Re: ATM Percent illumination question

[Andy Saulietis <iss@pvtnetworks.net>]

Mel writes,

>A 'tall focuser' needs to be designed like the old Cave Astrola giant
>focusers where the base was some 4 inches in diameter.  This to prevent
>vignetting by the focuser drawtube.
>
>A 'tall focuser' has benefits for baffling and for better off-axis
>illumination.  
>
>A 'short focuser' has benefits for larger 100% illuminated field.
>
>I use a short focuser on my 6 inch f/4 and a medium height focuser on my
>20 inch f/5.
>
>You just have to play with the numbers and see what works best for you. 
>Clearly, a short height focuser is not best for every situation.

Good advice for a visual-only scope. If you plan to do
film or CCD astro-imaging, having some extra distance
between the scope & focal plane makes it much easier
to have off axis guiders, focal plane rotators, filter
wheels, cameras etc. The 24" & 2 32" scopes I built
all have a 7" distance from the mounting surface to the
focal plane. This does cost having to use a bigger secondary
mirror. BTW the 7" distance was originally for the 16" f11 
S-C I used at NASA, it allowed me to use its accessories
with my own scope & vice versa.


Andy Saulietis
ISS Enterprises
39 Silver Fox Trail
Mayhill NM 88339
505-687-3067 voice
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e-mail: iss@pvtnetworks.net