Hi Richard & All,
Quote:
Originally Posted by croweater
Excellent post Les. Is there an equation to work out the smallest secondary mirror size possible for a certain aperture and focal ratio without losing light. Cheers, Richard.
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The best advice I can give you is to obtain the long booklet "Newtonian Notes" by Peter Francis that speaks about and gives examples of calculations pertain to this sort of thing. Once upon a time (back 30-40 years ago) nearly every astronomical society had people who knew and understood this stuff thoroughly, but like the sub-editors at the newspapers, they have largely disappeared.
Woo hooo !! I found a .pdf of this handy booklet on the web:
http://www.alpo-astronomy.org/jbeish/NEWTNOTE.pdf
There used to be a freeware called Newt that was like a CAD for Newtonian telescopes and calculated all this stuff automatically and you can see that by altering this, that or the other parameter, what effect it had on the size of the 100% field, 75% field and whether the focuser or the secondary was vignetting the telescope, secondary mirror offsets (if desired -- I don't bother with that -- I centre everything).
Hold on, I found it! It's now web based and is here:
https://stellafane.org/tm/newt-web/newt-web.html
All this stuff is one of the nice things about actually sourcing components for yourself and assembling a Newtonian telescope that best suits your own observing (and other needs regarding portability) needs -- you can, by varying a few components, f/ratios, focusers, secondary sizes etc etc, optimise the telescope for visual observing (all-round) planetary observing or richest-field type telescopes -- or indeed photographic Newtonians. There are a few reasons why some other designs (R/C or other exotic astrographs) (in a perfect world where funds were unlimited) work better or at least more easily than a Newtonain photographically, but they can still be quite good for photography too.
Best,
L.