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De Rotation

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Mounting largish telescopes alt az wise offer many many advantages. The big drawback is frame rotation. If you look at a group of stars though a Dobsonian type telescope for a reasonable length of time you may note that the field of view rotates with respect to the focuser. For visual observing this is not a problem. However if a camera is put into the focuser the this rotation can cause objectionable trailing of stars in 30 seconds or less exposures.

The amount of rotation is heavily dependant on the position the object is in the sky and the observers' location. Chucks web pages describe the effect well. Mel's telescope control system is able to control a frame-derotator to counter act this effect. Basically the focuser is rotated in step with the frame rotation. Again for an f3.8 it is important not to add more height to the focuser. The frame derotator I put together is only 15mm thick and was made using the standard tools (router, drill, jigsaw).

The de-rotator is made of 3mm thick aluminum sheet. Briefly, 2 square pieces and a rectangular piece were aligned and the centre marked by drilling a small hole. The router was then used to make 1mm deep circular grooves for a ball bearing race. Groves either side of the centre piece and on 1 side of both the outside pieces. The router was then used to make the outside pieces circular. Holes were drilled for the bolts that hold the outside pieces together and against the inner piece. Finally used a jigsaw to make a hole large enough for the focuser tube to pass through and mounted the focuser. Initially I had planned to use a belt to drive the de-rotator but had problems with slippage. As a temporary fix I have used epoxy putty to make a ring gear. This has worked well enough that finding a proper gear ring is not a priority.

 

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Copyright 2006 Steve Chambers. All Rights  Steve@pmdo.com

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