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GEM Reflector
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20" Goto Scope
20" Goto Dob








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GEM Reflector

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What I wanted was a scope capable of astrophotgraphy and that could be moved when not in use. The  only option seemed to be to use a German equatorial mount.  To get away with a cheaper mount the weight of the tube needed to be kept as low as possible.

An aluminum tube was ordered 4" longer than necessary, the extra length was removed with tin snips and used to make the cradles

The cradles are lined with felt and the tension required to hold the tube applied with a nut and bolt.

Weight was again a major consideration when making the mirror cell.   The cut outs both save a few grams and allow for quick thermal equilibration of the mirror.

The spider is made if 2 strips of steel with square nuts welded to each end.  Bolts in the tube allow some flexibility in positioning of the mirror.  A bolt welded to the middle for the spider is threaded into a nut on the secondary holder and provides the rotational and vertical adjustment.  The inset shows a ring of 1/4W resistors which can provide a small amount of heat to the secondary to prevent condensation forming,

The Mount itself is a standard German equatorial mount.  I have added stepper motors to each axis which are controlled by a PC running Mel Bartels telescope control software.

The polar axis scope makes aligning the mount relatively painless.  Although those living further south would not have to bend over so far.
The 12V 200 step steppers are connected to 25:2 gear boxes; this gives a theoretical precision of 0.18 arc sec per micro step.  Though just as importantly the gearboxes increases the toque available from the motors.  2 Tips:- 1, When delivered the mount had been put together with grease which had the consistency of glue.  When this was replaced with a lighter grease the motors were much less likely to stall.  2,  Using proper flexible motor linkages to connect to the slow-motion controls instead of car fuel piping much reduced the backlash in the system.

The electrical components required to run the motors is contained in a box with a large ammeter on the front.  It is useful to know the current drawn by the motors when adjusting the software parameters.  Also a hand controller is required in order to slew and guide the scope.  The super Nintendo controller could of been made for the purpose.

The software has no end of nice features including  PEC, automatic drift computing  and interfacing with planetarium software.  Also it isn't fussy about the spec of machine its running on ( a second hand 386 in this case).

The big advantage with this system is being able to enjoy a all night observing session in the warm as the water in the garden pond freezes to a depth of 2".  The computer on the right controls the telescope while the left hand one has the planetarium software and controls the ccd camera.


CCD Imaging




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See the full range of Atik cooled CCD cameras for astronomy on the Atik Website


Copyright 2006 Steve Chambers. All Rights

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