hints and tips for DS 20

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

  I'm working on some of the DS data right now, and hope
to write up a Tech Note sometime soon.  Let me explain
a few of the things I've noticed so far, since they may
help Michael Sallman or others to save a little time.

      - the box flats on disks 2 and 3 are similar 
            to night-sky flats derived from disk 6.
            Large-scale differences are at most 3-4 percent.

      - the I-band flats show faint circular arcs
            (probably the arcs mentioned by Michael S.).
            These are common in CCD chips at long wavelengths:
            I believe they are caused by small variations
            in the thickness of the silicon or coatings.
            The circular shape is a result of the manufacturing
            process.

      - the dark frames on disk 6 (only three for each camera)
            probably shouldn't be used for reductions; 
            I found what looks like a light leak in the
            V-band darks.  Strangely enough, I see no hint
            of this pattern in the V-band night-sky images
            for that night.

      - the night of disks 6, 7, 8 was not perfectly clear.
            I found that clouds came and went throughout the
            night.  A relatively clear patch runs from
            images 2239820 - 2239835, with the best set of
            3 consecutive images being 822,823,825.

      - the I-band data has a FWHM almost 1 pixel larger than
            the V-band data

      - when comparing Mark IV to Tycho-2 photometry, use only
            stars in the range
                       7.7 < V < 10.0
                       8.0 < I < 10.0
            The Mark IV data is saturated (or nonlinear) at
            brighter magnitudes, and the Tycho-2 data is ratty
            at fainter magnitudes

      - it appears to me that the V-band camera is very close
            to standard Landolt V (color term approx 0.01);
            the I-band camera has a color term of about -0.17:

                   Landolt I  =  TASS I  -  0.17*(TASS V - TASS I)

            (but this is preliminary)

      - the frames on disks 7,8 are centered at RA=71.2 deg, Dec=7.4 deg,
            which includes the middle of the shield of Orion

      - the images drift by about 1.5 pix E/W and 0.5 pix N/S between
            exposures on disks 7,8

  More later.

                                         Michael