Remote Focus Control for LX90 and Similar Telescopes
Submitted: Monday, 5th July 2010 by Dennis Greeve
This article will be better understood if it is read in conjunction with To Build An Observatory, May 2010.
When Astrophotography became an interest, focusing my Light Bucket 10 inch Dob Scope, with high magnification, was frustrating with shake while focusing and being in two places at one time – at the telescope and the computer. The problem was solved with a simple motor drive and O ring belt on the rack and pinion focuser and a remote control box - see earlier article, The Light Bucket.
The LX 90 tube focus arrangement presented a different approach and the 2 diagrams below show the early focus drive gripping the focus tube with a spring jawed chuck, which was satisfactory until I decided to add a focus meter to provide approximate focus settings for the DMK Web Cam and the Canon Camera, plus EP added magnification - I have no doubt there will be many who will smile at this attachment as being unnecessary - It works for me and was well worth the effort.
The modification to the original design took into account that the focus tube would not be moved manually and the chuck would be locked to the focus tube replacing the spring grip method. This can be seen in the attached pics and if copied, the slide arrangement for the motor mount becomes obsolete and easier to make.
Two matching gear/wheels from the junk box (not cogs!) and an old wristwatch was chosen and the design arranged around them. The drive gear/wheel is 56 tooth and the second gear/wheel driving the wristwatch winder knob is 36 tooth. Any ratio will work just as well, providing that the smaller gear provides sufficient clearance for the chuck.
Wrist watches have odd winding ratios, anything from an hour for a single turn to 40 minutes and so on, and the most disappointing feature with all, is loss of motion when reversed. But all of these features can be accounted for with a simple chart logged for Clockwise movement and Counter Clockwise movement. Note: Final focus is made independent of this metering device.
The modified chuck is fixed with 2 grub screws to the focus tube and the number of full or part revolutions of the focus tube is recorded as time in both the clockwise and counter clockwise rotation providing a close approximate focus.
A one-shot circuit with fine and course flip switch and clockwise and counter clockwise slide switch. On and off switch and red led power indicator. Power is supplied from the telescope rechargeable 12v lead acid battery via Red ICA cable and focus via the yellow ICA cable.
The web cam, eyepieces and Canon Camera, all with different magnification, require different focus settings, which I found rather daunting. A simple time log for clockwise and counter clockwise rotation provides a close estimation for each setting.
The Motor is a Jaycar WG 2734 36 rpm.