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    *             Introduction to PPM South 4.91              *         
    *                                                         *         
    *                       April 1991                        *         
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Contents                                                                
========                                                                
 1. Scope of PPM South 4.91                                             
 2. Construction of the catalogue                                       
 3. Properties of PPM South 4.91                                        
 4. Practical usage                                                     
 5. Explanation of the machine-readable version                         
    References                                                          
                                                                        
Preface                                                                 
=======                                                                 
This is a short explanation of the version 4.91 of the southern         
part of the PPM Star Catalogue. It briefly describes the way it         
is constructed and the contents of the catalogue on magnetic            
tape.                                                                   
                                                                        
PPM South is a joint effort of U. Bastian, S. Roeser, R. Wielen         
at Astronomisches Rechen-Institut, Heidelberg; D.D. Polozhentsev,       
Kh.I.  Potter, L.I. Yagudin at Pulkovo Observatory, Leningrad;          
Ya.S. Yatskiv, Kiev Observatory; and V.V. Nesterov, Sternberg           
Institute, Moscow.                                                      
                                  U. Bastian and S. Roeser              
                                  Astronomisches Rechen-Institut        
                                  Heidelberg, April 1991                
                                                                        
                                                                        
1. Scope of PPM South 4.91                                              
==========================                                              
This version of the southern part of PPM gives positions and            
proper motions of 144787 stars south of -2.5 degrees declination.       
At this border a continuous transition between the northern and         
southern part of PPM was tailored in such a way that no overlap,        
nor gaps, nor double entries occurred. The main purpose of PPM          
South 4.91 is to provide as early as possible an accurate, dense        
and convenient net of astrometric reference stars on the southern       
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. Until recently, only      
the SAO Catalog was available as a reference star catalogue on          
the southern sky. Reasons for its replacement have been given in        
the Introduction to PPM (north) and will not be repeated here.          
The high present-epoch accuracy of PPM South 4.91 is due to the         
inclusion of the recently completed FOKAT-S and CPC-2 catalogues.       
FOKAT-S is a fourfold coverage of the southern sky, photographed        
around 1984 in a Soviet-Bolivian cooperation. CPC-2 also is a           
fourfold coverage of the sky, photographed around 1966.                 
PPM South 4.91 is a preliminary catalogue for two reasons:  first,      
it was not possible to derive proper motions for all 203135 stars       
of FOKAT-S because early-epoch observations of these stars are          
lacking.  Second, most stars brighter than about sixth magnitude        
are missing from this version.  Both will be changed for the final      
PPM South. A new reduction of the southern part of the                  
Astrographic Catalogue will be performed, which will then give old      
observations of almost all FOKAT-S stars. In addition, we shall         
incorporate into that final version all stars of FK5 and its            
extensions, and of the catalogues SRS, GC and SAO.                      
The data files following this introduction are analogous and            
complementary to the resp. files of PPM (north). Combining the          
corresponding files will provide the user with a catalogue of           
326518 reference stars on the whole sky.                                
                                                                        
                                                                        
2. Construction of the catalogue.                                       
=================================                                       
The construction of the catalogue proceeded in much the same way        
as described in the Introduction to PPM (north). Only few changes       
to that procedure were made, and will be mentioned below.               
 Step 1: Compilation of the star list.                                  
We chose, for the time being, the 203135 stars of the FOKAT-S           
catalogue (Bystrov et al., 1989) as star list for PPM South 4.91.       
No attempts were made to incorporate stars that were not                
contained in FOKAT-S. This will be changed for the final version,       
as mentioned above.                                                     
 Step 2: Identification of stars in the source catalogues               
The list of source catalogues is given in Chapter 3. 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 effect of       
grossly incorrect positions in the star list, etc.                      
 Step 3: The system of positions and proper motions.                    
It is well known that the construction of an extension of the           
system of FK5 to higher star densities and fainter magnitudes on        
the southern hemisphere is a troublesome task. It is difficult          
because of the poor observational history of the southern               
hemisphere and the problem of availability of old catalogues free       
of magnitude equation. So we adopted the recently published SRS         
with its preliminary proper motions (Smith et al., 1989) as the         
"system" of positions and proper motions. By a catalogue                
comparison between SRS and the CMC catalogues we convinced              
ourselves that, at least in the declination range between -40           
degrees and the equator, no serious systematic differences occur        
at present epoch, either depending on right ascension,                  
declination, magnitude, or spectral type.                               
 Step 4: Reduction of source catalogues to the "system".                
Zonal systematic deviations as well as magnitude- and                   
colour-dependent systematic deviations between each of the source       
catalogues and the "system", were determined and eliminated.            
                                                                        
