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In article <1145533308.088905.293330@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
<tony_flanders@xxxxxxxxx> wrote:
>Note in particular that conventional erecting prisms -- whether
>built-in
>as with a spotting scope or add-on as with an astro refractor --
>introduce lots of optical abberations. So if you're willing to put up
>with the mirror reversal and 90-degree viewing angle of a star
>diagonal, you can cut a lot of nasty elements out of the optical
>train.
A conventional porro-prism image erector has two prisms, while a star
diagonal has one prism.
Now, if two prisms introduce "lots of optical aberrations", as you
here claim, perhaps the optical aberrations of the single star diagonal
prism cannot be regarded as negligible ??? Therefore, for optimum
image quality, one shouldn't use a star diagonal either.
Of course, it depends on the quality of the prisms.
--
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Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/
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