Re: Tycho-2 catalog

[Stupendous Man <richmond@a188-l009.rit.edu>]

  Tom asks:

> Uhhhhhh!  Just how much do we know about the accuracy of the magnitudes in 
> the tyc2 catalog that we are using for our reference catalog?

  I find several papers with information about Tycho-2.  First,

    "The Tycho-2 catalogue of the 2.5 million brightest stars",
    by Hog et al., Astron. Astrophys. 355, L27-L30 (2000),
    http://adsabs.harvard.edu/cgi-bin/nph-bib_query?
            bibcode=2000A%26A...355L..27H&db_key=AST&high=3e1d8ce0f003848

which states in Section 4:

    4. Photometric accuracy

  Internal standard errors of the Tycho-2 photometry are shown in Fig. 2b. It
  is concluded in H2000a, from a comparison with ground-based photometry, that
  the ratio of the external and internal standard errors is close to 1.0 for
  stars brighter than magnitude 7.5 and then gradually rises to at most 1.5 at
  magnitude 10 and fainter. The Tycho-2 magnitudes are systematically correct
  down to magnitudes 13 and 12 in [FORMULA] and [FORMULA], respectively. Fainter
  magnitudes are not trustworthy, and magnitudes in the catalogue fainter than
  respectively 15 and 14 should be understood to mean that the star was not
  detected in that colour.  


  Another good source of information is 

       "Construction and verification of the Tycho-2 Catalogue",
        by Hog et al., Astron. Astrophys. 357, 367-386 (2000).
        http://adsabs.harvard.edu/cgi-bin/nph-bib_query?
            bibcode=2000A%26A...357..367H&db_key=AST&high=3e1d8ce0f003848

This paper compares the Tycho-2 measurements against ground-based 
measurements in the B and V passbands.  It concludes that for bright
stars, defined as Bt < 11.8 and Vt < 10.7, the Tycho-2 magnitudes are
accurate (i.e., match ground-based magnitudes).  Over this range,
they should also be precise (i.e., have small scatter, less than 
0.10 mag in each band).

  When I created the "refcat" subset, I tried to include only 
stars which would be well behaved:

          magnitude      Bt  >  1.0    (discards incorrect "zero" magnitudes)
          magnitude      Vt  >  1.0    (discards incorrect "zero" magnitudes)
          uncertainty in Bt  <  0.20
          uncertainty in Vt  <  0.20
          color   -0.2 < (Bt - Vt) < 1.8
          no other Tycho-2 star within 20 arcsec
          "solution" flag does not indicate double-star astrometric solution

  So Tom's experience:

> I have looked at the difference between what the pipeline measures for the 
> refcat star magnitudes and the magnitudes listed in refcat for 
> DS-24-3.  This has been done frame by frame.  Some of the stars are 
> measured 2 magnitudes off from their catalog values.  It is obvious that 
> each maverick star will affect the measurement depending on whether or not 
> it is used for a particular frame.

puzzles me as much as it puzzles Tom.  Something is rotten in the
state of Denmark...

                                    Michael Richmond