions and proper motions are derived from two AC                
            measurements. It is a companion to AGK3 +9 2184.                    
                                                                                
            166000 +2 2269  :                                                   
            Inspection on POSS shows: Both AGK2 and AGK3 are wrong.             
            There is a star in the Astrographic Catalogue (AC) at               
            position: 18 30 53.79 +2 5 27.6, Epoch 1910.0                       
            AGK3 data are copied. Do not use them.                              
                                                                                
            168632 +4 2602  :                                                   
            Inspection on POSS shows: Both AGK2 and AGK3 are wrong.             
            Positions and proper motions are derived from the                   
            measurements in AC and by Lopez (1987).                             
                                                                                
           6. List of Remarks on Individual Stars                               
           ======================================                               
           The following list gives, for each star included: The PPM number,    
           the AGK3 number (if any) and a remark.  In contrast to the           
           Critical Comments these remarks are not of direct relevance to       
           astrometry. Most remarks refer to peculiar DM designations and to    
           variable-star designations.  Any positions given in the comment      
           refer to equinox J2000.0.                                            
                                                                                
           All stars included in this list carry the 'R' flag.                  
               736 +85 0058    BD +85 0059 austr                                
               737 +85 0059    BD +85 0059 bor                                  
              4213 +85 0399    BD +84 0540 bor                                  
              5978 +70 0252    BD +70 0353 bor                                  
              5979 +70 0253    BD +70 0353 austr                                
              6564 +73 0195    BD +73 0362 bor                                  
              6565 +73 0194    BD +73 0362 austr                                
              8570 +71 0401    BD +71 0677 bor                                  
              8571 +71 0402    BD +71 0677 austr                                
             10800 +76 0836    BD +75 0780 austr                                
             10801 +76 0837    BD +75 0780 bor                                  
             11191 +70 0777    BD +70 1233 austr                                
             11426 +78 0551    BD +77 0879 austr                                
             11427 +78 0551a   BD +77 0879 bor                                  
             12044 +62 0026    BD +62 0029 austr                                
             13092 +61 0153    BD +60 0272 bor(AGK2)                            
             13093 +61 0154    BD +60 0272 austr(AGK2)                          
             14737 +64 0258    BD +63 0489s1                                    
             14738 +64 0259    BD +63 0489s2                                    
             16337 +65 0408    BD +65 0592 bor                                  
             19128 +61 0873    BD +61 1454 bor                                  
             19129 +61 0872    BD +61 1454 austr                                
             19225 +67 0640    BD +67 0857 austr                                
             19226 +67 0639    BD +67 0857 bor                                  
             22874 +60 1381    BD +60 2239 bor                                  
             22875 +60 1382    BD +60 2239 austr                                
             22923 +65 1105    BD +64 1552s1                                    
             24229 +64 1186    BD +64 1764 austr                                
             24230 +64 1187    BD +64 1764 bor                                  
             24348 +68 1014    BD +68 1366 austr                                
             24349 +68 1013    BD +68 1366 bor                                  
             24660 +62 1473    BD +61 2506 bor                                  
             24661 +62 1474    BD +61 2506 austr                                
             25060 +51 0012 SS Cas. On one AC plate it has a companion (also    
                            variable? ) at position  0 9 36.63 +51 33 58.2 .    
             25788 +56 0096    BD +55 0191D                                     
             25791 +56 0097    BD +55 0191ABC                                   
             26075 +50 0132    BD +50 0228Aa according to AGK2                  
             27500 +54 0285    BD +53 0509 austr                                
             27501 +54 0286    BD +53 0509 bor                                  
             28515 +55 0346    BD +54 0684s2                                    
             29017 +57 0457 This AGK2 star was omitted in the printed           
                            version of AGK3.                                    
             29111 +50 0459    BD +50 0985 bor                                  
             29236 +52 0443    BD +52 0848 bor                                  
             29237 +52 0444    BD +52 0848 austr                                
             30659 +58 0568 This star is wrongly identified in AGK3 to be       
