                "PHOTOMETRIC CATALOGUE AROUND M42"           
                           Mati Morel 
       An electronic reincarnation of A.D. Andrews's 1981 Atlas.

  My project is aimed at restoring most of the basic data in David Andrews'
monumental 1981 Atlas of the Orion Nebula region to computer-readable form.
It was decided to update his B1950.0 equatorial coordinates to the J2000.0
epoch, as a charting tool, and would allow unambiguous matching to modern astrometric and survey catalogues such as Tycho-2, UCAC2 and CMC14.

Ref:
1) Source of star numbering/coordinates/BV data :
   "A Photometric Atlas of the Orion Nebula", Andrews, A.D. 1981,  Armagh 
   Observatory. Atlas and Catalogue. Epoch 1974. It documents B1950.0 
   equatorial coordinates and photographic UBVI magnitudes for >16000 stars
   numbered 1-16777 in a sky region of 21.5 sq centred on the Orion Neb. 
   (M42).  Atlas extends from (1950) 5h23m to 5h42m RA, -3 to -7.5deg dec.
   Photographic UBVI data was derived from scans of plates secured by the
   SRC/UK Schmidt telescope, Siding Spring NSW. A complete listing of these
   stars appears in hardcopy form in Andrews' Atlas, but sadly the computer 
   readable files had been destroyed shortly after publication. 

1. The Challenge.
   As indicated above, the coordinates in Andrews' Atlas are derived from
scans on the GALAXY machine at the Royal Greenwich Observatory. It appears
that each scan strip spans 8.192mm = 36.864sec RA.    This means that some
scan strips fall entirely within two adjacent minute-lines of RA, and other
strips lie astride particular minute-lines. Moreover, the stars in his Atlas
are largely numbered according to the scan strip in which they fall, rather
than being ordered strictly by Right Ascension. Also, the scans progress from
East to West, which is the opposite direction to that normally found in most
astrometric catalogues. It appears that 31 scan strips were required for each
UK Schmidt plate. The densest regions of H II emission are omitted. 

2. Transformation of Coordinates.
   In the course of 113 days Andrews' data were was entered into a spreadsheet
with separate columns for 
*  Star number
*  1950 position
*  B magnitude
*  V magnitude
   This was done one "panel" (150 stars) at a time, from the printed pages.
Each batch was processed and saved before moving on to the next one. Certain 
magnitudes (U and I) were skipped, being far from complete, with numerous  
large gaps. My spreadsheet was embedded with formulae, which corrected all 
positions automatically, from B1950.0 to J2000.0. These formulae, it turned out, returned positions which were close to,  but slightly off correct. The small residual, or offset value, was internally consistent for each block of 150 stars, and was determined by identifying 15-20 stars, in range 10-13V in catalogues such as Tycho-2, CMC14 or UCAC2, and comparing those positions with my own. A mean offset was then calculated, typically of the order 0.15s RA and 5" dec., meaning that RAsec was systematically too small by this amount, and the dec. seconds were too far north by this amount, roughly,  (This step was also useful in picking up egregious keystroking errors, which proved to be very rare). This mean correction was applied to each star in the spreadsheet, and it is expected that final corrected positions conform to the Tycho-2 reference frame to, typically, +/-0.03s RA, +/-0.5" dec.
   Over the life of the project at least 2152 stars from Tycho-2/CMC14/UCAC2 were looked up in determining positional offsets, CMC14 being much favoured. For ALL stars brighter than 10.0V, positions from Tycho-2 have been adopted.  
This work is closer to epoch 2000, than Andrews' original Atlas, but it must be empahasized, in general NO PROPER MOTIONS HAVE BEEN TAKEN INTO ACCOUNT. Except for stars brighter than 10.0V - all positions are adopted from Tycho-2.
3. Magnitudes.
   Andrews' Atlas provides V magnitudes for approximately 95% of his stars; B magnitudes appear for a similar number, but slightly less. However, there are several hundred stars for which no BV data is given at all. I looked up 291 such stars in CMC14, and using known values for r', J and Ks, V was derived for each, using the standard formula
            V = 0.628 x (J-Ks) + (0.995 x r')
   This sample of 291 stars yielded a mean value of V = 16.507. 
   The overwhelming majority of V so derived lay betwen 16.00 and 17.00. These derived values of V have been inserted in table ANDREWS.DAT, and are followed by the letter "C".  There are however at least an equal number of magnitudes which have yet to be transformed. A placeholder value of 16.50: has been inserted, this being the approximate limit of the catalogue in V.          

