Re: [ATM] Figuring help for Lutfu

["Jerry" <wa4guu@verizon.net>]

Lütfü

Do you have any telescope making books available in your language?

I hope you have good luck searching on the internet. I found a little on
building a tester and some on how to use the test. I didn't see much to
recommend. I hope you find what you need in your language.

I will look some more.

If you are going to make mirrors you want to learn the Foucault test and the
Ronchi test. You have done OK at seeing the shape with the Ronchi test. The
Ronchi test shows the quality of the curve well and the Foucault quatifies
it better. The Ronchi test can reveal slope changes that would be difficult
to see in the Foucault test but might not quantify the changing slope so
well. 

I think the Ronchi shows TDE better than Foucault and maybe even reveals the
quantity (depth) of the turn down better too. (I'm sure many will not
agree.)

For small mirrors of typical focal length the Ronchi shows degree of
correction well enough to get something close to 1/4 wave wavefront
accuracy. If the bands bend smoothly and bend to approximately the proper
degree the mirror should be "good". Of course that would be dependent on
your judgment of what the right degree of bending would be for your size and
focal length. To be good at this you would probably need a fair amount of
experience with both Foucault and Ronchi testing. Then you have no reason
for not measuring by Foucault test. Well... I guess if you just don't have a
Foucault tester, that would be a reason.

I think you can find a tester on Mike Lockwood's web page. A good clear
explanation of using it may be more difficult. I hope you can find something
in your language. If you can find Jean Texereau's book in your language
would be great.

I had a response to my question about Thomas' stroke to get a sphere from
another ATM. He says that will make a sphere and in time make a good edge. I
agree with Mike about the TOT and larger W. He does just about the same size
as I do but I recommend you start smaller and enlarge it in increments until
you find the size that makes something close to a sphere for you. 

I think I sensed a consensus from the responses. I'll give it a name,
"hovering". If you hover around a sphere for a while the edge will get
better. If it quits getting better and it is not good enough for you, you
will have to change one of those many factors I mentioned (phase of the
moon) and continue to hover at the sphere to see if the edge will get
better.

I agree with Mitch in that I think you will find it is a combination of
strokes that does the trick. Some COC strokes or near COC (narrow W) that
will make a very slight turned up outer zone about 3/4" to 1" wide and then
a narrow turned down edge. Then some larger W strokes MOT to take the outer
zone down and in doing that the edge will get better. It takes the correct
amount of both strokes. If you did the short strokes right and set up good
for the larger W, you have to do it for the right length of time. If you do
it too long you will take the outer zone down too far and then fixing that
will make the edge just like it was before. You can do things too long and
never know that the surface went through the desired shape. 

If I have made something of the other's comments that they don't agree with,
I hope they will correct me.

You have learned about the importance of the lap, proper channels, and
keeping it trimmed, in order to make the work predictable. If you keep the
lap in good condition, the shape on the glass will not go bad. You will not
be doing anything so much different than what you have done that has put a
fairly good surface on the mirror. If you go wrong, it won't be far.


Jerry



-----Original Message-----
From: lütfü çakmak 

I understand now it's time to make a foucault tester.
But, unfortunately I don't know how is made it and
foucault testing how is done. First of all, I should
search on the internet and some other source.



_______________________________________________
ATM mailing list http://www.atmlist.net/