Re: [ATM] 1.25in Versus 2inch focuser.

[John Lynch <john_lynch2006@yahoo.com>]

Marco,

Choice of focuser and secondary size will affect the
75% illumination field.  My recommendation is to use
Texereau to find your optimum secondary size, find the
closest one that is commerically available, and then
run some numbers through NEWT to find out how changes
to these two variables affect your outcome.

John Lynch

--- Marco <marco.miglionico@btinternet.com> wrote:

> Which should I consider for the following scope:
> 
> 20in F5 mirror. I ran SEC and consulted my local
> mirror
> supplier and 83mm (3.27in) seems to be the best
> match
> in both Kriege/Berry's book and in SEC.
> 
> However in Berry's book it is recommended that in a
> scope of this
> size, parameters give a practical limt of around 0.5
> degrees FOV.
> 
> Doing some calculation tells me that I only need a
> 1.25in focuser
> as a 24mm panoptic will supply me with a field of
> around 39 arcmin
> which is more than ample.
> 
> My question is, what advantage would I get from
> fitting a 2in focuser if
> the 1.25inch eyepiece gives me my maximum practical
> field of view.?
> 
> For that matter, is there any point in fitting a 2in
> focuser to anything F5 
> or faster?
> 
> any illuminating ideas?!!
> 
> Marco. 
> 
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> ATM mailing list http://www.atmlist.net/
> 


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