Re: [APML]: Focusing a Takahashi Epsilon astrograph (was: New photos)

[Wil Milan <wmilan@airdigital.com>]

The Astro-Photography Mailing List
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Gene Horr wrote:
> 
> Well, some larger sized images would be greatly helpful.  They
> are quite small.  But the scan quality and colour values look
> quite good.

Gee, and I thought some of the images would be too large for
folks with slower modems :-). But you're not the only one to
request some larger images, so I've uploaded a larger version of
the North American - Pelican image. See
http://www.airdigital.com/NGC7000_and_Pelican_Nebula_Tak_E130_Pro100_30m_Sunglow_Jun98_larger.jpg

> Do you have the r&p or the helical focuser?  Your focus is very
> good and the E-XXX series is one of the most difficult instruments
> that I have ever seen to focus.

I have the rack-and-pinion focuser, and yes it is a challenging
instrument to focus. It's a two-pronged problem: The star images
from this astrograph are teeny-tiny points, so small and sharp
that they disappear into the "grain" of a focusing screen. But
the very fast focal ratio (f/3.3) makes focus very critical, with
the depth of focus at the film plane only a small fraction of a
millimeter. I had a lot of problems with focus when I first
started using this instrument, but I've finally developed a
"formula" for nailing the focus fairly reliably:

- Use an 8x focus magnifier (I made one which slides in as a
replacement finder in my Nikon F4)

- Focus on a star about 1/3 - 1/2 of the way from center of the
FOV

- Rotate the focuser to frame the shot, lock the rotation, *then*
focus

- Frame, focus, and shoot with the same camera so as not to risk
shifting focus by changing cameras

- Finally, it may be a bit paranoid, but the only camera I use
with this scope is a Nikon F4. On many 35mm cameras the film
plane and focus screen are not always in perfect alignment; I
figure the F4 is likely to be one of the most accurate in this
regard. 

I've thought about using knife-edge focus with this scope, but
I'm concerned that swapping cameras (or the focuser for a camera)
may shift focus. The procedure above seems to nail it dead-on
just about every time, and I've been very pleased with the
results.


Wil M.      
mailto:wmilan@airdigital.com
Astrophoto web site: http://www.airdigital.com/astrophoto.html
---
"When I consider your heavens, the work of your fingers, 
the moon and the stars which You have set in place, 
what is Man that you are mindful of him, 
or the son of Man that you care for him?"  -- Psalm 8