Re: ATM Re: Small Schmidt Cass

[Aplanatic@aol.com]

Hi Jerome:

> I am planning on making a small Schmidt Cass for bird watching and game
>  viewing. Portability and weight are my main criteria.

You're in for an adventure.

>  Would I be better of making a 4 or 5 inch scope. How small is
>  practicable....... 3 ......3.5 inch?

What size do you want?  The issue with smaller will be the unvignetted field 
and the size of the secondary obstruction.  But 3.5" and up should be just 
fine.

>  The 3 mm plate is also float .... very, very flat. The wedge is less than
>  0.0002 inch. It has the obligatory blue green tint (very slight) but should
>  not be bothersome for daylight viewing.
>  
>  I plan to use the vacuum method for the plate but am not sure whether   the
>  vacuum will work on a small plate say 100mm diam by 3mm thick. The ratio
>  should be about 50:1 but this is closer to 33:1. Should I opt for a 2mm
>  thickness or will the corrector be too thin to work properly?

See my page on the vacuum pan method:

http://members.aol.com/drowesmi/schmath2.html

>From it you should be able to calculate the pressure needed to deflect a 
plate of any size or thickness.  It would be desirable to have a plate that 
can be properly deflected with between 1 and 4 PSI of pressure difference.  
The thickness for such a plate will strongly depend on the desired aperture 
and the focal length of the telescope.

If you are wondering, yes, I have made a Schmidt corrector plate.  See:

http://members.aol.com/drowesmi/schmath2.html

Dave Rowe.