RE: [APML] Using a Mitsuboshi Knife-edge Focusers?

[John Mirtle <spampit@shaw.ca>]

Wade,
	Check out Gerry's page on KE focusing:
http://www.astropix.com/HTML/I_ASTROP/I08/I0815/I0815.HTM 
Generally the shorter the FL length the tougher this gets, especially with
fast optics. It should be easy at 300mm but at 45mm....?

John Mirtle
Calgary, Ab. Canada
http://www.astrofoto.ca 

-----Original Message-----
From: astro-photo-bounces@seds.org [mailto:astro-photo-bounces@seds.org] On
Behalf Of Thomas W. Earle
Sent: Wednesday, December 22, 2004 8:56 PM
To: astro-photo@seds.org
Subject: [APML] Using a Mitsuboshi Knife-edge Focusers?

I recently purchased a Mitsuboshi Knife-edge Focusers for my Pentax
67.  Tonight was the first night I tried using it (i.e. testing
purposes only).  I had it mounted to a Pentax 300 mm EDIF f/4.0
lens.  Using a regular lens at least gives you an idea where to
start so I set the "focus" just short of infinity to try to get the
star (i.e. Vega in this case) to appear as stated here:

http://sciencecenter.net/hutech/mitsub/focuser.htm

Unfortunately, I don't really have any idea how the system is
supposed to behave.  Is the star supposed to look like a quarter
moon just for an instant while it crosses the knife edge or what?
The star I saw was generally round while it was visible (i.e. within
the FOV but not at the knife edge).  Basically what I saw is a
distorted out of focus star.  At times, it almost looked like the
picture, although it never was half gone.  I did get it to where it
would disappear rather quickly when move away from the knife edge.
I was using the keypad to my AP 900GTO mount.  Is this the key (i.e.
you have perfect focus if the star disappears instantaneously when
it moves across the knife edge)?  As you can tell, I'm pretty
clueless about what I'm actually supposed to be looking for.  Can
the knife edge focuser be used on any lens or telescope no matter
what the focal length (e.g. 45 mm lens)?  Any suggestions, comments
or techniques you would like to share would be greatly appreciated.

Thanks in advance,

Wade


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