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Subject: Re: OFF TOPIC:LX200 Photography Focusing
From: Bill Arnett
Reply To: mapug@shore.net
Date: Mon May 06 00:24:16 1996

At 2:44 PM 5/5/96, Fred Park wrote:
>Get a piece of cardboard cut to fit the inside of the 8" scope with a hole
>in the middle to fit over the center obstruction. On either side of a center
>line on the cardboard and two inches from the outer edge scribe a mark. That
>will be a mark on both sides of the center. Cut a 1 1/2 (approximate) hole
>with the center on the mark inscribed. With this focusing tool you will see
>two objects when out of focus. Two moons or two stars...

I believe this device is formally known as a "Hartman aperture mask"; it is
sold commercially as "Kwik Focus" (almost $50 for a 12"!). I made one the
other day from cardboard. My first attempt was very sloppy -- the holes
were not well centered and were grossly out of round. But it seems to work
just fine anyway. But I find that it doesn't really improve my focusing
ability. It is somewhat easier to focus with the mask but a focusing
carefully on a star normally gets just a close. Question: if I tried again
and made it very carefully would anything look different thru the eyepiece?
Would it be possible to get a better focus? I made my two holes about 3.5
inches in diameter (nearly as large as possible with a 12" aperture and a
4" obstruction); would I be better off with smaller holes? (I would guess
that smaller holes would work better with bright stars whereas the large
holes would be better for dim stars.)

BTW, Philip Harrington otherwise excellent "Starware" says that this device
works only with extended objects, not stars. Seems to me it works just
fine with stars.

---
Bill Arnett                 "Science is a way of trying
San Jose, CA  USA            not to fool yourself."      -- Feynman
billa@zNet.com                              <URL:http://www.seds.org/billa/>