[ATM] Sizing you secondary mirror

[Francis J. O'Reilly]

A great answer!
----- Original Message ----- 
From: "Richard" <richard1941 at gmail.com>
To: "'Corona Australis'" <corona_australis at westnet.com.au>; 
<atm at atmlist.net>
Sent: Tuesday, August 31, 2010 8:12 PM
Subject: Re: [ATM] Sizing you secondary mirror


> 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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