[ATM] Sizing you secondary mirror

[Richard]

It all depends on what you mean by "optimum".   

If you want a wide field low power scope for visual photometry, you must
have zero vignetting.   In that case, see Rutten & Van Venrooij, Telescope
Optics, formula 5.4.2.   It is not at all harmful to have a slightly larger
diagonal than the formula suggests.

If you want a high power scope for splitting double stars and studying
planets, diffraction becomes important.  You want to minimize the central
obstruction (also try to minimize the vanes, as with piano wire).  You need
a minimal diagonal as given in formula 5.4.1.   Actually, you can get by
with an even smaller diagonal than that, if you are willing to have the
focal point deep inside the tube and use a relay lens... but that is off the
beaten path and is likely to get a lot of flaming on this list.

If you want a cheap scope, the smallest diagonal you can find is the
cheapest, but not cheap enough to cover the cost of a relay lens.   

If you want a light weight scope, the smaller the diagonal, the lighter.

I repeat, it all depends on what you mean by "optimum".  In real-world
engineering, there are a lot of compromises and trade-offs that depend on
your individual intent.  Simplistic stock answers like those given by NEWT
are likely to mess you up.


-----Original Message-----
From: atm-bounces at atmlist.net [mailto:atm-bounces at atmlist.net] On Behalf Of
Corona Australis
Sent: Tuesday, August 31, 2010 4:23 PM
To: atm at atmlist.net
Subject: [ATM] Sizing you secondary mirror

Hi folks,

What is the formula for working out the optimum size of your secondary
mirror?
I'm looking for full illumination. I used to have it somewhere but can find
it, any
help greatly appreciated. The mirror being used is as follows

Diameter 254mm 
Focal Length 1200mm 
F/ratio F/4.7

Btw, this doesn't need to be turned into a great debate on the pro and cons.
The 
formula would do nicely.

Cheers,CS

Rob




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