Re: ATM Rumak Fabrication Tolerances (long)

[Aplanatic@aol.com]

I was, of course, fully expecting Bratislav to respond to my post and figured
I'd learn a lot about the right way to make a Mak....

>  In all cases, MAKE THE MENISK FIRST !!! It is the most difficult part of 
>  the project, and unlike mirrors, cannot be changed easily (especially
>  thickness - once it's gone, it's gone :-) 
>  As long as the menisk follows the required R1-R2 (to make it achromatic), 
>  there will ALWAYS be a design that is stigmatic. You may introduce a bit of
>  coma or something, but will be absolutely insignificant. After menisk is
>  completed, MEASURE its parameters as best as you could, redesign if
>  necessary (ray trace program absolutely required) and proceed with making
>  the mirrors.

Now, this is an interesting proposal.  I can't prove to myself that for *any*
meniscus (close to the one desired), there is a reasonable set of *spherical*
mirrors that does not, when introduced into the system, have spherical
aberration, and has about the desired f/ratio with the desired back focal
length.  How does one show this?  If this is so, then I would agree, "tuning"
the mirrors is easier than figuring the corrector.  Of course, we still have
that nasty measurement problem:  How do you characterize the meniscus, anyway?
By direct measurement?  This may beg the question, eh?
  
>  And finally, aspherizing of Maksutov Cassegrain is something you would
>  definitely try to avoid. If it cannot be avoided, I'd recommend front
>  surface of the corrector. Off axis images nonwithstanding, it will have
>  the LEAST effect on roughness and scatter from butchering with a smal
>  tool (because of the very nature of refraction, for a given surface
>  roughness final Strehl is affected 16 times (!) less than on a
>  mirror).  This telescope has great visual potential - we should think
>  about on-axis quality and place image size in the corners of the 35mm
>  film a distant last, IMHO.

I agree that working on the corrector is a better approach, if possible.  But
smooth, highly aspherical secondaries are made for Cassegrains and can be made
to perform well, apparently.  (I have heard this, somewhere. <g>)

>  PS I don't understand why people are so hung on Rumak as a visual
>  telescope. It has great off axis performance, that's for sure, but you
>  pay for thet with huge obstruction (>0.3). I'd rather have little bit
>  more curved focal surface and a faster primary and try to keep the
>  obstruction below 0.25. You won't cover 35mm unvignetted in any case,
>  and who wants f/15 astrograph anyway ?

Yes, I totally agree.  My post was, perhaps, misleading.  I was trying to
point out that the native design has exceptionally good off-axis performance,
not that it should be considered an astrograph.  I also believe that the
secondary size should be minimized (for visual use) and that the instrument
should be treated as a beautiful but exceptionally challenging design.  Not my
cup 'o tea, at the moment.

Dave Rowe
Torrance, CA