Re: [ATM] Paul-Baker, Gregorian variety

["vladimir sacek" <v959@msn.com>]

  Jerry Reddell wrote:

  >This description sounds a lot like the Stevick-Paul configuration, which
  uses a normal primary, spherical convex, spherical concave, and then a
  typical flat diagonal to bring the image out for viewing.<

  The similarity is in the same parent design, and in exceptional image quality,
  but there are differences. The SP is unobstructed, nearly-flat field, but with 
  tilted components, and impractical for apertures larger than ~8 inch due to
  somewhat awkward shape and length. On the other hand, the arrangement
  I described has small c.obstruction, field curvature (correctable) and greater
  compactness, with no particular limitations aperture-wise.

  The arrangement is similarly sensitive to miscollimation to the SP;
  primary is about as sensitive as in a comparable Newtonian, secondary
  only about 1/3 as much, and spherical tertiary is fairly insensitive. 
  Probably the most practical way to collimate it is to do it separately, primary 
  and the first diagonal on one side, and the side tube with four elements on the other,
  and then have them propely positioned. 

  Wonder why isn't Paul-Baker more popular as an astrograph. It can be made around
  any good parabola. A 12" f/4.5, with ellipsoidal (K=-0.6) convex secondary and concave 
  spherical tertiary gives a perfect flat-field f/6.1 astrograph (the image can be made 
  accessible by inserting a diagonal). Even with spherical secondary it still has perfect
  center field and (mainly) astigmatic blur smaller than the Airy disc (8 micron) 0.25 degrees 
  off-axis. Astigmatism does induce some field curvature (~1700mm), which probably can be 
  nearly corrected with some more tweaking.

  Question came to my mind, how wide well corrected visual field we really do need?
  If the criterion is having Lambda/D being not more than 1 arcmin in the eyepiece,
  the size of needed diffraction-limited field is approximately that of a field stop of an 
  eyepiece with f.l. of 2F in mm (F=telescope f.l./D). So, an f/5 system would need 
  10mm-15mm diffraction-limited field diameter, depending on the eyepieces' FOV.
  The 1 arcmin criterion may too demanding; probably half as large field would be 
  still good. If so, a perfect field is not even necessary: an f/7 parabola already has it.

  Vlad



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