Re: [ATM] Secondary Cage Calculations

[Nils Olof Carlin <nilsolof.carlin@telia.com>]

Arjan, Richard:

>>>>>>>>>
For a given aperture you'd expect the ring ID to grow with decreasing 
focal 
length...

> DW> ring ID = aperture + 1% of focal length
>
> Without knowing the focal ratio and the design field of view that
> statement is meaningless (like a lot of "rules of thumb")
>>>>>>>>

I wonder, Arjan....
Assuming the distance mirror-ring is about one focal length (often it 
is a little less), that 1% would buy you a 0.01 radian or about 0.6 deg 
field free of vignetting of the primary. It would be centered in the 
FOV only with the correct offset from the focuser of the secondary (the 
old Obsession wisdom, I believe, was not to offset the diagonal at all, 
meaning the field free of vignetting from the *secondary* would not be 
even nearly centered!). 
Then again, vignetting from the cage is so mild as to be negligible, 
but if you want a "rule of thumb" with some safety factor built-in, 
instead use ID = aperture + focuser diameter.

Nils Olof
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