Re: [ATM] Figuring flats

[Mike Lockwood <melockwo@uiuc.edu>]

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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