Re: Formatting [was: Re: Grammar Crackdown]

Hatunen <hatunen@xxxxxxx> wrote
On Thu, 31 Jul 2008 11:51:29 +0100, Paul Wolff
<bounceme@xxxxxxxxxxxxxxx> wrote:

Chuck Riggs <chriggs@xxxxxxxxxx> wrote
On Wed, 30 Jul 2008 18:15:24 +0100, Paul Wolff
<bounceme@xxxxxxxxxxxxxxx> wrote:


I have a telescope with a cloudy mirror, but there are no corresponding
clouds in the image plane. The defect just affects the proportion of
received starlight that makes it to my retina. I decided that further
work to cure the poor polish would likely destroy the near-perfect
parabolic figure that I'd achieved.

Have you considered sending the mirror off for re-silvering?

Sensible idea, but that's not the problem. During manufacture the
mirror blank was ground with progressively finer grit as the shape
approached perfection. The idea was to be polishing it brilliantly at
the time that the paraboloid was finally formed. My timing was out and
the mirror tested to near-perfect before polishing was complete. I was
advised that polishing it out would spoil the figure, and elected to
keep a fine shape at the expense of perhaps 20% light loss overall,
rather than polish and lose only 10% of light but with optical errors.
The result is a highly accurate glass surface marred by millions of tiny
pits, but these cover only a small area, cumulatively. I have a
six-inch aperture for resolving purposes with the light-gathering power
of a mirror of 10% smaller area (5.7-inch equivalent).

The pits don't cause any diffraction problems?

They just show up, on inspection, as a slight haze on the mirror surface. When observing, the eyepiece is focussed on a virtual image two inches away, not on the mirror surface forty inches beyond that. If there's any diffraction at the pits it doesn't affect the image in the eyepiece, as far as I can tell.

My percentages given above are really for the sake of example, not the result of measurement. On reflection, I really don't think that 10% of the surface is spoilt, or it would look far worse than it does.
--
Paul
.

Relevant Pages

  • 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]
    ... 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)
  • Re: Formatting [was: Re: Grammar Crackdown]
    ... Have you considered sending the mirror off for re-silvering? ... the mirror tested to near-perfect before polishing was complete. ... keep a fine shape at the expense of perhaps 20% light loss overall, ... pits, but these cover only a small area, cumulatively. ...
    (alt.usage.english)
  • Re: Formatting [was: Re: Grammar Crackdown]
    ... 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)
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