From: "Jerry" 
To: , ,
        
Reply-To: "Jerry" 


  This mirror is not bad
already, it would be easy to make it worse.



I don't agree. I think there is about an even chance of being this close
without any effort at figuring at all. It is bad enough that I would not
worry about messing it up. You need to be about twice as good before
worrying about messing it up.

Consider that an 8" f7 spherical mirror only differs a tiny bit more
than 1/2 wave on the wave-front from the desired parabolic mirror.

Look at the zone knife readings. Lets look at the comparison of the desired
movement from one zone to the next. From the innermost zone to the next is
almost exactly twice the desired reading. This is the same size error as if
the mirror were left spherical except on the other side of the parabola.
Then to the next zone is better but still 1.6 times the desired knife
movement. And then to the next zone the correction is actually slightly
negative.

What all that means on the glass is that roughly speaking is that from
about 50% out is slightly oblate and the inner 50% is a hyperbola that I
would rather just call a hole. I will say that this seems to me to be the
tendency for me and others on the first mirror with the classical
parabolizing method too work the center way too fast and the outer zones
too slow or not at all. One needs to learn to adjust the long wide W to
work the outer areas along with the central areas. That means more strokes
near the center of the W and fewer on the sides of the W.

Now I would say that if Aditya has run out of patience the mirror is good
enough to put in a telescope. But I think it would be easy to make better
and not really all that much chance of making it worse. And if he did make
it worse, as I said at the beginning, I think there is about an even chance
of just polishing a while and then testing to find a curve as good as he
has now.

But of course I would recommend a systematic approach to bring the knife
reading difference from one zone to the next as close as possible to the
desired readings. Getting them within .125mm would be great but within .2
would probably still make for a very nice mirror. And within .25mm would be
fairly easy to attain on a first mirror and would still be good.


I recommend that in testing to look at the desired knife movement from each
zone to the next. Test the mirror. Take your numbers and determine the
actual knife movement from one zone to the next. Compare the actual with
the desired to determine if the actual is larger (overcorrected) or smaller
(under-corrected) than desired. You don't need to run the numbers through a
program every figuring session. You do need to get a feel for how to change
the numbers and for how close you can get them to target. The closer to
target you get them the harder it is to do a figuring session without
making them worse. That is the challenge. Learn to get them to move in the
right direction and learn how close you can expect to be able to get them
to the target number.

Jerry

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