Re: ATM Star test

[astillma@nit.AirTouch.COM (Anthony Stillman)]

Hugues,

Regarding astigmatism.

Astigmatism can arise from a number of source.  It could be the glass figure 
or a result of the mounting.  I assume, from your e-mail, that you have
determined that it is a result of the mirror mount.  I also infer that the 
mirror is thin (flexable) and that you have a three point mount.  If this is 
true, replace the points with pads, or larger pads.  Some people have used an
infinite point mount (read flat spungy material (cork, sponge, soft pitch ...)
)  If you test near azmuth you don't need clips and there is no great
compression.

Now it may be that your mirror mount is distorting the mirror.  I've seen this 
when silicone is used to glue a mirror in to place or when a band is place 
around a mirror and the tension is oneven.

Please forgive me members of the group 70 optical team, but I'm about to reveal 
our dark secret.  When we right the 1.8 meter mirror for test, we attach two curved steel 
bands to lift the mirror.  The bands have a series of wooden blocks
along there length which press against the mirror.  As we tighten the bands 
astigmatism is introduced into the mirror and to releave it, we pound on the 
bands with hammers. There I said it, I feel better, no more shame.  OK, so
this isn't the best solution for supporting the mirror.  But the glass weighs 
2450 lbs and we need to know that it is secure.  Further, pounding has not 
harmed the mirror and it does releave the induced astigmatism.

Perhaps Hugues, you could give som additional details on your current mirror
mount and mirror.

Anthony