[vsnet-chat 5847] flatfields and photometric reference catalogs

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

  Maciej asked some questions in recent posts:

> -maybe flats are not the best. Can anyone tell me how,
> step by step, make good twilight flats. Which one will
> be better twiligt flats or light box flats?

  If you are using a system which takes images with a very
wide field -- more than one degree on a side -- then it is
very difficult to create accurate flatfields.  There are
real gradients in both the twilight sky and even the night
sky of several percent over such large fields, so any flats
you make from sky images will have gradients in them;
those gradients will systematically ruin your photometry.
Using a light box is probably a better idea, but it's 
difficult to avoid introducing scattered light.

  In the TASS project, which uses lenses and CCDs with a field
of view 4 degrees on a side, we have discovered gradients of
roughly one percent per degree in night-sky images which are
due to skyglow.  I guess it would help if we only looked
very near the zenith :-(  We're still trying to figure out
a proper method for creating flatfields ...

> I just examined one image using Tycho-2 catalog.
> Here are results vs previous GSC:
>         GSC    TYC2
>         PSCVY  6.08V  6.44V
>         ARIZZ  8.06V  8.41V

  Now, remember this: the GSC is based on scans of photographic
plates with very long exposure times.  Stars brighter than,
um, around twelfth (?) magnitude are very saturated.  I don't
see how one could use the GSC for stars brighter than tenth
magnitude with any confidence in the result.

  The Tycho2 catalog, on the other hand, is based on measurements
of stars with the Tycho satellite's photometer, which performed
best on bright stars.  Its response wasn't exactly the standard
shape for the B, V passbands, but the people who created Tycho-2
worked hard to transform the measurements to the standard
system.  I'd prefer the Tycho-2 values for stars this bright.
Below about magnitude 10, however, the random errors in Tycho
measurements become very large.

                                    Michael Richmond

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