Re: [ATM] Final figuring advice?

["Bob May" <rmay@nethere.com>]

Two tings to consider.
First is that you aren measuring the mirror the same each time.
Go and do several iterations of the test and see if you repeat
yourself each time.  If you are and you are truthful with
yourself, you then have the second problem.
Second, you have some astigmatism in the mirror.  This can be
best checked with the Ronchi test.  A grating from Willman-Bell
is quite cheap and will show up such things readily at ceertain
angles of the error.

For testing, I'd suggest that you do up a set of masks with one
zone on each mask.  This will insure a much higher accuracy of
measuring each zone.  In addition, you know that the center of
the mirror, the zone 1 is going to be the same no matter the
orientation of the mirror so use that as the distance zero point
and measure the other zones relative to it.  Use other angles
than 90 degrees as you can hide the astigmatism that may be
present by placing the mirror at the certain angles on the stand.
I like to use TEX for my evalusation of a mirror as it also does
a Monte-Carlo analysis of the readings which can show how good or
bad your results are.  It will also show how important accuracy
of the measuring of zones is when you do a mirror.  You're at the
point where making a 0.005" error in a measurrement will degrade
the quality of a mirror by as much as you have stated.
I have found tho that with a mirror on top approach that astig.
is not very common and it is usually more likely that measurement
errors are the cause of different readings.
Be true to yourself when doing the test and don't look at the
readings to see if the numbers are the right numbers but rather
make sure that the gray levels are the same for each zone.  I
also like to have a little bit of light on the mask from some
area source (a window that has a pane unblocked or  a bare light
that is non-directional( to produce a general gray level that I
can also compare the gray of the zone against.  I will note that
I exclusibely use a camera and a Black and white monitor for
testing so I don't have to worry about the colors that may mask
the brightness levels.  I find tht I am repeatable to about two
thousandths of an inch when doing this.  Blinking a zone also
helps as the motion of the light from light to dark also nicely
shows up in the eye's responseas it will tend to show up as a
motion which the eye is sensitive to.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net


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