                            BD +58 961a. The correct identification is          
                            +58 961. The star is HD 237537 and has              
                            spectral type K2 according to HD.                   
                            Consequently, spectral type M9, given in AGK3, is   
                            wrong. This spectrum belongs to BD+58 961a =        
                            HD 47929, a star not contained in PPM.              
             36165 +59 1134    BD +59 1842 bor                                  
             36166 +59 1133    BD +59 1842 austr                                
             36357 +50 1269    BD +50 2520 austr                                
             36358 +50 1268    BD +50 2520 bor                                  
             37314 +53 1220    BD +53 2228 bor                                  
             37315 +53 1221    BD +53 2228 austr                                
             37610 +52 1296    BD +51 2645p2                                    
             37611 +52 1297    BD +51 2645p1                                    
             37667 +51 1344    BD +51 2661 bor                                  
             37668 +51 1345    BD +51 2661 austr                                
             38331 +51 1432    BD +51 2821p2                                    
             38332 +51 1433    BD +51 2821p1                                    
             38663 +52 1418    BD +51 2898 bor                                  
             38664 +52 1417    BD +51 2898 austr                                
             41324 +57 1624    BD +57 2657 austr                                
             41350 +52 1707    BD +52 3341 bor                                  
             41950 +55 1690    BD +55 2981s1                                    
             42988 +48 0047    BD +48 0141 bor                                  
             42989 +48 0048    BD +48 0141 austr                                
             43816 +40 0119    BD +40 0260a seq                                 
             44379 +43 0208    BD +43 0356 bor                                  
             44469 +47 0208    BD +47 0508s1                                    
             45477 +46 0288    BD +46 0633p1                                    
             45582 +44 0291    BD +44 0589 bor                                  
             45583 +44 0292    BD +44 0589 austr                                
             47415 +44 0472    BD +44 1036p1                                    
             48885 +44 0639    BD +44 1402 austr                                
             48886 +44 0640    BD +44 1402 bor                                  
             50828 +49 0822    BD +50 1575 bor                                  
             50829 +49 0821    BD +50 1575 austr                                
             55572 +48 1261    BD +48 2421a prec                                
             55574 +48 1263    BD +48 2421a seq                                 
             58003 +49 1511    BD +49 2959b seq                                 
             58005 +44 1577    BD +44 3078 bor                                  
             58006 +44 1578    BD +44 3078 austr                                
             58174 +44 1594    BD +44 3107s1                                    
             58554 +39 1976    BD +39 3832 bor                                  
             58555 +39 1975    BD +39 3832 austr                                
             58885 +44 1669    BD +44 3261p1                                    
             58886 +44 1670    BD +44 3261p2                                    
             59824 +43 1820    UW Cyg                                           
             63398 +44 2119 This AGK2 star was omitted in the printed           
                            version of AGK3.                                    
             63400 +44 2118 CMC has observed AGK2 +44 2119.                     
                            +44 2119 = BD +43 4305 = LHS 3853 = EV Lac.         
                            It is a flare star and astrom. binary,              
                            P=28y, a= 0.04".                                    
             64939 +39 2543    BD +38 5114 bor                                  
             64941 +39 2544    BD +38 5114 austr                                
             66259 +35 0130    BD +35 0266 austr                                
             66260 +35 0129    BD +35 0266 bor                                  
             67031 +33 0203    BD +32 0390 austr                                
             67032 +33 0204    BD +32 0390 bor                                  
             67168 +37 0259    BD +36 0453AB                                    
             67839 +34 0295    BD +33 528B in AGK3                              
             68514 +35 0353    R Per                                            
             68820 +34 0408    BD +34 0744 bor, but austr according to AGK2     
             69837 +38 0522    BD +38 1001s1                                    
             76100 +36 1089    BD +37 2218 bor                                  
             76101 +36 1090    BD +37 2218 austr                                
             78728 +36 1364    BD +36 2626CD                                    
             82523 +34 1814    BD +34 3463 austr                                
             82668 +35 1753    BD +34 3494s1                                    
             83206 +35 1822    BD +35 3703AB                                    
             83207 +35 1823    BD +35 3703s1                                    
