Re: [ATM] What is that Image size in Calculations

[Alexis Cousein <al@sgi.com>]

James Sturtevant wrote:
>  The choice of a secondary size for a visual scope is easily made much 
> more complicated than it needs to be.  You are trying to accomplish two 
> things: 1. provide a reasonable level of illumination (not necessarily 
> "full illumination") for the edges of the field of view and, 2. minimize 
> diffraction effects from the secondary.  The simple rule of thumb to 
> reach these somewhat contradictory goals is to choose an elliptical 
> secondary with a minor axis equal to 20% to 25% of the primary.

That's a rule of thumb that's only good for a certain f/ratio and
aperture range. If you build a 43" scope even at f/4, 20% obstruction
will give you a humongous fully illuminated field, and 25% is simply
too large (unless you also come up with a 4" 160° AFOV 31mm eyepiece
that can handle an f/4 light cone and doesn't weigh a ton).

f/ratio has a large influence, and so has size.

Size? Because the focuser width doesn't scale. And it doesn't
scale because there is an exit pupil limit (because the fully
dilated eye pupil of the observer doesn't scale), which means
there is a minimum magnification, and with 82° AFOV for eyepieces
that translates to a maximum usable field stop (yes, there is a
100° AFOV eyepiece these days, but I doubt you'd want to scale it
from 13mm to 31mm ;) ).

And there's the position of the focal plane, too: low profile focusers
will require smaller secondaries, but baffling will be harder and
some eyepieces may not focus. You may say that's being unnecessarily
complicated, but someone will, I assume, design the location of
the focal plane to lie *somewhere*, and once you've made that choice
the rest kind of takes care of itself.

There's little point for rules of thumb when you have an excellent 
calculator you can use:

http://www.bbastrodesigns.com/diagonal.htm

All you need is the aperture, the focal length, and the design
location of the focal plane. Punch in a few numbers, and look at
the illumination at the edge of your favourite low power eyepiece.


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