Re: Formatting [was: Re: Grammar Crackdown]

On Fri, 1 Aug 2008 12:18:25 +0100, Paul Wolff
<bounceme@xxxxxxxxxxxxxxx> wrote:

Chuck Riggs <chriggs@xxxxxxxxxx> 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).

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.

That's a shame. But you did at least one thing better than me: you
achieved the intended focal length, while I overshot and ground it down
to 42 inches. Not that it mattered much, I suppose.

True, the more important aspect, by far, being the shape. Achieving a
parabolic shape instead of a spherical one is an art, I am told. I've
only done the one mirror, so I had to carefully follow the
instructions in the book recommended to me, using a flashlight to
bounce a beam off the mirror, then comparing the light and dark
patterns on the surface to the ones in the book. Then I'd polish some
more and repeat the process.
--

Regards,

Chuck Riggs
Near Dublin, Ireland
.

Relevant Pages

  • Successful Mirror Grinding
    ... in the subject of grinding (and polishing and figuring) one's own ... mirror for a newtonian telescope. ... How big a piece of glass should a novice first attempt? ...
    (sci.astro.amateur)
  • 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)
  • 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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