Re: [ATM] Re: Aplanatic Gregorian

["john sherman" <atm@mail.johnspics.com>]

Hi Tom,

I believe the Ritchey-Cretien Cass is a "compensating system" like you describe. Surely there would be a Gregorian version? 

I recall from an early Sci-Am article a folded Gregorian. Near the focus point of the primary is a 45-degree flat mirror (like in a Newt). The Greg secondary mirror is outside the tube, facing in perpendicularly to the primary. It re-aims the light past the flat once more on the way to focus, So the flat is an obstruction to both mirrors. However, the flat can be very small compared to a normal Greg or Cass secondary. And you can now come up with a design that requires a very large secondary, 50% or more, since the secondary does not obstruct light.

The finished scope is in the shape of a "T". The primary at the bottom, the secondary at top left, the flat at the intersection, and the eyepiece at the top right. Best to move the flat a bit away from the focal point of the primary, so dust does not get imaged into the eyepiece.

Cool, eh?

John




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