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Old 17-04-2019, 05:34 PM
ausastronomer (John Bambury)
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Join Date: Mar 2005
Location: Shoalhaven Heads, NSW
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Quote:
Originally Posted by Wavytone View Post
Mirko then I guess you haven’t tried a premium Erfle in a newtonian !
Well Nick I guess you haven't tried a premium Erfle in an F5 refractor, because you end up with the same thing, an "AWFLE"

The problem with Erfles is their inability to handle the steep light cone of today's faster F-ratio telescopes. The original Erfle was designed in 1921 and for many decades thereafter, the typical astronomical telescope had an F-ratio somewhere between F6 and F15. They just weren't designed to handle the steep light come of telescopes in the F3 to F7 range, which are commonplace today. Erfle's also suffer from astigmatism, which gets worse as the eyepiece focal length gets shorter. This is the reason you will never find an Erfle with a focal length less than about 12mm and usually not less than about 15mm. A good Erfle in an F8 to F10 Newtonian works very well.

Cheers
John B
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