             83534 +39 2011    BD +39 3910s1                                    
             83538 +39 2012    BD +39 3910s2                                    
             83664 +32 1842 A faint double star not measured in AC.             
             83719 +37 1855    BD +37 3651 bor                                  
             83720 +37 1856    BD +37 3651 austr                                
             85463 +30 2192    BD +29 4160 austr                                
             87076 +34 2235    BD +33 4359 austr                                
             87077 +34 2236    BD +33 4359 bor                                  
             87448 +30 2471    BD +30 4617 bor                                  
             87449 +30 2472    BD +30 4617 austr                                
             87475 +31 2276    BD +30 4622 austr                                
             87962 +39 2417    BD +38 4808 austr                                
             87963 +39 2418    BD +38 4808 bor                                  
             88329 +32 2277    BD +32 4546 bor                                  
             88330 +32 2276    BD +32 4546 austr                                
             91866 +28 0295    BD +28 0448 bor                                  
             91867 +28 0294    BD +28 0448 austr                                
             91890 +26 0264    BD +26 0443 austr                                
             91891 +26 0265    BD +26 0443 bor                                  
             92457 +24 0288    BD +23 0449 austr                                
             92458 +24 0287    BD +23 0449 bor                                  
             92765 +25 0331    BD +25 0593 austr                                
             92766 +25 0332    BD +25 0593 bor                                  
             94304 +20 0496    BD +19 0905 bor                                  
             94305 +20 0497    BD +19 0905 austr                                
             94779 +28 0546    BD +28 0868 austr                                
             95670 +27 0624    BD +27 1035 austr                                
             96431 +27 0713    BD +27 1206 austr                                
             96432 +27 0712    BD +27 1206 bor                                  
             96463 +23 0713    BD +23 1477 austr                                
             96464 +23 0712    BD +23 1477 bor                                  
             97653 +22 0897    BD +23 1744 austr                                
             97654 +22 0898    BD +23 1744 bor                                  
             97837 +28 0827    BD +28 1440 bor                                  
             97991 +28 0845    BD +28 1474 austr                                
             99247 +26 0987    BD +26 1885 bor                                  
             99248 +26 0988    BD +26 1885 austr                                
             99374 +28 0979    BD +28 1707 austr                                
             99455 +24 1065    BD +24 2067 bor                                  
            100883 +27 1117 Double star in AGK2: +27 1117 and +27 1118.         
                            The AGK3 measurement refers to the centre and       
                            was not used for PPM.                               
            100884 +27 1118 This AGK2 star was omitted in the printed           
                            version of AGK3.                                    
            101958 +24 1314    BD +25 2518 bor                                  
            101975 +21 1297    BD +21 2429 bor                                  
            102684 +23 1318    BD +23 2584 bor                                  
            102685 +23 1319    BD +23 2584 austr                                
            104347 +25 1597    BD +25 2973 bor                                  
            104348 +25 1598    BD +25 2973 austr                                
            105039 +25 1673    BD +25 3108 bor                                  
            105040 +25 1674    BD +25 3108 austr                                
            105654 +21 1678    BD +21 3063 bor                                  
            106982 +25 1901    BD +25 3473 austr                                
            106983 +25 1902    BD +25 3473 bor                                  
            109767 +21 2053    BD +21 3927 bor                                  
            110507 +21 2112    BD +21 4073s1                                    
            110508 +21 2113    BD +21 4073s2                                    
            111510 +20 2287    In printed AGK2/3 BD +19 4464p1                  
            111960 +20 2329    BD +19 4525 austr                                
            112396 +20 2371    BD +20 4822 bor                                  
            112397 +20 2372    BD +20 4822 austr                                
            113400 +28 2523    BD +27 4131 austr                                
            114011 +28 2614    BD +28 4282 bor                                  
            114012 +28 2615    BD +28 4282 austr                                
            115008 +27 2697    BD +26 4540 austr                                
            115977 +22 2581    BD +21 4999 bor                                  
            116009 +24 2583    BD +23 4830p2                                    
            116010 +24 2584    BD +23 4830p1                                    
            126928 +10 1274    BD +10 2072p1                                    
