[ATM] FW: Figuring flats

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

Mike,

	Thanks for the quick response. Yes, all three disks are plate/float glass,
1" thick. Their backs were not ground flat prior to fine grinding the fronts
because I didn't fancy having to polish them out too! Perhaps I should have.
However, judging by the ware pattern as I fine ground the fronts, they were
only very slightly out of flat, producing a slight saddle-pattern between
the ground and un-ground areas. This disappeared very quickly. I am using a
1/2" thick foam-rubber camping mat to support the work on top of my work
stand (because it is water-proof and washable) and a piece of carpet,
covered in black cloth, to support the plates in the tester.

	I have a 14" f/15 sphere (the primary mirror for the folded Stevick-Paul)
that can be pressed into service for a Ritchie-Common test. The only problem
with that is having to build a test tunnel 10.5m long to control the air
currents. It is devilishly sensitive to air movement... and I live in a big
old draughty house... with central heating at this time of year! I will
*try* and knock up a Ritchie-common test and capture some images this
weekend. I'll also have a play with FringeXP.

	Thanks again,

		Phil.



-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net]On Behalf Of
Mike Lockwood
Sent: 09 March 2007 18:36
To:
Subject: Re: [ATM] Figuring flats

Philip,

Philip Lardner wrote:
> I've made lots more progress on the flats front and have updated my
figuring
> log. I managed to beat plate B into submission... but I think I may have
used
> a bit too much violence - it's showing a whacking great zone from 70% out
to
> the edge!
> Sooo... now I need some advice on where to go from here. Would Mike L or
> anyone else like to offer their 2c worth? :-)

Sounds like you're starting to see why a reference sphere and the
Ritchey-Common test is a good thing.  :)

Some basic questions:
  How thick are the disks?
  Are they Pyrex?
  Are the backs flat?

Since fringes can be seen through them I wonder if they are plate.  If
they are plate and the back is not flat then gravity and the weight of
the two disks may be distorting one of them.

I see two things that concern me:

1)  The shims seem to be altering the fringe pattern, as shown in the
last four images on the web page.  (That's why I asked how thick the
disks are.)  I say get rid of the shims.  You will have to solve the
dust problem and you should not slide the disks while they are in
"contact".

2)  The latest set of fringe images shows some astigmatism.  You can
import one of the images into FringeXP to get a look at the difference
in shape that the fringe pattern represents.

Since you have the three disks getting within maybe a wave of each
other, you need to start looking at zones of each flat, rather than
the overall curvature.  The astigmatism will make this rather
difficult, though.  The only cure for astigmatism is to polish for a
while with proper technique, and proper support of the blank.  If the
glass is thin with an uneven back then this will have to be fixed.

To tell if a zone is high or low, press on the center of the top disk
when they are forming fringes.  You will see the fringes move
perpendicular to themselves.  In the top left images of the most
recent web page:
   http://ochra.110mb.com/testing_and_figuring_flats_page_2.htm

Press on the center, and let's say the fringes move towards the top of
the image.  The fringes move towards the wider air gap, because when
you press on the center you reduce the air gap.  So, if this were
true, the wider air gap is at the top of the image.  A wider air gap
implies that there is more distance between the pieces.  So, because
the fringe curves toward the top of the image (toward the wider air
gap) in the outer ~20% of the disk, then that zone would be low.  The
center would also be low in this case, with a high zone at the ~70% zone.

If the fringes move toward the bottom of the photo when you press,
then the 70% zone is low on one or both of the disks, or it is REALLY
low on one disk and high on the other.  All you can determine is the
difference between the disks.

Search the archives for my posts about reading fringes.  I posted some
messages on that topic recently, I believe.

Gotta know more about the disks themselves before I can say more.
I'll be off email until Monday.

        Mike Lockwood

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