Re: ATM 16" F/33 valid design?

[Michael Peck <mpeck1@ix.netcom.com>]

At 20:12 1/15/2002 +0100, Dominic-Luc Webb molmed wrote:

>The February issue of S&T is out and there is an
>articule there about Emile Schaer's telescope and
>the "F" ring of Saturn. I have not seen this ring
>through any scope, but it seems Schaer did, with
>a 16" F/33 Cass (i.e., fl = 528"). Interesting
>thought... I have an 18" mirror, soon to become
>a slow Newtonian. Maybe its actually justified to
>core this beast and give it a Cass secondary, with
>a system EFL around F/30. I guess I could still use it

To answer the question in your subject header, sure it's a valid design, 
but that doesn't mean it's necessarily a wise design choice.

That telescope was probably driven by a couple design criteria. First, just 
eyeballing the relative length to diameter of the tube I'd guess the 
primary was around f/4, which would have been unusually short for that era. 
That would make the secondary magnification around 8.25, and the minimum 
required secondary obstruction would be about 12.5%, which is quite small 
for a cassegrain and presumably chosen to maximize theoretical contrast.(*)

The other issue is that eyepieces of the day would have been poorly 
corrected and had poor eye relief compared to what's available now. This 
would dictate a narrow light cone for the telescope and, in order to get a 
high magnification without risking corneal abrasion a long focal length 
eyepiece matched to a long focal length telescope would be favored.

I think the comment you quoted about the focal length of an observatory 
telescope was probably related to matching image scale to seeing conditions 
for photography, which is a different subject altogether.

Mike Peck

(*) What's the downside of a large secondary magnification, you might ask? 
Well the sensitivity of focal plane position to mirror separation is the 
secondary magnification squared (or m^2+1) - assuming a secondary 
magnification of 8.25 a 1mm change in mirror separation changes the focal 
plane position relative to the primary by approximately 70mm.

_________________

Michael Peck
email mpeck1@ix.netcom.com
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