Disk 17 FITS headers

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

  I'm trying the reduce the images on Disk 17.  Here's what I've found
concerning the FITS headers:

     1. some keywords have changed (relative to old Mark IV keywords)

            CRVAL1   is now   RA
            CRVAL2   is now   DEC
            UT       is now   UTSTART
           
     2. some keywords are new -- the ones I am using are

            IMAGETYP
            EXPTIME

  Warnings for others who are using Disk 17:

     a. the RA value is in sexigesmial notation, formatted as a string:
              '13:03:43.4'

     b. the Dec value is NOT in sexigesimal notation, but is formatted
        as a string:
              '1        '

     c. the EXPTIME values have a significant scatter.  What I mean
        is that the values in the FITS headers are (for example)

                 101.8, 101.4, 100.7, 101.6, 99.4, 100.2, etc.

        when one might have expected identical values of "100" in
        each case.  This causes problems for me when I try to 
        create master dark frames.  I'd like to combine dark images
        with the same exposure times ... but no two images have
        (exactly) the same exposure time.  Moreover, I'd like to
        use the 100-second darks to subtract from the 100-second
        object frames ... but again, the exposure times aren't
        all 100 seconds.

        I'm dealing with this by binning all exposure times to
        the nearest 10 seconds before I try to group frames together.
        It's a minor pain.  

        Do we know if the variations are real?  That is, was the
        exposure time for a frame which says "100.4" in the FITS
        header really 100.4 seconds?

     d. the IMAGETYP values are often incorrect.  I found that
        the dark frames are misidentified:

           frame            has IMAGETYP         but is really
         -------------------------------------------------------
           2747                dark                 object
           2749                object               dark

           2763                dark                 object
           2765                object               dark

           2779                dark                 object
           2781                object               dark

           2795                dark                 object
           2797                object               dark

           2810                dark                 object
           2812                object               dark

         Apparently, there's an "off-by-two" error in the assignment
         of IMAGETYP keyword.

  On another note, the V-band master flatfield (made from 34 object
frames) shows small-scale features: blurry spots (perhaps due to
spots on the lens?) and vague dark shapes near two of the edges 
(tree branches?).

  I'll continue to work on the data over the weekend.

                                                     Michael Richmond