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

["Ahwang" <parsec32@adelphia.net>]

When using the mitsuboshi kef the star will look like a big out of focus
star at first.  When it is in perfect focus the star will disappear
suddenly like someone turned the lights off when the edge covers the
star light cone.  As you get closer to focus the star will appear to
turn gray and slowly blink out like a shutter in slow motion as the edge
covers the stars light cone.  It takes a while to learn what to look for
and you will be in an awkward position trying to look up through the
kef.  Mitsuboshi used to have an adapter that allowed a diagonal to be
used so that you didn't have to sit on the ground and look up through
the kef but I could never get it to work.
Allen

-----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 11:21 PM
To: 'Discussion of Film Astrophotography'
Subject: RE: [APML] Using a Mitsuboshi Knife-edge Focusers?

John and Steve,

Thanks for the links.  Unfortunately, the Mitsuboshi Knife-edge
focuser doesn't use a Ronchi screen, although it would have been
nice if they had chosen too, so the behavior is a little different.


I went out again and finally started seeing what's in the diagrams.
I think the first time I went out, I didn't get the star close
enough to the knife-edge to see the desired behavior.  I was able to
get the star to disappear suddenly.  In addition, the 300 mm goes a
little "beyond" infinity so I was able to vaguely see the "star"
reappear on the other side of focus.  I have an additional question.
Is one in perfect focus when the star disappears suddenly or is
perfect focus in the middle of the darkness (i.e. between the inside
and outside focus when the star reappears)?.  It would make sense
that the latter would be true; however, I just want to be sure.
Finally, I was most successful when I got the star as close to the
knife-edge as possible using a button rate from 64x to 1x.  Using 1x
I was able to get the star to disappear, and then I tweak the star
back into view.  Once here, I moved the focus ring until the star
disappeared suddenly which should be pretty darn close to perfect
focus.  Please let me know if this technique is incorrect.

Thanks,

Wade  


_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo


_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo