Re: ATM RE: 12" refractor

["k.honders" <k.honders@worldonline.nl>]

-----Oorspronkelijk bericht-----
Van: Roger Ceragioli <rogerc@dakotacom.net>
Aan: k.honders <k.honders@worldonline.nl>
Datum: zondag 11 juli 1999 15:32
Onderwerp: Re: ATM RE: 12" refractor


>Hello Klaas,
>
>Actually, if you raytrace the situation you'll find that for tilt angles of
>about 6 degrees--what is needed to make this design work--convexities or
>concavities of several waves will remain undetectable.  It is only for the
>severe case of a Newtonian flat, where the tilt amounts to 45 degrees, that
>you need a flat good to 1/8 wave or better.  Otherwise, you produce
>astigmatism in the wavefront.
>
>Of course, in either case your flat must have a smooth, zone-free, surface,
>or else you'll introduce zones in your wavefront.  A decent optician with a
>machine or by hand can pretty easily produce a large flat, smooth and good
>to 1/2 wave, which will prove fully adequate to folding the refractor's
>path.  I am in the process of doing just this for a folded 8" refractor.
>So in a few months, I will have experimental confirmation (or refutation!)
>of these statements.  In either case I will send a note to the list.
>
>Best regards,
>Roger
>
>>hi Roger
>>            Just read your proposal to fold the lightpath of this 12" ref.
>>with 2 flat mirrors
>>In my opinion they should be flat better then 1/10th wave if not there
will
>>be astigmatism .Although a small amount may be unnoticed due the quite
>>severe chromatic abb. in a 12"  F 12 refractor.(With standart glass)
>>
>>Greetings,
>>Klaas Honders
>>k.honders@worldonline.nl
>>
>>
>>
>>
>>
>>
>>
>
>

Hi Roger,

            Well I checked your comments and a little bit to my astonishment
you are right
I took two flat mirrors to fold one of my designs a 175mm.achromat F=
3200mm.
in  a "Z" form angles maximum 4 degrees.Took radiļ both 40mtrs.first one
convex and the second concave there was a little bit detoriation of image
quality but no real problem only with both surfaces convex or concave there
was a clear detoriation but even then the image would surely be very
acceptable!
A radius of 40 mtrs. corresponds to a sagitta of about 0.03mm.(100mm diam)
This is quite easy to measure with a sferometer.A few month ago I made a
flat using my sferometer during grinding.After the first polishing session
it showed .3 wave convex.It would have been enough for the purpose (Folding
flat in a Schiefspiegler design) but after correction it is now better then
0.1wave.

Greetings,
Klaas Honders
k.honders@worldonline.nl