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Re: Refractor:Spotting Scope

Subject: Re: Refractor:Spotting Scope
From: Paul Schlyter
Date: Thu, 20 Apr 2006 16:13:48 GMT
Newsgroups: sci.astro.amateur
In article <1145541324.135940.252300@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
 <tony_flanders@xxxxxxxxx> wrote:
>Paul Schlyter wrote:
>> 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.
>Yes, for fast scopes, prisms *inherently* introduce chromatic
>aberration, regardless of how high the quality is. That's why
>it's (in theory, anyway) better to use a mirror diagonal at
>f-ratios of f/6 or faster. Or, as you say, you can use the
>scope straight-through.
>Having said that, two prisms *are* twice as bad as one.
>Moreover, there's one reflection in a star-diagonal prism
>and two in each of a pair of Porro prisms, and the light
>path through the glass is longer in a Porro prism. So there
>are a lot more opportunities for things to go wrong in a
>Porro-prism erector than in a prism-based star diagonal.
>    - Tony Flanders

So if you have a system with "lots of optical aberrations", and you then
manage to cut down the aberrations by a factor of 3 or so (e.g. by
replacing the porro prisms with a star diagonal), I would say the
aberrations would still be noticeable, although of course smaller.

Btw, fast telescopes are inherently more aberration-prone than scopes
with a larger f-ratio.  So if your demand for image quality is so high
that you advice against using porro image erecting prisms because of their
possible degradation of the image quality, then it's time to switch to a
slower scope IMO.

Paul Schlyter,  Grev Turegatan 40,  SE-114 38 Stockholm,  SWEDEN
e-mail:  pausch at stockholm dot bostream dot se

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