[ATM] gregorian considerations

[Douglas S Angle <Douglas.S.Angle@can.dupont.com>]

Thanks to all that replied to my earlier post about gregorian baffling and 
corrector plate.  At this point I'm really just exploring various design 
options.  If I choose a fast primary, small secondary, wide fully 
illuminated field, and short BFL, then I run into baffling problems - 
either stray light can enter the focusser directly or I vignette part of 
the field.  Several of the replies have helped me think through this some 
more, but I think ultimately there's a tradeoff in those parameters. While 
I appreciate all replies, there are a few comments that I'll address 
specifically:

Mike Lockwood wrote:
> Using the tertiary to direct the light path to the focuser on the top of 
the mirror box in itself performs some baffling, because the view of the 
sky through the eyepiece is limited to the reflection off the tertiary. 
Otherwise, the eyepiece sees the bottom of the mirror box.

It looks like a great scope.  I hadn't thought of this approach of a 
tertiary mirror before the primary, and others suggested similar 
approaches.   It also allows more flexibility in the secondary-to-focus 
distance.  I can't see that using a tertiary mirror is any different than 
a baffle of the same dimensions at the same location though.


Kevin MIchael Zabbo wrote:
> Why put a corrector in front of a Greg, anyway?

I'd like to have a reasonably fast system so was considering a primary 
shorter than f/4, maybe even as short as f/2.  I sure I couldn't figure a 
parabola that short, so was looking at a spherical primary with corrector 
plate instead.

> You are going to put this on an EQ platform with a nice drive, right?
Actually, if I build this instrument, it will be on a permanent equatorial 
mount.  The design objectives are at the "foolishly optimitic" stage now. 
It will likely turn into a dob before I'm done.


Jarvis Krumbein wrote:
> The Gregorian optical system was once the favored type for terrestrial 
telescopes during the 18th century.  The only baffle used was a stop 
located at the exit pupil of the eyepiece. 

That's what the one in the Ingalls ATM book looks like.  I'm finding that 
hard to reconcile with the discussion about baffling newtonians, and in 
general keeping stray light out of the eyepiece to improve contrast. 
Especially since the high f/ inherent in the design implies a planetary 
scope where you would not want to sacrifice contrast.  Ditto for a small 
secondary, although as I'm finding, the two may be mutually exclusive. 
I've never seen a gregorian, much less looked through one, so don't have 
anything to compare to.  Anyone have direct experience they'd like to 
share? 

-Doug

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