A hyperbola over-corrects for spherical aberration. The hyperbolic mirror still needs a corrector flattener but it is a relatively simple design - two lenses made of standard glasses with simple spherical radii on 4 surfaces to give a wide corrected photographic field.
By comparison, the parabolic coma corrector is more complex. The ASA Keller correctors often cost more than the parabolic reflector owners attaches it to.
However I believe that a hyperbolic astrograph is just that - an astrograph. Can't be used for visual observations because the corrector needs to be quite close to the focal plane.
Something like an f4 parabolic newtonian is a more flexible combo in that it can be used with :
- a 1x coma corrector and eyepiece as an f4 visual scope
- a 1x coma corrector as an f2.8 astrograph
- a 0.73x keller coma corrector as an f2.8 astrograph
My comments above relate to newtonian style optical configurations. There are other cassegrain type designs where the hyperbolic primary is matched with a spherical secondary and a field flattener to yield a wide flat field.
Keep in mind that to take full advantage of the full corrected wide field, you will probably spend a lot more on a large format CCD camera. Some of these designs hark back to the days of film photography where people wanted to use medium format film - 60 x 70mm or large format 4x5 inch sheet film on amateur size scopes or large plates on professional scale scopes.
Might be worth your while spending $30 on this book before spending any serious money.
Telescope Optics by Harrie Rutten and Martin van Venrooij
http://www.willbell.com/tm/tm6.htm
or
Telescopes, Eyepieces and Astrographs
by Gregory Hallock Smith, Roger Ceragioli and Richard Berry
http://www.willbell.com/tm/Telescope...trographs.html
Joe