                                                                        
Step 5: Weights for the source positions.                               
In a least-squares adjustment, weights have to be assigned to the       
individual measurements according to their mean errors. Mean            
errors for the source catalogues were preferentially taken from         
Eichhorn (1974) with a few exceptions:  the FOKAT-S catalogue was       
assigned a mean error of 0.13 arcsec.  Investigation of the Yale        
zone catalogues yielded that some of them, especially the latest        
ones, were in fact much better than estimated by Eichhorn. This         
is because the final catalogues were not available when Eichhorn        
wrote his book. We assigned 0.23 arcsec rms error for Yale 31 and       
32, and 0.20 arcsec for Yale 12 to 17. The data in the Cape             
Astrographic Zone stem from measurements of the Astrographic            
Catalogue (Cape Part).  From our experience with the Astrographic       
Catalogue on the northern sky, we assigned 0.28 arcsec for the          
mean error of a position.  For the CPC-2 we used 0.11 arcsec.           
All these values were chosen to be conservative estimates, in           
order to ensure that the accuracy of PPM South 4.91 will                
certainly not be overestimated by us.                                   
 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 ascension and declination.  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' or 'Q' in        
column 127 of File 2). This occurred in about 3100 cases.               
 Step 7: Transformation to FK5, J2000.                                  
The positions and proper motions derived so far were transformed        
from the preliminary system (SRS, nominally on B1950/FK4) to the        
new IAU (1976) coordinate system, as defined by FK5. This               
transition followed exactly the same procedure as was used in the       
construction of FK5 (Fricke et al., 1988).                              
                                                                        
                                                                        
3. Properties of PPM South 4.91                                         
===============================                                         
                                                                        
Random errors                                                           
-------------                                                           
The table below shows a summary of the error budget of PPM South        
4.91.  Each line of the table gives the following data for              
different declination zones:  the number of stars in the zone, the      
average number of source positions per star, the average number         
of stars per square degree, 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 SAOC are given for comparison.       
                                                                        
         Table 1: Error budget of PPM South 4.91                        
     Zone      No.   No.  Dens. mean ep.  mean err.   mean err.         
              stars  obs. /sq.d. 1900+    prop. mot.  pos. 1990         
 -2.5 to -30  58931  4.0  6.3  67.6 67.5  0.39 0.39   0.12 0.12         
 -30  to -40  24189  5.2  8.2  64.8 65.9  0.38 0.39   0.12 0.12         
 -40  to -52  25310  4.1  8.5  64.0 64.0  0.35 0.35   0.12 0.12         
 -52  to -64  20844  4.9  9.2  66.7 67.3  0.43 0.44   0.12 0.12         
 -64  to -90  15513  4.8  7.4  68.4 69.3  0.52 0.54   0.13 0.13         
     total   144787  4.4  7.3  66.5 66.8  0.40 0.40   0.12 0.12         
  SAOC,south 126000  2.0  6.3  30   30    1.5  1.5    1.2  1.2          
                                                                        
Note that (on average) about 4.4 measured positions are available       
per star. Although this situation is better than in the case of         
SAOC, it is not sufficient to resolve all problem cases. Among          
the 144787 stars of PPM South 4.91 there are 3088 such cases (see       
chapter 2, step 6). They carry the 'P' or 'Q' flag mentioned            
above. For the final catalogue the situation will improve, when         
the data from the Astrographic Catalogue are incorporated.              
This will give two additional positions per star.                       
                                                                        
Source catalogues                                                       
------------------                                                      
Almost 640 000 source positions (i.e., right ascensions and             
declinations) were used for PPM South 4.91.  The following table        
lists the various groups of source catalogues used and the total        
number of positions taken from each group. More information on          
many of the catalogues as well as bibliographic references, are         
given in the book by Eichhorn (1974).                                   
                                                                        
