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Re: [OM] focusing screens

Subject: Re: [OM] focusing screens
From: Kennedy <rkm@xxxxxxxxxxxxxxxxxx>
Date: Wed, 8 Apr 1998 12:55:18 +0100
In article , Whipple.Art <Whipple.Art@xxxxxxxxxxxxxxxxxxx> writes
>I have a question about the 1-12 screen.  It is always described as a cross
>hair on a clear field but if you look closely the field is not clear.  The
>field is actually a fresnel lens pattern.  The central circular region that
>contains the cross hairs is indeed clear but the rest of the field is made up
>of fine concentric circles.  At least this is true for any of the 1-12 screens
>that I have seen.  I use this screen for high resolution solar photography
>(a focal length of about 14 meters at f/140) and at this focal ratio the
>fresnel pattern is useless.  The field outside the clear circle shows up as
>concentric light and dark rings.
>
You need the fresnel to ensure that all the light rays across the field
actually get through the eyepiece - in other words to stop vignetting.
I think this would be less of a problem with the extreme f/# that you
are using but for general astronomic work at f/10 or so it is necessary,
as you can tell if you simply remove the screen completely.
-- 
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers         (replace 'nospam' with 'kennedym' when replying)

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