            126930 +10 1275    BD +10 2072p2                                    
            129303 +10 1567    BD +10 2466p1                                    
            129304 +10 1568    BD +10 2466p2                                    
            133569 +12 1829    BD +12 3253s1                                    
            133571 +12 1830    BD +12 3253s2                                    
            134095 +19 1680    RY Her                                           
            134248 +11 1993a This star is a close companion to 134249. In       
                            AGK3 this pair had only one entry (centre of        
                            light?). It is unclear which of the companions      
                            is HD 165475.                                       
            134249 +11 1993 This star is a close companion to 134248. In        
                            AGK3 this pair had only one entry (centre of        
                            light?). It is unclear which of the companions      
                            is HD 165475, but from a remark in GC it is         
                            clear that 44864 = SAO 103373 = GC 24655            
            134415 +10 2192    BD +10 3434s1                                    
            134416 +10 2193    BD +10 3434s2                                    
            136159 +19 1867 This AGK2 star was omitted in the printed           
                            version of AGK3.                                    
            136160 +19 1866 Double star in AGK2: +19 1866 and +19 1867.         
                            The AGK3 measurement refers to +19 1867 and not     
                            to +19 1866.                                        
            148223 +05 0540    BD +05 0776p1                                    
            148226 +05 0541    BD +05 0776p2                                    
            149628 +05 0689    BD +05 1043p1                                    
            149631 +05 0690    BD +05 1043p2                                    
            150058  +1 0662 This AGK2 star was omitted in the printed           
                            version of AGK3.                                    
            151093 +05 0860    BD +05 1373p1                                    
            151095 +05 0861    BD +05 1373p2                                    
            151096 +05 0862    BD +05 1373s1                                    
            151097 +05 0863    BD +05 1373s2                                    
            151581 +03 0887    BD +03 1471p1                                    
            151582 +03 0888    BD +03 1471p2                                    
            151709 +00 0825    BD +01 1635p1                                    
            152202 +03 0956    BD +03 1609p1                                    
            152207 +03 0957    BD +03 1609p2                                    
            152338 +04 0950    BD +04 1652p1                                    
            152339 +04 0951    BD +04 1652p2                                    
            152441 +03 0987    BD +03 1651s1                                    
            152442 +03 0988    BD +03 1651s2                                    
            154155 +02 1086    BD +02 1916s1                                    
            154161 +02 1089    BD +02 1916s2                                    
            159258 +07 1684    BD +08 2654s1                                    
            159259 +07 1685    BD +08 2654s2                                    
            159970 +07 1756    BD +08 2747s1                                    
            159973 +07 1757    BD +08 2747s2                                    
            161865 +05 2052    BD +05 3070s1                                    
            161866 +05 2053    BD +05 3070s2                                    
            163056 +06 1957    BD +06 3288p1                                    
            163059 +06 1958    BD +06 3288p2                                    
            164086 +08 2122    BD +08 3427s2                                    
            165886 +07 2419    possibly BD +7 3700                              
            167690 +06 2469    BD +06 4103s1                                    
            167691 +06 2470    BD +06 4103s2                                    
            168472 +08 2604    BD +08 4190p1                                    
            168475 +08 2605    BD +08 4190p2                                    
            168783 +08 2638    BD +08 4237s1                                    
            168787 +08 2639    BD +08 4237s2                                    
            169828 +09 2732    BD +09 4492p1                                    
            169830 +09 2733    BD +09 4492p2                                    
            171945 +07 3164    BD +06 4855s1                                    
            171946 +07 3165    BD +06 4855s2                                    
            173580 +6 3146  Inspection on POSS shows: Both AGK2 and AGK3        
                            are correct. This star is not measured in AC,       
                            but there is a star in AC nearby at position:       
                            22 56 41.10 +6 36 36.4, Epoch 1918.77.              
                            On POSS the AC star has a faint companion.          
            173772 +06 3158    BD +05 5135p2                                    
            173773 +06 3157    BD +05 5135p1                                    
                                                                                
