Re: [APML] Genral Focus Questions and a pathetic plea for help

["D. Kodama" <kodama@alumni.caltech.edu>]

I think the real issue is the *quality* of the Ronchi screen.
One of my cameras (Japanese Planet Town 6x7 vacuum camera) came
with a 1250 LPI metal-on-glass Ronchi screen.  Even though I
use the Mitsuboshi knife edge for my other cameras, I personally
prefer to use the Ronchi screen and am able to consistently
focus just fine (and easily) at f/4.  At 1250 LPI, one of the
dark bars is just filling the view when I'm at focus.  If the
seeing is bad, I back off until I can see more lines, note
the focuser position, then go the other direction until I
can see the same number of lines.  Exact focus is 1/2 way
between the points with the same number of lines.

I think the problem with the Ronchi screens usually supplied is
that they have a low line count and are usually more cheaply
made (perhaps printed lines on plastic), so even using the
bars as knife edges doesn't work very well.

Dave



Date: Tue, 3 Jun 2003 11:56:46 -0400 (EDT)
From: "Frank Barrett" <frankb@celestialwonders.com>
Subject: Re: [APML] Genral Focus Questions and a pathetic plea for help


I have to agree with Herm.  Focusing at f/4 is tough to do with ronchi
screens.  When I bought my STI I bought the optional knife edge for it.
It wasn't long before I stopped using the ronchi screen altogether...much
more consistent results with the knife edge.

Frank
http://celestialwonders.com

 > I have a similar f4 newt, the only method that consistently worked for
 > me was the knife edge .. tried several ways including several ronchi
 > screens without luck. I was reluctant to try the knife edge method since
 > it sounded so difficult to do, but its actually quite simple.. and
 > foolproof.
 >
 > "William R. Mattil" <wrmattil@ix.netcom.com>  wrote:
 >

 >>I have been using a 253mm f/4 Newtonian @ prime focus and using a STI
 >> ronchi screen set-up to focus this and have had nothing but terrible
 >> results. Richard has provided me with multiple versions to no avail.
 >> The last trip out was the back breaker.

 >
 > Herm
 > Astropics http://home.att.net/~hermperez

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