RE: [APML] Focusing OM-1 - but what about the Nikon F2?

["Lane, Jason R" <jason.lane@navy.mil>]

Hi Wei-Hao,

Actually, knife-edge or Ronchi screens work in spite of the scope being less than diffraction limited or even just diffraction limited.  That is, the more perfect the scope the better the knife-edge works, whereas with the magnifier the opposite is limited by accommodation and your ability to see the Airy disc.  However, in our non-ideal world, either technique (high magnification or knife-edge) are equal in accuracy.

-Jason

-----Original Message-----
From: astro-photo-bounces@seds.org
[mailto:astro-photo-bounces@seds.org]On Behalf Of Wei-Hao Wang
Sent: Sunday, November 14, 2004 11:21 PM
To: Discussion of Film Astrophotography
Subject: Re: [APML] Focusing OM-1 - but what about the Nikon F2?


Hi Jeff,

Thanks for sharing your experience.  The reason you can see the 
diffraction ring easily is that the Hartman mask decreases the effective
aperture of the telescope and so the diffraction ring becomes a few
times larger. This actually decreases the accuracy of focusing.

I usually use a 60x magnifier (or you can call it a microscope) to focus
on the focal plane, without a Hartman mask.  On my F5.1 telescope, I
can only see the diffraction ring with magnifications  greater than 40x.  
Because I'm using the native aperture but not stepping it down with a 
Hartman mask, I'm sure if I see a diffraction ring, I'm on perfect focus.  
No matter how good Ronchi screen or knife-edge is, it cannot be better 
than diffraction-limited, right?

The 60x magnifier is Tak's FM60.  It comes with a ground glass that can
be used on both 35mm and 120 format cameras.  I'm very satisfied by
this setup so far.  I usually choose a bright star at about 30 degree 
elevation.  This makes me neck feel much better.

Wei-Hao

> 
> Yesterday afternoon I made (with the help of my better half) a ground glass
> focusing screen for the focal plane of the pentax 67.  It took us about
> 30 minutes.  Now, we had all the supplies and tools, so ymmv.  We simply
> took a peice of glass, cut it to sit on the focal plane (about the
> same size as the film).  Then we masked one side of the glass, and
> applied a "frosting" etcher to the other side of the glass.
> 
> I tried it out last night with with an 8x loupe and a hartman mask.
> It seemed easier to focus than on the viewfinder ground glass.
> Visually, it was easier, but it was more difficult on the neck!
> I also noticed that after th e hartmann mask dots converge, I
> could actually see diffraction rings.  I'm suspecting this
> was perfect focus.
> 
> I dont have the results yet - film still in the camera.  But I'll
> followup when I get the results.
> 
> I'm pretty sure the ronchi would simplify focusing; If I had one
> that was the correct thickness for the film, I'd use it.
> 
> 


-- 
________________________________________________________________
Wei-Hao Wang  :)

Institute for Astronomy at University of Hawaii

Address:                       
2680 Woodlawn Drive         Personal Website:
Honolulu, HI 96822             http://www.ifa.hawaii.edu/~wang
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