[ATM] Dall-Kirkham Coma Reduction?

[mpeck1 at ix.netcom.com (Michael Peck)]

At 10:26 14-02-04 -0800, John Spero wrote:
>for a planned Dall-Kirkham cassegrain project.  I've been looking at 
>Takahashi's Mewlon series of D-K's for ideas for my scope, and several of 
>the reviews of these telescopes claim that Takahashi masks the primary to 
>reduce the coma inherent in the D-K design.  Although the Mewlon's do use 
>an oversized primary (the Mewlon 250 uses a 260mm primary stopped down to 
>250mm), I don't understand how stopping down a D-K primary can reduce 
>coma. Am I missing something?
>

A telescope with a clear primary diameter of 250 mm. will have a little 
less coma than an otherwise identical one with a clear diameter of 260mm, 
but I'd hardly call that a strategy for coma correction. Chances are 
Takahashi has other reasons for using a 260mm blank to produce a 250mm 
mirror, such as perhaps allowing for edge defects.

If the review you're referring to is the one on "cloudy nights", remember 
that having a lot of money to spend on optical toys doesn't make you an 
expert on optics. The guy who did the Mewlon 250 review once had to be 
informed (on usenet!) that a DK cass is not a variant of a SCT.

Mike Peck


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