Re: [APML]: [APML] GM-8 Polar Alignment Scope

[Rene Goerlich <goerlich@tfh-berlin.de>]

The Astro-Photography Mailing List
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John F. Wall wrote:
> 
> The Astro-Photography Mailing List
> ------------------------------------
> 
> Just made some major equipment changes.  Lost the LX-50, Aquired a GM-8.
> Lost the 201XT. Aquired (not here yet) an ST-4.  Can't wait to get things
> up and running BUT-
> 
> the reticle in the polar scope on the GM-8 appears to be set to far forward
> in the polar scope tube.  As a result the three star alignment cannot be
> achieved since the reticle pattern is to large.  I believe its made by
> Celestron.  Has anyone had any experience with these polar scopes. Can they
> be adjusted or does it have to go back to Celestron.
> 
> John Wall

John, 

three star alignment ? Polaris and a "second star" (Delta UMi ??) are needed for the
Northern hemisphere. They should be located where "Place ... here" is etched on the
reticle.

But yes, the pattern of my (G-11) polar finderscope is somewhat too large or the focal
length of the objective is somewhat short. This can't be adjusted. 
You can adjust the objective so that it's focal plane is on the reticle. Then you can
adjust the eyepiece for a sharp image of reticle and star. Just do that during daytime
while looking at a distant object.

You _could_ adjust the internal place of the reticle. I wouldn't recommend this pain. The
threads have bad fitting and are full of grease. Just a bit on the reticle and you can
take it out and clean it. And every piece of dust or dirt will be magnified when looking
at the eyepiece and illuminated nicely.

If your eyepiece has to be screwed out somewhat, you can insert a o-ring used in plumbery
to keep it at the right distance instead.

Apropos illuminated: the brightness of the illuminator is way to high to see the second
star. I've glued a 2.5 kiloohm resistor in a line switch and use it to dim the
illuminator. My accumulators are attached with velcro to the inside of the mount. I use
the same device for my illuminated reticle eyepiece.

Returning to Celestron/Losmandy: I would recommend it if you could be sure to get a better
suited pair of reticle/objective. But I doubt you would. If you plan
this polar aligning for visual use, the accuracy of the finder borescope is good and
adequate, even if you want to use DSCs. 
If you want to use it for shortening setup/drift alignment time for photography, things
are a bit more complicated. It is important that the pole point where the lines are
crossing is in the middle, at the optical and mechanical axis. First I thought that you
could use the three little Allen screws along the ring where the illuminator sits to
adjust the reticle after a careful drift alignment. But that's wrong. You'll get close but
some error will remain if the reticle size and the objective focal length do not match. 

If you intend the latter, check if you can remember the position of the two stars with an
exactly aligned mount.

Good luck,
René