           7. Short Explanation of the PPM                                      
           ===============================                                      
           The PPM catalogue contains 2 physical files.  The records            
           within each file have constant length.  They are grouped into        
           physical blocks of various constant lengths depending on how         
           the tape has been prepared.                                          
                                                                                
           The two files are:                                                   
            - File 1: The text of this Introduction to PPM                      
            - File 2: The catalogue as an easily readable table, very           
                      similar to the format of the printed catalogue.           
                                                                                
           The remainder of this chapter gives very short descriptions of       
           these two files. It describes only the formats of the files. The     
           precise meaning of the individual data items is not described        
           here, but in the Explanation of the Printed Catalogue (Chapter 8,    
           below). In the present chapter the individual data items are         
           denoted by mnemonic abbreviations which are the keys to the          
           explanations in Chapter 8.                                           
                                                                                
           File 1: The Introduction to PPM                                      
           _______________________________                                      
           This is a text file, given in exactly the same format as the         
           printed Introduction to PPM. Record length is 80 bytes, but only     
           76 bytes are actually used (the remaining being always blank).       
                                                                                
           File 2: The PPM catalogue as an easily readable table                
           _____________________________________________________                
           This is the main file giving the catalogue proper. Record length     
           is 133 bytes. The format is very similar to that of the printed      
           catalogue. You can use this file to produce a rough printed          
           version of PPM by just submitting it to a printer capable of         
           printing 133 characters per line.  No page headings, form feed       
           characters etc. are included, of course.                             
                                                                                
            PPM     1 -   6   I6                                                
            DM      8 -  18   A11                                               
            Mag    20 -  23   F4.1                                              
            Sp     25 -  26   A2                                                
            R.A.   28 -  39   F2.0,1X,F2.0,1X,F6.3  hours, minutes, seconds     
            Sign   42 -  42   A1  Sign of declination ('+' or '-').             
            Dec.   43 -  53   F2.0,1X,F2.0,1X,F5.2  degrees, arcmin, arcsec     
            PMA    56 -  62   F7.4       sec/year                               
            PMD    64 -  69   F6.3       arcsec/year                            
            N      71 -  72   I2                                                
            SA     74 -  75   F2.0       0.01 arcsec                            
            SD     77 -  78   F2.0       0.01 arcsec                            
            SPMA   80 -  83   F4.1       0.001 arcsec/year                      
            SPMD   85 -  88   F4.1       0.001 arcsec/year                      
            EPA    90 -  94   F5.2       years since 1900.0                     
            EPD    96 - 100   F5.2       years since 1900.0                     
            SAO   102 - 107   I6                                                
            HD    109 - 114   I6                                                
            AGK3  117 - 125   A9                                                
            Notes 127 - 131   5A1                                               
                                                                                
           Be careful not to miss the sign of the declination (byte 42)         
           while transforming this formatted file into a numeric file on        
           your computer.                                                       
                                                                                
           8. Short Explanation of the Printed Catalogue                        
           =============================================                        
           The stars are arranged in bands of 10 degrees width in decli-        
           nation.  Within each of these bands they are ordered according to    
           right ascension.                                                     
                                                                                
           Column 1, PPM     Serial no. of the star in PPM.                     
                                                                                
           Column 2, DM      Designation of the star in the Bonner              
                             Durchmusterung. Doubtful identifications           
                             are omitted (blank). The character 'x' denotes     
                             peculiar DM suffixes, which are given in the       
                             List of Remarks on Individual Stars.               
                                                                                
           Column 3, Mag     Photographic magnitude, copied from AGK3.          
                             Visual magnitude for FK5 stars, copied from FK5    
                                                                                
           Column 4, Sp      Spectral type, copied from AGK3; for FK5 stars     
                             copied from FK5.                                   
                                                                                
           Column 5, R.A.    Right Ascension for the equinox and epoch          
                             J2000.0, on the system of FK5, given in            
                             hours, minutes and seconds of time.                
                             For FK5 stars original FK5 data are given.         
                                                                                
