Re: Post flat processing

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

  Mike G. has described his plan to make corrections to his TASS data
by using Tycho stars as standards.  I think that his method of 
dividing all scans into 4 Dec zones, and assuming that the correction
is a simple linear function within each zone, is a good one.

  I have one quibble, perhaps due to my imperfect understanding 
of the data.   If we use the Tycho catalog to make these adjustments,
then we can really only correct the "V" measurements: the Tycho 
instrument only measured B and V magnitudes (yeah, I know, it really
measured Bt and Vt, but the differences are small and easily removed).

  It is _not_ clear to me how well this plan will work for correcting
the I-band TASS data.  My guess is that _if_ we have "I" band measurements
for stars in the Tycho catalog, those are just estimates based on
the "B" and "V" magnitudes of the stars; that is, they are extrapolations,
not real measurements in through a reddish filter.  In that case, I think
it very possible that we could _add_ a new systematic error into the
TASS dataset by using the Tycho I-band measurements.

  Maybe I'm wrong -- maybe there are real, ground-based I-band
measurements of a large number of stars in the Tycho catalog.
In that case, the plan should work fine for TASS I-band measurements.

  If not, then I would suggest that we might do better to

        a. make no correction
        b. make a very rough correction based on the VERY few Landolt
              stars with true I-band measurements which fall into each
              scan (but this number is likely to be < 10 stars!)

  Again, if you've already taken this into account, Mike, my apologies.

                                      Michael Richmond