Re: [ATM] Secondary Cage Calculations

[Alexis Cousein <al@sgi.com>]

James Crombie wrote:
> Hello all
> 
>    For a nice visual method, download a copy of NEWT and input your 
> specs for the scope, the program will allow you to tweak the design to 
> avoid vignetting at the tube and focuser.
> http://www.dalekeller.net/atm/newtonians/newtsoft/newtsoft.htm
> 
The trick there is to know what to ignore. It flags things in RED!
for vignetting of the 75% illuminated zone (determined by the secondary)
by the front and the focuser that are actually not that bothersome,
given that clipping by the secondary dominates everything anyway.

Especially on fast scopes, sizing the secondary for a given fully illuminated
fields may tend to make the 75% illuminated field focuser-limited - that's
a fact of life like death and taxes, so you can safely ignore that almost
100% of the time, at least if you're already using a 2" focuser.

Avoiding vignetting by the front by making the front wider makes the
secondary more distant from the focuser, which decreases the fully
illuminated field. So the rule of thumb to make sure to size
the front so that the 75% illuminated field is only determined
by secondary clipping is a choice.

You don't necessarily have to make the same one, so it's fine to ignore
some warnings.


-- 
Alexis Cousein                                  al@sgi.com
Senior Systems Engineer/Solutions Architect     SGI/Silicon Graphics
--
<If I have seen further, it is by standing on reference manuals>

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