[ARC5] WHINK and crystals - plate glass.

[J. Forster]

I believe that some tests for telescope mirrors require large flats.

-John

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>
> ----- Original Message -----
> From: "J. Forster" <jfor at quikus.com>
> To: <kgordon2006 at frontier.com>
> Cc: "ARC-5 Maillist" <arc5 at mailman.qth.net>
> Sent: Friday, June 28, 2013 12:37 PM
> Subject: Re: [ARC5] WHINK and crystals - plate glass.
>
>
>> Your point is well taken.
>>
>> Some people use a 'figure 8' pattern and rotate the plate
>> frequently. Even
>> if the surface of the plate eventually does become a bit
>> dished, the
>> curvature is over a much, much larger area than the
>> crystal, so minor
>> dishing of the plate still looks locally flat to ther
>> crystal.
>>
>> It's mostly a matter ow how much is too much.
>>
>> On a related topic, I was just at a Michaels Craft Shop.
>> They have several
>> things for glass etching. There is an etching cream. I(
>> think the stuff
>> could be made into a water solution and used for xcrystal
>> etching. The
>> brand was something like Armor or Armour. A few ounce
>> bottle was about
>> $12. It was an ammonia-flourine compound. If you are
>> interested, Michaels
>> has a web site.
>>
>> -John
>>
>    I expect you could find a lot of useful information in
> books on amateur telescope making.  I think people still
> grind their own mirrors.  The difference is that one wants
> the surfaces of the crystal to be flat and parallel while
> the mirror is parabolic.
>
>
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com
>
>