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

[Ben Gomes-Casseres <ben@alliancerevolution.com>]

The Astro-Photography Mailing List
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Wil,

I notice that Gene mentioned the benefit of calibration numbers on his 
helical focuser. I picked up a little calibration gizmo in Japan for my Tak 
refractor, which has an r&p focuser, as you know. It's a dial micrometer 
that you attach with a bar to the focusing lock and the measuring pin rests 
against the front ring of the focuser. It then shows any movement of the 
focuser. Though I use a magnifier (ground glass at film plane) or the B&K 
focuser, I focus several times in a row before shooting and check the dial 
each time. That way I am sure that my "final" focus is not an outlier; the 
true focus is where the measurements converge to. This worked well for me 
on the solar eclipse, when it was very tricky to focus on fuzzy sunspots 
and the boiling limb. Something like this might help as an addition to your 
already-great focus when seeing conditions are poor (which doesn't happen 
in AZ, I'm sure...).

Regards,

Ben

-----Original Message-----
From:   Gene Horr [SMTP:genehorr@swbell.net]
Sent:   Friday, July 10, 1998 10:16 AM
To:     astro-photo@nightsky.com
Subject:        Re: [APML]: Focusing a Takahashi Epsilon astrograph (was: New 
photos)

The Astro-Photography Mailing List
------------------------------------

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.

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.

> 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

VERY NICE!!!  I like how you were able to get a black background
while still keeping fine detail in the nebulae.

Gene Horr
genehorr@swbell.net