Re: [ATM] RE:anealing

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

Scott,

If everything is squared on properly, one would not see such apparitions as
you suggest; go look at my article on stopping down. Nothing is tilted.
Simply covr the sub aperture with your subaperture flat, and locate a light
source, and, voila- you have an autocollimator showing part of the mirror
only.

If it was good enough for George Ritchey when he figured the 40" mirror at
the U.S. Naval obsergvatory, I would bet good odds that it would be good
enough for some of our clever mechanicians. The only real challenge in the
whole project would be proper squaring on of the mirrors. The reason an
excentrically placed entrance pupil usually works so well is that there is
nothing to square on properly. Go check my diagrams, in this article; you
will see, intuitively, that aberrations that were not in the whole mirror
cannot be in part of it, either. Just make sure that the flat is squared on.
The flat would go where you see the sub aperture mask on the front end of
the tube. I made excellent drawings for the article: The exxtra focal
diffraction rings, at nearly 1,000X, were textbook perfect, with it stopped
down. A flat right where the aperture is, and a pinhole light right at
focus, would provide an artificial sky and an artificial star.

It worked real well for GeorgeWillis Ritchey. I did not invent the method.

http://www.atmsite.org/contrib/Harbour/StoppingDown.html

Just get the flat perfectly squared on, no false astigmatism, nor coma.
Light source centered at focus. Autocollimation with a sub aperture flat.

Dave


----- Original Message -----
From: "Scott Milligan" <starzkey@charter.net>
To: "'David Harbour'" <scarab2@cox.net>; "'ATM LIST'" <atm@atmlist.net>;
"'Mike Lockwood'" <melockwo@uiuc.edu>
Sent: Tuesday, May 24, 2005 10:39 PM
Subject: RE: [ATM] RE:anealing


Just a thought, but an eccentric section of an ellipsoid or Hyperboloid, or
in fact, ANY conic section other than a paraboloid will look, to a first
approximation, like a toroid.  So will the paraboloid if aligned off-axis
(i.e. light source not a focal point, but translated off to the side a bit.)
So with a 1/2 aperture flat, both spherical aberration and coma look a lot
like astigmatism (yes, this is an approximation, but a useful one).  How
would you propose the optician distinguish between off-axis coma and
residual spherical aberration in such a test?

Scott Milligan

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
David Harbour
Sent: Tuesday, May 24, 2005 7:59 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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