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

["paolo gramigna" <p.gram@tin.it>]

Another hint: if you have the star very close to the knife, you can just
push (gently) with your finger against the  lens, and the star will blink in
and out. Take note of the direction the knife goes in, move the focus a bit
until the knife comes in from the opposite direction, and keep nudging
gently while redicing the range in the focus. After a bit of practice, it
will become as easy as a kiss.

Cheers,
Paolo


> -----Messaggio originale-----
> Da: astro-photo-bounces@seds.org 
> [mailto:astro-photo-bounces@seds.org] Per conto di Ahwang
> Inviato: giovedì 23 dicembre 2004 8.34
> A: 'Discussion of Film Astrophotography'
> Oggetto: RE: [APML] Using a Mitsuboshi Knife-edge Focusers?
> 
> 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  
> 
> 
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