Re: [ATM] RE:anealing

["David Harbour" <scarab2@cox.net>]

But I agree, whole heartedly.

It was PE who said they needed a whole diamter flat, not me. It was PE who
evidently either:

(1) Did not understand testing at center of curvature; or:

(2) Just needed an excuse that the public wouldn't understand so they could
save face.

No argument from me; I would have chosen Foucault, too. I wrote the Primer
on it, the definitive one, remember?

They just got caught with their pants down, and had to make excuses-the
notion that autollimation would have been the only other test they could
have made is, as you all know, hogwash.

All I'm saying, is, they whined that they couldnt affor a 92" diameter flat
on their budget. Well, then: Howabout a 36" flat?

A subdiameter flat worked for Ritchey worked for Ritchey for the Naval
observatory's  40" project. Ritchey also preferred Foucault; but I rather
imagine that he wanted to be on safer ground with autocollimation and null
figuring the primary for the first large RC reflector).

Dave

Autocollimation allows null figuring, which even brain dead PE opticains can
do; Foucault does not. That's the difference you ask about.


----- Original Message -----
From: "Richard Schwartz" <richas@earthlink.net>
To: "'David Harbour'" <scarab2@cox.net>
Sent: Tuesday, May 24, 2005 10:21 PM
Subject: RE: [ATM] RE:anealing


> What colossal negligence did PE do that could have been avoided with a
flat?
> The mirror was not parabolic, so autocolimation would not have been any
> easier than an ordinary center-of-curvature test.   And that ordinary
> Foucault test would have immediately found the discrepancy in the
> correction.
>
> Eastman Kodak made a duplicate of the PE mirror, and theirs came out with
> the correct correction.   I wonder how they tested it.
>
> . . . Richard
>
> -----Original Message-----
> From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
Of
> David Harbour
> Sent: Tuesday, May 24, 2005 4:59 PM
> To: ATM LIST; Mike Lockwood
> Subject: Re: [ATM] RE:anealing
>
> Mike, all:
>
> It IS NOT NECESSARY  to use a full aperture flat to test a primary mirror.
> This was Perkin Elmer's excuse for their criminal negligence. Nor is it
> necessary to poke a hole in a subdiameter flat flat to use it to creat an
> "artificial sky". I have two Perkin Elmer flats (ultra long radius
spheres)
> 7" on their long axis (they are rectangular). FORGET LOOKING FOR
ASTIGMATISM
> IN YOUR WORK. It takes someone who does not understand anything
mechanical,
> optical, or physical in at all to grind, polish, or figure in astigmatism.
> IT NEVER HAPPENED TO ME, AND I AM SORT OF AN IDIOT.
>
> If we may assume that astigmatism is the easiest thing to avoid when
> figuring, then we may assume it is only necessary to test that portion of
> the mirror's aperture that lies between the edge of the secondary's shadow
> on it, and the peirmeter of the mirror.
>
> Where is the need for a Kolosssal flat?
>
> Dave
>
>
> ----- Original Message -----
> From: "Mike Lockwood" <melockwo@uiuc.edu>
> To: <atm@atmlist.net>
> Cc: "Julio Sanchez" <jsanchez@skipanon.com>
> Sent: Tuesday, May 24, 2005 10:22 AM
> Subject: Re: [ATM] RE:anealing
>
>
> > 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
> >
> > _______________________________________________
> > ATM mailing list http://www.atmlist.net/
>
> _______________________________________________
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>

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