Re: New Reference Catalog

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

  Tom wrote:

> What I need first is a catalog that covers -6 to +18.  I have been taking 
> data there since last November, and I have proper flats and darks for most 
> of it.  There are 10 to 20 measurements for many stars in this region 
> covering nearly 180 degrees in RA.  Enough for a good test, I think.

  Okay, so I went ahead and pared down on the Tycho2 reference catalog.
The new conditions are more stringent than the old ones:


    magnitude      Bt  >  1.0    (discards incorrect "zero" magnitudes)
    magnitude      Bt  < 11.8    (discards faint stars)
    magnitude      Vt  >  1.0    (discards incorrect "zero" magnitudes)
    magnitude      Vt  < 10.7    (discards faint stars)
    uncertainty in Bt  <  0.05
    uncertainty in Vt  <  0.05
    color   -0.2 < (Bt - Vt) < 1.8
    no other Tycho-2 star within 50 arcsec
    "solution" flag does not indicate double-star astrometric solution

  There are about 350,000 stars in this subset, which means about 8
per square degree if evenly spaced, which means about 120 per Mark IV
field.  

  After I went to the trouble of making this subset, I discovered that
I already had one.  In fact, the original reference catalog I distributed
with the Mark IV pipeline was just about this subset.  Since it didn't
have enough stars for astrometric matching purposes, however, I suggested
people use a bigger reference catalog, with more (but fainter) stars.
Sigh.

  Anyway, one can find this data file at

       http://spiff.rit.edu/tass/tycho/

The files are

           tyc2_photom.dat        20  MB, plain ASCII
           tyc2_photom.dat.gz     6.2 MB, gzipped

  I also include a sub-subset, just the stars between Declinations
-10 and 20, since Tom requested them.  

           tyc2_photom_eq.dat     3.8 MB, plain ASCII

  Tom (and others), you can awkwardly test this new subset on your
images by 

         a) running the current pipeline up to (but not including)
                  the "photom" step, with the big reference
                  catalog in place

         b) copying the new, smaller catalog over the big one,
                  in the same directory and with the same name

         c) running the "photom" step

  Yes, the next version of the code will allow for two separate
catalogs for these two separate purposes.

  Tom, if you are able to do this on just a single night's data,
could you let us know how it goes?

                                Michael Richmond