                                                                        
          Table 2: Source Catalogues                                    
     FOKAT-S                        144 787                             
     CPC-2                          120 342                             
     "AGK1"                         122 936                             
     CPC                             60 221                             
     Cape Astrographic Zone and                                         
     Cape Faint Stars                25 363                             
     Yale zone catalogues           124 611                             
     Melbourne Gen. Cat. 3 to 5       7 712                             
     Perth Cat. of Astrogr. Ref. St.  8 913                             
     Sydney SSC                      21 631                             
     Total                          639 526                             
                                                                        
Here, "AGK1" stands for the different catalogues of the "Catalog        
der Astronomischen Gesellschaft, Zweite Abtheilung" and its South       
American extension, the catalogues Cordoba A to D and La Plata A        
to F.                                                                   
                                                                        
Numbering.                                                              
----------                                                              
The assignment of running numbers to the stars is not a mere            
continuation of PPM (north), but starts with number 500001 for          
the first star and ends with number 644787. This ensures that a         
star in this preliminary catalogue can be definitely identified         
by its number, and no confusion with the final PPM number need be       
feared. As mentioned earlier, the final catalogue will contain          
roughly 200 000 stars on the southern hemisphere compared to            
144 000 at present.  Its numbering will be a consistent                 
continuation of the PPM (north) numbering.                              
                                                                        
Magnitudes and spectral types.                                          
------------------------------                                          
The magnitudes are taken from FOKAT-S. They are approximate             
visual magnitudes. The spectral types, if available, were taken         
from the SAOC. Efforts will be taken for the final catalogue to         
publish magnitudes on a homogeneous system and to search for            
spectral types for stars having none in the present version.            
                                                                        
Proper motions.                                                         
---------------                                                         
PPM South 4.91 contains 3435 stars whose proper motions are based       
on only two measured positions (see bytes 71-72 in file 2). In          
these cases the possibility of misidentification exists, and a          
coarse error in one of the positions immediately leads to an            
erroneous proper motion.                                                
                                                                        
                                                                        
                                                                        
4. Practical usage                                                      
==================                                                      
PPM South 4.91 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 the catalogue 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. In general, this adds up to about        
an arcsec in position and several tenths of an arcsec per century       
in proper motion. In the region between -60 and -80 degrees             
declination, the difference between the FK4 and the FK5 system is       
especially large. It amounts to 1.6 arcsec in right ascension at        
epoch 2000, and to about 1 arcsec/century for the proper motion         
in right ascension.                                                     
                                                                        
Application of proper motion and precession still gives mean            
positions. In order to derive apparent positions for PPM objects,       
one must also apply nutation, aberration, stellar parallax, and         
relativistic light bending (see 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.                         
                                                                        
                                                                        
5. Explanation of the machine-readable version                          
==============================================                          
The machine-readable version of PPM South 4.91 contains 2               
physical files.                                                         
The three files are:                                                    
 - File 1: The text of this introduction to PPM South 4.91              
 - File 2: The catalogue as an easily readable table, 144787            
           records, in the same format as PPM (north).                  
The remainder of this chapter gives very short descriptions of          
these two files. It describes the formats of the files together         
with a short description of the contents.                               
 The tables defining the individual files give the following            
information for each data item:                                         
 - name of the item (mnemonic)                                          
 - position of the item within each record (first and last byte)        
 - FORTRAN format                                                       
 - explanation                                                          
 - maybe a remark                                                       
In addition, there is information on record length, etc., at the top    
of each table.                                                          
                                                                        
File 1: The Introduction to PPM South 4.91                              
------------------------------------------                              
This is a text file, given in exactly the same format as the            
printed Introduction to PPM.  Record length is 72 bytes.                
                                                                        
