Re: Formatting [was: Re: Grammar Crackdown]

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).

I see.
When I ground and polished my 6-inch mirror, the end shape was a bit
off from a parabola, but after the endless hours I'd spent on it over
a summer holiday, I figured it was close enough to be sent off for
silvering. Except for a couple of small scratches that occurred during
the final stages of polishing, it was a pretty good mirror, having
nearly the 48 inch focal length I'd been aiming for.
I was happy with the resulting telescope and mount until the telescope
was stolen one semester while I was away from my parent's home, at
college. So it goes.
--

Regards,

Chuck Riggs
Near Dublin, Ireland
.

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