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Surplus shed 114mm f/8 mirrors

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#1 ccaissie

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Posted 26 June 2013 - 01:54 PM

Anyone evaluated these mirrors? If smooth with a good edge, they might make nice flex mirror scopes for christmas gifts...

For $25 what is so wrong here?

C

#2 saemark30

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Posted 26 June 2013 - 03:46 PM

They say they don't know if its spherical or parabolic.
So they may be good candidates for budding mirror makers to refigure.

#3 highertheflyer

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Posted 26 June 2013 - 09:39 PM

Just curious why in the world they would not know if the mirrors were spherical or paraboloidal? A simple Ronchi test would achieve the answer.
But gosh, you are correct in a means to teaching the ways of polishing and reconfiguring!
Jim

#4 Arjan

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Posted 27 June 2013 - 03:34 PM

At 114mm and f/8 a spherical mirror would be borderline OK at 80% Strehl.

#5 saemark30

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Posted 27 June 2013 - 03:37 PM

They surely tested some. You get what you pay for I am thinking.

#6 ccaissie

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Posted 04 July 2013 - 07:38 PM

I'll probably spring for one and find out. A friend had one in a finished commercial scope, and it had astigmatism. The cell was not at fault, so I got the mirror replaced.

#7 Tom and Beth

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Posted 04 July 2013 - 08:30 PM

This could be a great start for you as a mirror maker. While a bit more expensive (by a few bucks) for a blank, it's already been ground and polished. My bet is that it's close to Spherical. You could build a basic Foucault tester and try to determine it's shape and if the surface is smooth.

And if you found the mirror was NOT up to a nice handheld scope, make a pitch lap and take a swing at correcting it. The Aluminum coating is so thin it would be gone after a few minutes of polishing.

EDIT: I see by other posts you've done a mirror or two ;) but considering this Surplus Shack mirror as a quick start to a nice little scope still stands.

#8 ccaissie

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Posted 06 July 2013 - 06:52 AM

I think the price, if it's serviceable is the kicker. I've got enough blanks...cervit, silica, pyrex, plate for a lifetime. What makes it, is that it may be eligable for building a scope quickly and cheaply for friends who would like a scope.

If it's smooth, and already coated (saves $), it could make a nice one. I like f/8-f/12 in this size. Moon and planets are reasonably satisfying for most casual users.

I'm not much interested in going thru the motions :lol: of making a small mirror unless it was a schief or other special type.

If it's spherical, but a good one, a simple center pull flex cell would take care of the borderline correction. Might be good as-is..dunno. Maybe return any that are poor.

I have an edmund 4-1/4 f/11 that is my grab & go. It was way undercorrected, but I've got it well corrected and smooth now, with very small diagonal, and Mars' polar cap and Cassini are easy.

Just want my friends to have one too.

#9 highertheflyer

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Posted 06 July 2013 - 03:51 PM

I've resurfaced these glass blanks and as long as they are of good quality, the reground could be quite simple.
Are the mirrors Pyrex?
Jim






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