From: "Jerry" 
To: "'hermit'" , "'Tom
Krajci'" 
Cc: 
Reply-To: "Jerry" 




I'm with Tom on this one.

The star test might be used to test the assembled system to see if the
mirror is warped in it's cell, the diagonal and tube current/thermal
problems. But I just can't see figuring by star testing for all the reasons
Tom states. Judging the grey is not that hard. And the star test requires
some judgment too.

I don't think any manufactures of telescopes "figure" using star
test. If they did and I was buying one of their telescopes I would have to
test it on the bench first. I would not trust star test figured optics.

Keep in mind that telescope manufacturers are making products to sell. If
it is too expensive they won't sell many. When you see a manufactured
telescope with lesser quality of optics it is not because they tested it
indoors on an optical bench rather than with a star test. It is because if
they took the time to make them better they would have to charge a higher
price. Most have studied their market and know what the price has to be.
And most of those scopes are pretty good for the money.

For most mirrors you can just calculate the desired knife movement form one
zone to the next using 4 zones and get those numbers as close as you can
and you will find that, lo and behold, it will pass muster no matter which
program you use. And it wouldn't make it any worse if you didn't even
calculate the error!

Heck! You should be able to make most amateur mirrors, lets say between
around 6" f4 to around 12" f6 or longer, to about 1/16 - 1/20
wave on the wavefront without data reduction. If your knife readings for 4
to 5 zones are within a few thousandths of an inch and the curve is smooth
you will be about that close. I'll bet that will get the hornets swarming!

Jerry

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