           Column 6, Dec.    Declination for the equinox and epoch              
                             J2000.0, on the system of FK5, given in            
                             degrees, minutes and seconds of arc.               
                             For FK5 stars original FK5 data are given.         
                                                                                
           Column 7, PMA     Proper Motion in Right Ascension for epoch         
                             and equinox J2000.0, on the system of FK5,         
                             given in seconds of time per Julian year.          
                             For FK5 stars rounded FK5 data are given.          
                                                                                
           Column 8, PMD     Proper Motion in Declination for epoch and         
                             equinox J2000.0, on the system of FK5,             
                             given in seconds of arc per Julian year.           
                             For FK5 stars rounded FK5 data are given.          
                                                                                
           Column 9, N       Number of individual published positions           
                             used for the derivation of the position            
                             and proper motion given (cols. 5 to 8).            
                             For FK5 stars N is set to zero.                    
                                                                                
           Column 10, SA     Mean error of right ascension at the mean          
                             epoch of right ascension, multiplied by the        
                             cosine of declination, given in units of           
                             0.01 seconds of arc.                               
                             For FK5 stars rounded FK5 data are given.          
                                                                                
           Column 11, SD     Mean error of declination at the mean              
                             epoch of declination, given in units of            
                             0.01 seconds of arc.                               
                             For FK5 stars rounded FK5 data are given.          
                                                                                
           Column 12, SPMA   Mean error of proper motion in right ascension,    
                             multiplied by the cosine of declination,           
                             given in milliseconds of arc per Julian year.      
                             For FK5 stars rounded FK5 data are given.          
                                                                                
           Column 13, SPMD   Mean error of proper motion in declination,        
                             given in milliseconds of arc per Julian year.      
                             For FK5 stars rounded FK5 data are given.          
                                                                                
           Column 14, EPA    Weighted mean epoch of the individual published    
                             positions used for the derivation of right         
                             ascension and proper motion (cols. 5 and 7),       
                             given in years since 1900.0.                       
                             For FK5 stars original FK5 data are given.         
                                                                                
           Column 15, EPD    Weighted mean epoch of the individual published    
                             positions used for the derivation of decli-        
                             nation and proper motion (cols. 6 and 8),          
                             given in years since 1900.0.                       
                             For FK5 stars original FK5 data are given.         
                                                                                
           Column 16, SAO    Designation of the star in the SAO Catalog.        
                             Doubtful identifications are omitted.              
                                                                                
           Column 17, HD     Designation of the star in the Henry Draper        
                             Catalogue.                                         
                             Doubtful identifications are omitted.              
                                                                                
           Column 18, AGK3   Designation of the star in AGK3.                   
                             Doubtful identifications are omitted.              
                                                                                
           Column 19, Notes  A number of notes on the individual stars are      
                             given in these columns. Notes P, C and D mark      
                             objects that should preferably not be used as      
                             astrometric reference stars.                       
                                                                                
               Column 19.1   P    problem case, preferably not to be used as    
                                  astrometric reference star.                   
               Column 19.1   C    a critical remark on this object is given     
                                  in the List of Critical Remarks, it           
                                  should not be used as astrometric refe-       
                                  rence star.                                   
               Column 19.2   D    double star, preferably not to be used as     
                                  astrometric reference star.                   
               Column 19.3   H    member of the High-Precision Subset (HPS)     
                                  of PPM.                                       
               Column 19.4   F    member of FK5, high precision stars,          
                                  mostly very bright stars.                     
               Column 19.5   R    a remark on this star is given in the         
                                  List of Remarks on Individual Stars.          
                                                                                
