Re: Making off-axis optics

From: Thomas Womack (twomack_at_chiark.greenend.org.uk)
Date: 10/08/04 

Date: 08 Oct 2004 17:00:08 +0100 (BST)

In article <ck6bil$n7b$1@zot.isi.edu>, Brian Tung <brian@isi.edu> wrote:
>Thomas Womack wrote:
>> The ATM book I last read was very clear that you had to trepann the
>> mirror for a Cassegrain design before polishing and figuring, because
>> otherwise "released stresses" would completely ruin the figure. How
>> come this doesn't apply for producing off-axis mirrors by cutting
>> chunks cookie-cutter-style from a large paraboloid; is there some
>> physical property which makes cutting out the centre uniquely
>> damaging?

>I'm not sure, but I observe that in the case of the Cassegrain, you
>use the donut part of the mirror, whereas with the off-axis, it's the
>donut hole that you end up using. That might have something to do
>with it.

I was imagining that you'd get several (if I've got my conception of
the geometry right, six) small off-axis mirrors from one paraboloid,
so all but the first would be using the donut part too. If that's not
the case, it goes a long way to explain the pricing of OA telescopes!

Tom

Relevant Pages

  • Re: Making off-axis optics
    ... >I'm not sure, but I observe that in the case of the Cassegrain, you ... >use the donut part of the mirror, whereas with the off-axis, it's the ... >donut hole that you end up using. ...
    (sci.astro.amateur)
  • Re: Formatting [was: Re: Grammar Crackdown]
    ... Have you considered sending the mirror off for re-silvering? ... the time that the paraboloid was finally formed. ... the mirror tested to near-perfect before polishing was complete. ... I was happy with the resulting telescope and mount until the telescope ...
    (alt.usage.english)
  • Re: Formatting [was: Re: Grammar Crackdown]
    ... the mirror tested to near-perfect before polishing was complete. ... When observing, the eyepiece is focussed on a virtual image two inches away, not on the mirror surface forty inches beyond that. ...
    (alt.usage.english)
  • Re: Formatting [was: Re: Grammar Crackdown]
    ... Have you considered sending the mirror off for re-silvering? ... the time that the paraboloid was finally formed. ... the mirror tested to near-perfect before polishing was complete. ... I was happy with the resulting telescope and mount until the telescope ...
    (alt.usage.english)
  • Re: Making off-axis optics
    ... Thomas Womack wrote in message ... > mirror for a Cassegrain design before polishing and figuring, ... > come this doesn't apply for producing off-axis mirrors by cutting ...
    (sci.astro.amateur)