4. Statistics.
  The stars in Andrews' Atlas are numbered up to 16777. Upon keystroking his data I find that the actual number is 16398, as 379 numbers do not exist (skipped).  The 16398 numbered objects in ANDREWS.DAT include the following:    
- 23 duplicate pairs, identical in every respect
- 41 faint galaxies in the PGC (Principal Galaxy Catalogue) 
- 1 bright galaxy with an NGC number
- 1 reflection nebula
- 2 objects which appear to be non-existent (not found on POSS plates).
  Most of the PGC identifications appear real, owing to very close coincidence in positions, but it cannot be excluded that some are actually faint foreground stars superimposed on distant galaxies.

5. Layout.
  "Photometric Catalogue around M42" consists of three files:
- README_1.txt   file you are now reading
- andrews.tdf    required file for GUIDE8, to read/display the .DAT file
- ANDREWS.DAT    the main table of real, numbered objects. All objects have been re-sorted, according to Right Ascension, and therefore the highest numbers tend to appear close to the top, lowest numbers near the end (with few exceptions).
  The columns are read as follows, from the short extract, below.
- 1  :  Star number from Andrews' Atlas
- 2  :  RA hour
- 3  :  RA min
- 4  :  RA sec
- 5  :  Dec. degree
- 6  :  Dec. min
- 7  :  Dec. sec
- 8  :  B magn.
- 9  :  V magn.
-10  :  Remark
------------------------------------------------------------
  1    2  3  4     5  6  7     8     9     10
------------------------------------------------------------ 
16105  5 25 33.82 -7 30 13.6  15.54 13.74
16143  5 25 34.72 -7 15 47.2  13.91 13.32
16147  5 25 34.86 -7 14 20.1  15.78 15.82
16187  5 25 35.28 -6 55 42.0  13.05 12.26
16129  5 25 35.60 -7 21  3.7  15.60 15.33
16110  5 25 35.88 -7 29  9.8  15.24 14.15
16189  5 25 36.15 -6 55 18.1  11.74 11.17
16179  5 25 36.29 -6 58 19.4  14.64 14.49
16308  5 25 36.76 -6  8 13.2  14.18 13.29

6. References to Catalogues of Precision.

Abbrev.      Catalogue       Citation
================================================================================
CMC14       CMC14           http://www.ast.cam.ac.uk/~dwe/SRF/cmc14.html
                            http://vizier.u-strasbg.fr/viz-bin/Cat?I/304
GSC2.2      GSC2.2          (2001GSC2..C......0S, I/271)
Hip         Hipparcos       (1997HIP...C......0E, I/239)
T2          Tycho-2         (2000A&A...357..367H, I/259)
U           UCAC2           (2004AJ....127.3043Z, I/289) 
PGC         PGC             (Paturel, G., Bottinelli, L., Gouguenheim, L. 1989.
                            (1989A&AS...80..299P) 2nd ed. PGC CD-ROM (1996).
2MASS       2MASS           (2006AJ....131.1163S, II/246)

7. General Remarks.
  How would one characterize "Photometric Catalogue around M42"?  Having compared the updated positions with those from Tycho-2, CMC14 etc, I find that my positions are consistently reliable, though just slightly "soft". There is no ambiguity in matching with those other catalogues, which MUST be consulted in order to take proper motion into account. Similarly, the BV data, when compared with the very latest photometry, is shown to be somewhat more noisy, but generally reliable to +/- 0.1mag at least, and perhaps slightly better. I regard this catalogue as a charting tool when run on GUIDE8, as one can get a good sense of the BV magnitude of every star in the field, down to 16th mag., while also being able to layer data from other catalogues.

8. Acknowledgement.
   I acknowledge the use of the Vizier service of the Centre de Donnees Stellaires, Strasbourg, in downloading data from their online catalogues.

Editor: Mati Morel,  Morel Astrographics
        6 Blakewell Road, Thornton NSW 2322 Australia
        mmorel7@bigpond.com

2011 November 27
2011 November 29 (revised)    