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:
> 
> Wil Milan wrote:
> 
> > I have the rack-and-pinion focuser, and yes it is a challenging
> > instrument to focus.
> 
> My hat is off to you!  I have the helical which has two
> advantages:  Calibration numbers on the side and an extremely
> fine thread (not quite 1000tpi, but it sure seems like it!)
> The biggest negative is having to spin the camera forever
> for minor focus adjustments.

That's the trade-off, I guess. I had a helical focuser on another
scope and I liked the fine focus adjustment, but having to spin
the camera to focus drove me nuts. The rack-and-pinion focuser
requires a very fine touch, but I've gotten used to it and I
really like the fact that the camera does not rotate as I focus.
That allows me to frame the shot first, and therefore don't have
to rotate the whole focuser to frame the shot after focusing. I
know the Takahashi focuser can supposedly be rotated without
affecting focus, but I tend to distrust such things when the
adjustment is so critical (it's paranoid, I know).

The best solution, I think, would be a helical focuser that does
not rotate the camera. I've seen one of those in the Lumicon
catalog; I wonder how well it works.

> I have the 6X magnifier and it is very helpful for rough
> focus.  Instead of a knife-edge I use the Tak FM-60.  This
> is an extremely handy device for just about any film camera.
> It consists of a focusing screen about the size of a microscope
> slide with a miniature microscope.  Once you get rough focus
> you open the camera back and lay the screen across the film
> plane.  You then use the microscope to examine the star
> image when fine focusing.  It works GREAT!  The street
> price is ~US$200.

That sounds like a great gadget, and even better because it
sounds like it would be very easy to build such a thing. The only
problem I see is that one has to open the camera back to focus,
which means removing the film each time (a big pain) or switch
cameras for the shot (risk of shifting the film plane slightly
from one camera to the next). Still, I think I'll try it; I've
got a damaged Nikon body that could be made into an experimental
focuser.


Wil M.      
mailto:wmilan@airdigital.com
Astrophoto web site: http://www.airdigital.com/astrophoto.html
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"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