Re: [ATM] Higher readings on one axis (2)...

[John Moore <jamoore42@yahoo.com>]

Hi Jean;
 
"STAR -TEST", then make a final decission!,
If the astigmatism is there,, star test will show-it!
 
John Moore.


Jean-yves Lauture <jylauture@sympatico.ca> wrote:
I want to thank all of you who replied.

John Moore and Scott Ewart:
I did a small amount of regular polishing with more strokes across the 
"higher axis" (John I did that before receiving your e-mail suggesting to go 
along the higher axis... and of course made the wrong decision). I did 2 
turns (8 positions rotating the mirror only on the same spot of the tool) 10 
double strokes on 6 of the positions and 25 double strokes across the higher 
axis). See results below.

I will first try to "even out" the surface and then follow the unanymous 
suggestions (also from Jim Burrows, John Sherman) for star/spot tests 
described on John Sherman's page.

Mike Lockwood:
(ATM Digest, Vol 19, Issue 34, Message 12. Re: Higher readings on one 
axis... (Mike Lockwood))
Mike, you are absolutely right. Lately I have noticed my inconsistency on 
taking readings on the wider z1 and will modify the masks if things don't 
get better as you suggestted. I was more careful on the latest sets of 
readings and they are much better.
I normally only rotate the mirror on its stand with two fingers and let it 
cool for 15 minutes in its new position before taking readings. After 
reading with the second mask, I go back to the first to make sure readings 
are the same. I make sure the masks are at exactly the same spot on the 
mirror.
The mirror/tool is BVC from ASM Products. Mirror thickness is around 1.5" 
and tool about 7/16". During polishing I worked only with MOT with the tool 
resting on plywood. I am at a fixed position, constantly rotating both 
mirror (8 positions) and tool (also rotating mirror with respect to the 
tool).

Here are the latest readings (see below for the mirror parameters):

1) 0 degree: 0, .501, 1.128, 1.970
2) 90 deg: 0, .580, 1.249, 2.228
3) 180 deg: 0, .529, 1.125, 2.048
4) 270 deg: 0, .602, 1.238, 2.227

Using FigureXP I get the following P-V Wavefront error: 1/8.37, 1/16.44, 
1/11.05, 1/17.94.
with an average (probably meaningless at this point) of 1/25.14

a) All zones are within tolerance except for the first z4 reading (1.970)
b) z2 is slightly higher on one axis
c) z3 is lower on one axis and right on the spot on the other
d) z4 is at lower limit on one axis and at upper limit on the other axis.
e) the differences between zones are within tolerance

Where does this come from? It looks as if one axis was slightly and smoothly 
more parabolized than the other. Can it be because I always start my session 
on the same position of the mirrorl, having more difficulty to move the 
mirror?

My choices:
1) leave it as is (??? please comment). What would be the consequences?
2) 2 turns polishing with more strokes along the higher axis and see the 
results
3) more standard polishing and parabolizing only on the "lower axis" (1 
position only out of the 8)... risky.
4) or whatever you gurus suggest.

Again here are my mirror parameters (in mm)

Diameter: 252mm

ROC: 2728mm

4 zones; Effective radii:
z1: 37mm
z2: 65mm
z3: 90mm
z4: 114mm
(I use two masks)

Moving Light Source

Ideal Readings:
z1: 0 (+/- .267)
z2: .523 (+/- .152)
z3: 1.234 (+/- .110)
z4: 2.131 (+/- .087)

Again, thank you all for your support.

Jean-Yves


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