Re: Distortion

[aah@nofs.navy.mil]

Tom asked how much distortion we can stand.  The answer: a fair amount.
Chris already mentioned that the astrometric solution can include the
proper terms for the normal distortions, like pincushion and barrel.  There
are enough reference stars in the 4-degree field of view to correct for some
pretty high order terms, assuming the images themselves are clean.
  The photometry suffers, as you have to take several additional steps (see the
latest NOAO newsletter for a discussion of flatfielding with distorted
images).  However, my main suggestion is to get spot diagrams from the
optical solution and look at what the images are like at various places
in the field of view.  If they are smaller than 1-2 pixels, and if the
geometric distortions are simple cubics, then we can live with it.  Of
course, the smaller the amount of distortion, the better!
  Camera/filter combinations need not have the same focal length.  That
changes the scale for each wavelength, but since you are extracting the
data and putting them on an RA/DEC reference frame, the input scales
are unimportant (you lose a little at the edges, but you probably wouldn't
use those stars anyway).  Also, differing focal lengths would affect drift-scan
observations, but the mark IV is designed to be a tracking system.
  As for whether 400mm is the best focal length:  I'll try to write something
up (perhaps a technical note?) since I need such tables for my book anyway,
and get it posted in the next couple of days.  Bottom line:  it is a good
compromise in my mind, since it gives a good aperture, reasonable plate
scale, and easily-achievable focal ratio.
  Bessell (CCDA, Fall 1995 p. 20) gives the following bandpasses for filters,
where L = central wavelength (nm) and W = full width at half maximum (nm):
    filt  L     W
      B  436   94
      V  545   88
      R  638  138
      I  797  149
This is pretty close to your estimates.
Arne