Re: [ATM] RE:anealing

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Julio Sanchez wrote:
> My experience is that plate glass is not
> suitable for optical flats because of the long stabilization time after each
> figuring session. With pyrex I can let the flat rest for 1 hour before
> retesting. With plate glass I must wait for at least 12 hours. Plate glass
> is also softer and thus more likely to scratch.

I agree with you for larger flats.

I made a 4.25" dia., 1/2" thick flat out of plate for testing 
elliptical diagonals.  This flat seems to stabilize in 30-45 minutes.

Its weight is small enough that I can set it on top of the diagonal 
without fear or scratching or warping the diagonal.  The back is 
polished so I can see through it.  This way I can test the whole 
surface of an elliptical diagonal, including the "tip" and "heel". 
(If you put the diagonal on top of the flat, you usually can't see 
through the diagonal where the side of the blank interferes.)

> I buy from Newport because they are cheaper and have a greater selection.
> All in all their pyrex blanks have been OK for me (except for Frankenblank,
> and I don't really know the reason why this one failed). I buy other stuff
> from WB but their pyrex blanks are about twice the price of the ones from
> Newport (last time I checked).

Try United Lens and avoid the WB markup.  (WB gets their blanks from UL.)

A while back I talked to Newport and requested a catalog (I think). 
This was a year ago or so.  6-8 months later a large manilla envelope 
arrived in my mail.  I opened it and found Newport's complete catalog 
and a price list.  I didn't even remember asking for them, but I 
started looking through and was amazed at depth of the product line. 
If they could just deliver on time (United Lens does), Newport would 
have a lot more amateur business.

> Every ATM should try making a set of 10 or 12" optical flats (to 1/20 wave)
> using the ABC method. It is a humbling experience. What for? well, punch a
> hole in one of them and you can use it for autocolimation testing, use the
> other ones to make diagonals for your Newtonians, and so on. Foucault once
> said that a flat mirror is like an artificial sky.

If the ABC method is not appealing, or you don't need 3 flats, try the 
water test or Ritchey-Commons test (if you have a good sphere laying 
around).

	Mike Lockwood

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