Motorized sliding focuser

Original design 1995

This is my original sliding focuser design from 1995.

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• I decided to construct a sliding focuser for my 6" telescope. I did it for several reasons:
  1. Since I am not equipped for metal work, I was looking for a design that could be made of wood.
  2. I wanted to have a small diagonal. Thus the focuser had to be low.
  3. Since this is the first telescope I built, I wanted as much focuser travel as possible to allow for small errors in determining the location of the focuser on the tube.

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• • The 100x190x10 mm focuser board is made of wood and rides on two aluminium rails (only one is visible on the photograph). The rails are 10 mm thick. The brass 58mm ID eyepiece tube is screwed and epoxied to the focuser board.
  • To accommodate 1 1/4" eyepieces, I made an adapter from a wood ring and 33mm ID brass tubing. Together with the eyepiece tube adapter, the focuser has a total height of 40mm. The 2 spider vanes of the secondary holder are fixed to the 58mm brass tube.
  • To allow lateral adjustment along the optical axis, the focuser board can be slanted 5mm in each direction.

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• • The focuser is moved by pulling a centrally mounted M5 threaded rod. The rod itself is pulled by turning a M5 T-Nut installed in a wooden disk that is clamped between two L-shaped aluminium supports. A small motor turns the wooden disk and allows for focusing without introducing vibrations to the tube.
  • The motor and the two 9V Batteries are housed in a plastic box to protect them from dew. A potentiometer on the handpad allows for fine adjustment of the focuser speed.
  • In the newer version on the right, I added some gearing and removed the 9V batteries. The motor is driven by 4 x 1.5V AA batteries that reside on top of the blue box.

Updated design 2010

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• I revisited the sliding focuser design during the construction of my startest telescope. Since I needed a compact design, I decided to change the motor arrangement.
  Some time ago I acquired some small geared DC motors on EBAY that are ideally suited for the task to drive the sliding focuser carriage. In this case I used a 3V 50rpm motor that I run on 6V and final speed of 17rpm thanks to final pulley reduction.
  The cariage runs on small drawer rails and is driven by a M6 threaded rod that is supported by ball bearings at both ends.

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• As you can see on the photograph, the focuser has a digital caliber attached to measure its displacement. Since the startest telescope works exclusively with a camera, there is no eyepiece tube but only a coupling for the camera support rod.

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• Here you can appreciate the camera mount arangement, in this case with a laser collimator in place of the camera.