           References                                                           
           ==========                                                           
           Bastian, U., Roeser, S. (1990): in preparation.                      
           Bien, R. (1988): private communication.                              
           CMC (1985-1988): Carlsberg Meridian Catalogue La Palma,              
                  Numbers 1 to 4. Copenhagen University Observatory,            
                  Royal Greenwich Observatory and Real Instituto y              
                  Observatorio de la Armada, San Fernando.                      
           Corbin, T. (1978): The proper motions of the AGK3R and SRS           
                  stars. IAU Colloquium No. 48, Eds. F.V Prochazka and          
                  R.H. Tucker, 515.                                             
           Dommanget, J. (1988): in HIPPARCOS, Scientific Aspects of            
                  the Input Catalogue Production II, Proceedings of a           
                  Colloquium held at Sitges, Spain, 1988,                       
                  eds. J. Torra and C. Turon, page 191.                         
           Eichhorn, H. (1974): Astronomy of Star Positions, Frederick          
                  Ungar Publ. Co., New York.                                    
           Fricke, W., Schwan, H., Lederle, T. (1988): Fifth Fundamental        
                  Catalogue (FK5). Part I. The Basic Fundamental Stars.         
                  Veroeff. Astron. Rechen-Institut Heidelberg Nr. 32.           
           IAU (1976): IAU Transaction XVI B, Reidel Publ., Dordrecht 1977.     
           Lieske, J. (1979): Precession matrix based on IAU(1976) system       
                  of astronomical constants, Astron. Astrophys. 73, 282.        
           Lopez, C. (1987): The Proper Motions of AGK3 Stars. IAU              
                  Colloquium No. 100, Fundamentals of Astrometry,               
                  Beograd, Sep. 8-11, 1987, in press.                           
           Murray, C.A. (1983): Vectorial Astrometry, Adam Hilger Ltd.,         
                  Bristol, UK.                                                  
           Roeser, S. (1989): The System of PPM. IAU Symposium No. 141,         
                  Inertial Coordinate System on the Sky, Leningrad,             
                  Oct. 17-21, 1989.                                             
           Roeser, S. and Bastian, U. (1988): A New Star Catalogue of           
                  SAO Type, Astronomy and Astrophysics Suppl. 74, 449.          
           Roeser, S. and Bastian, U. (1989): Compilation of the PPM            
                  Catalogue, in: Star Catalogues, A Centennial Tribute to       
                  A.V. Vyssotsky, eds. A.G.D. Philip, A.R. Upgren,              
                  L. Davis press, Schenectady, N.Y., 1989.                      
           Roman, N.G. and Warren, W.H. Jr. (1984): Documentation for the       
                  Machine-Readable Version of the SAO Star Catalog,             
                  Version 1984, Astronomical Data Center, National Space        
                  Science Data Center, Greenbelt, Maryland                      
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           Acknowledgments                                                      
           ================                                                     
           Generations of careful and patient observers and plate measurers     
           have laid the base for the present work. Compared to their labour    
           all other efforts that went into the preparation of PPM - inclu-     
           ding our own - are small.                                            
                                                                                
           We gratefully acknowledge the help of Hamburger Sternwarte (C. de    
           Vegt and H. M. Steinbach) and of Observatoire de Strasbourg          
           (D. Egret and A. Fresneau) in providing the raw Astrographic         
           Catalogue on punched cards. Several tons of cards were moved with    
           the help of many hands in Strasbourg.  Also we thank T. Corbin of    
           US Naval Observatory for providing a magnetic tape containing        
           part of the Strasbourg punched data already.                         
                                                                                
           Heidelberger Druckmaschinen AG is gratefully acknowledged for        
           technical help in the reading of the punched cards.                  
                                                                                
           We thank the CMC group at La Palma for giving the CMC 4 catalogue    
           to us prior to publication.                                          
                                                                                
           Last but not least we have to thank many people at our institute,    
           especially E. Roehl and E. Mueller for careful checks and            
           completion of the Astrographic Catalogue data, M. Erbach, M.         
           Fleischer and T. Lederle for much valuable information on source     
           catalogues, H. Schwan for many fruitful discussions and for his      
           data on systematic differences between several catalogues, R.        
           Bien for the determination of weights for the CMC catalogues and     
           H. Bernstein for advice on mathematical statistics. Finally we       
           want to thank R. Wielen, director of ARI, for giving us the          
           opportunity to do this work.                                         
                                                                                
Modified 2/91 Susan E. Gessner, NSSDC, NASA/GSFC                                