                                                                        
File 2: PPM South 4.91 as an easily readable table                      
--------------------------------------------------                      
This is the main file giving the catalogue proper. Record length        
is 133 bytes. The stars are arranged in bands of 10 degrees width       
in declination. Within each of these bands they are ordered             
according to right ascension. This arrangement was also used in         
the SAO Catalog.  You can use this file to produce a rough              
printed version of PPM South 4.91 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  designation of the star in PPM South 4.91        
                         starting with No. 500001, see chapter 3.       
 DM      8 -  17   A10   designation of the star in the Bonner          
                         Durchmusterung (BD). South of -30 deg          
                         declination (1950): Designation in the         
                         Cape Photographic Durchmusterung (CPD).        
 Mag    20 -  23   F4.1  magnitude, see chapter 3.                      
 Sp     25 -  26   A2    spectral type, see chapter 3.                  
 R.A.   28 -  39   F2.0,1X,F2.0,1X,F6.3                                 
                   Right Ascension for the equinox and epoch            
                   J2000.0, on the system of FK5, given in              
                   hours, minutes and seconds of time.                  
 Sign   42 -  42   A1  Sign of declination ('+' or '-').                
 Dec.   43 -  53   F2.0,1X,F2.0,1X,F5.2                                 
                   Declination for the equinox and epoch                
                   J2000.0, on the system of FK5, given in              
                   degrees, minutes and seconds of arc.                 
 PMA    56 -  62   F7.4                                                 
                   proper motion in right ascension for epoch           
                   and equinox J2000.0, on the system of FK5,           
                   given in seconds of time per Julian year.            
 PMD    64 -  69   F6.3                                                 
                   proper motion in declination for epoch and           
                   equinox J2000.0, on the system of FK5,               
                   given in seconds of arc per Julian year.             
 N      71 -  72   I2                                                   
                   number of individual published positions             
                   used for the derivation of the position              
                   and proper motion given                              
 SA     74 -  75   F2.0                                                 
                   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.                                 
 SD     77 -  78   F2.0                                                 
                   mean error of declination at the mean epoch          
                   of declination, given in units of                    
                   0.01 seconds of arc.                                 
 SPMA   80 -  83   F4.1                                                 
                   mean error of the proper motion in right             
                   ascension, multiplied by the cosine of               
                   declination, given in units of 0.001 seconds         
                   of arc per Julian year                               
 SPMD   85 -  88   F4.1                                                 
                   mean error of the proper motion in declination       
                   given in units of 0.001 seconds of arc per           
                   Julian year                                          
 EPA    90 -  94   F5.2                                                 
                   weighted mean epoch of the measured positions        
                   used for the derivation of R.A. and PMA,             
                   given in years since 1900.0                          
 EPD    96 - 100   F5.2                                                 
                   weighted mean epoch of the measured positions        
                   used for the derivation of Dec. and PMD,             
                   given in years since 1900.0                          
 SAO   102 - 107   I6    designation of the star in the SAO             
                         Catalog                                        
 HD    109 - 114   I6    designation of the star in the Henry           
                         Draper Catalogue                               
 CoD   117 - 126   A10   designation of the star in the                 
                         Cordoba Durchmusterung                         
 Flag  127 - 127   A1    problem flag  -  possible values:              
                           N   no problems with this star               
                           O   no problems with this star, but          
                               final FOKAT-S data not available         
                           Q   problem case (see chapter 2)             
                           P   problem case, and final FOKAT-S          
                               data not available                       
Be careful not to miss the sign of the declination (byte 42)            
while transforming this formatted file into a numeric file on           
your computer.                                                          
                                                                        
                                                                        
References.                                                             
===========                                                             
Bystrov, N.F., Polozhentsev, D.D., Potter, Kh.I., Zalles, R.F.,         
   Zelaya, J.A. and Yagudin, L.I. (1989): On the                        
   FOKAT catalog. Astron. Zh., 66, 425                                  
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                              
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 Transactions XVI B, Reidel Publ., Dordrecht 1977        
Lieske, J. (1979): Precession matrix based on IAU(1976) system          
   of astronomical constants, A&A, 73, 282                              
Murray, C.A. (1983): Vectorial Astrometry, Adam Hilger Ltd.,            
   Bristol, UK                                                          
Smith, C.A., Corbin, T.E. Hughes, J.E., Jackson E.S.,                   
   Khrutskaya, E.V, Polozhentsev, A.D., Polozhentsev, D.D,              
   Yagudin, L.I., Zverev, M.S. (1989): The SRS catalog                  
   of 20488 star positions: culmination of an international             
   cooperative effort, in: IAU Symp. No. 141, Inertial                  
   Coordinate System on the Sky, Leningrad, Oct. 17-21,                 
   1989                                                                 
