Re: [APML]: Mounting a piggyback guide scope

[a.vannini@mclink.it (Alessandro Vannini)]

>>I recently purchased an 80mm refractor to use as a guide scope piggybacked
>>atop a Celestron Ultima-11.  Now the trick is exactly how to attach is
>>securely.  I had originally thought of using rings, but I am not too sure
>>they will be solid enough.  Nor do I want to risk scratching the brass tube.
>>I've sketched a few ideas for a two piece clamp arrangement that could be
>>bolted/unbolted that wrap around the circumference of both OTA's.
>>
>>I am curious how others have solved this problem before I go off and
>>reinvent the wheel.  Anyone know a good machinist?

Ok, here is the solution astronomers adopted at the Astronomical
Observatory of Campo Imperatore (Gran Sasso, Italy): the guider
refractor is linked to the main telescope at the fore end by a
metallic (stainless steel) plate, thin enough to allow torsion along
the axis which is perpendicular to the main telescope's surface
(anyway, the plate must be also thick enough to avoid indesirable
flexures!). At the rear end of the refractor, there is a normal ring
with 3 screws. The fore metallic plate completely avoid play in
linking the refractor to the main scope, and if well dimensioned can
also completely eliminate unwanted flexures.

I think that this solution can be useful for great and heavy
instruments, but for our class telescopes there are some problems in
the correct dimensioning of the flexible fore plate. I thought to
replace this plate with a ball-head link: this kind of joint haves a
great degree of freedom in movement, and also works like a steady
anchorage point for the fore end of the refractor. Here in italy it is
not difficult to find a steady and high-quality (even if cheap)
ball-head link, which is made by a very craft manufacturer, the
Manfrotto Italia (http://www.manfrotto.com/). This ball-head link can
be also locked in whichever position you choose with a *great*
stability. At the rear end of the refractor, you can use a traditional
ring and screws to adjust the alignment of the scope. Just use a well
dimensioned ring, right oversized if possible. Consider that the
ball-head link actually do a great part of the work, so you can trust
the rear ring with more tranquillity :-) To operate an alignment of
the refractor in order to find a guiding star, you simply unlock the
ball-head link, and set the direction with the rear ring; after having
found your star, lock the ball-head and start with the exposure. Quick
and easy! :-) Working with two rings, instead, is more difficult,
'cause you have to work with SIX screws, and having two tri-axial
systems you can spend a lot of time to properly align the refractor.

Your Schmidt-Cassegrain, like all the available SCTs, haves service
threaded holes along the fore and the rear cell, if I correctly
recall. You can link a rectangular-shaped shaft to these threaded
holes, and use this shaft to mount the ball-head link and the rear
alignment circle; this way you can safe the main tube from being
scratched (this is the solution I currently and successfully use on my
SCT). Choose an aluminum and not too much thin shaft, both to let the
whole structure as light as possible, and to avoid flexures.

Since I have access to a foundry, I've also melt a couple of standard
aluminum alignment circles, and then I've machined them to achieve a
very beautiful look. This way you one can freely choose the dimension
of the rings, but it's also possible to obtain good and working rings
by cutting an aluminum tube, taking care about the thickness of the
wall (the tickest you choose the firmest you get). Then it's a ten
minutes task to make the threaded holes; the screws should be not less
than five millimeters in diameter, to achieve a good stability.

Hope this helps, let me know! :-)

-------------------------------------------------------------
Alessandro Vannini (from Rome, Italy) - a.vannini@mclink.it
http://www.mclink.it/personal/MC7872/deepvoid/index.html

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