Re: [ATM] Fringe XP and flats

["Philip Lardner" <philiplardner@eircom.net>]

Yes, I worked three 10" x 1" plates against each other in pairs until there
was no sensible difference between them using a dial-micrometer on a beam
across their diameters. I then switched to pressing and polishing all three
plates simultaneously using just one lap (A - Tool/Mirror then M/T, B - T/M
then M/T, C - T/M then M/T...) until they were fully polished out. Working
this way, I managed to keep the radius of curvature more or less stable,
ending up with all three plates slightly concave - the difference was
visible only in the interference test and not when using the micrometer.
I'll update my Flats page shortly to include the method I used before
starting figuring.

As this is my first attempt at making a flat, please don't take my advice as
Gospel - I'm still on the steep part of the learning curve!

By testing all three plates in pairs against each other (A on B, B on C, C
on A) and then working just one plate at a time until you get it close to
flat (+/- a fraction of one fringe) you can restrict any *new* deformations
to the plate you are working. However, the interference test shows you the
*sum of errors* for the two plates under test. It tells you the size and
shape of the air gap between the two plates, not the shape of the actual
plates. That is why I asked the question that started this topic!

As far as I am aware, the Ronchi and Foucault tests can't be used on flats
without a reference sphere - as in the Ritchie-Common test - or a lens. I
have been trying to use an uncoated 14" f/15 as a reference, but so far the
images are so dim that I can't tell anything from them. I have noticed at
least three returning spots of light when lining up the test with a laser,
all of which produce incredibly dim images even using a video camera. My
slitless tester uses ultra-bright green and blue LEDs (switchable!)

Phil.

-----Original Message-----
From: Dominic-Luc Webb [mailto:dlwebb@canit.se]
Sent: 16 March 2007 09:39
To: Stephen Koehler
Cc: Philip Lardner; ATM
Subject: Re: [ATM] Fringe XP and flats

On Thu, 15 Mar 2007, Stephen Koehler wrote:

> > under test. In my case, I don't have a reference flat as I am making my
flat
> > by comparing three plates in pairs - all of unknown quality!

I'm curious. I have not yet produced a flat myself, but have some
glass set aside for such a project. If you have 3 plates (as I do), is
it not possible to confine a fault to 1 surface by comparing it to
each of the other 2 plates? Also, do other tests (Ronchi for instance)
not help to isolate the faulty surface(s)?

Also, did you produce all 3 plates simultaneously by switching grinding
between plates intermittantly (I've seen such methods)?

Dominic


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