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Subject: Re: Star diagonals
From: Rob Roy
Reply To: mapug@shore.net
Date: Sun Jun 11 03:42:25 1995

I know this isn't strictly LX200 stuff, but it does affect my LX200 (and maybe others.) You are the experts out there, so I'll risk the wrath and ire that may be forthcoming.

I need advise on a problem. The mirror on my 2" star diagonal has a bad case of astigmatism and it has to go. The housing is quite nice, so I just want to put in a new mirror. I spoke with a couple of my astroclub members and they said to buy the best flat I could get....It didn't make sense to spend over $300 on individual eyepieces and $35 on a soso flat. Supposedly, the best were made by E&W Optics who made 1/20 wave flats at about $100/inch minor axis.

My problem is 'what size to buy?' The current bad one is 2.14", but I don't think that it needs to be more than 1"! Here's my reasoning. The secondary is just less than 3" diameter (8" f10) and the light cone has to travel approx. 20" to get to the eyepiece (~15" of tube plus ~5" through the star diagonal.) At 10" towards the eyepiece, the cone should be <1&1/2" diam. As it leaves the scope in another 5", it should be <3/4" diam. A couple of inches later, in the middle of the star diagonal, a 1" flat could easily accommodate the ~1/2" cone of light going towards the eyepiece (saving $100++ USfunds.) The exit hole from the scope is <1&1/2" diam! Isn't this latter part of the scope acting like a 3" ~f7 scope (fed with extra light from the primary,) which could easily use a 1" diagonal? Am I right or am I missing something? Please comment. Maybe you also know a better place to buy one?

Thanks, Rob

************************************************************************ Rob Roy a5817394@mcmail2.cis.mcmaster.ca Maker of LX200 Joysticks ************************************************************************