Last updated: 12 March 2010
Subject:	ETX 125AT Electric Focuser
Sent:	Monday, March 8, 2010 13:45:26
From:	John Farrant (johnfarrant@gmail.com)
ETX & DS TECHNICAL TIPS/Modifications - ETX 125 AT Electric Focuser

Reading the recent note from Richard Birch about the Autostar 492
prompted me to write you about my electric focuser project using this
particular controller.

I picked up an ETX90 at a local flea market last month for 90 euros
($122 in your money!). As this came with both a 492 and 497 controller I
wondered what I could do with the redundant Autostar. Like all good
ideas, it came to me in a flash. Use the 492 as an electric focuser.

My first problem was how to fit three micro switches into the small
plastic case of a 492. Obtaining switches of this type is fairly easy. I
extracted them from a serial mouse, the three button variety. These are
readily obtained from second hand markets for a couple of euros. In
fitting them I had to butcher the 492 controller's pcb but as the image
below shows, it turned out not too messy. The switches work together in
combination to supply +/- 3 or 6 volts to the focuser motor from four
AAA batteries installed in the base. The motor was removed from a
security camera - again obtained from a flea market for about 10 euros.
This particular camera has two motors, for zoom and focusing. The zoom
motor was u/s but the other one was ok. It has a gear box bolted to the
rear end and was perfect for the job. A rectangle of plastic bent and
shaped under steam together with a suitable coupler attaches the motor
to the focus shaft.

However, there was a  more serious problem. What I didn't want was the
focus lead, from my modified 492 controller, creating a cord wrap issue
every time the scope slewed in azimuth. To avoid this I needed the
controller to plug into a socket at the base end. But how was I to get
the power from the batteries located in the base up to the motor? I
couldn't run two extra wires up the middle as Meade had done with the
four dec leads. This would have been nearly impossible and potentially
dangerous. Again, a flash of inspriration. Why not use the red dec lead
as positive and the 125s metal frame for my return path? But as I soon
discovered, the two halfs of the metal work are electrically isolated
from one another. I solved this problem by connecting the two metal
frames together with a crude but effective slip-ring arrangement. This
allows the ota to rotate completely without the frame-to-frame
resistance rising much above zero ohms. Both the Azimuth and Altitude
motors were unaffected by this modification.

How does it perform? Perfectly. I just need a clear night for a full

By the way, if you're wondering what the other lead is - visible in the
last image - coming from the Autostar 497 to the main unit, it's a
bluetooth to serial adaptor built into the base. But that's another

Thanks Mike.
Regards, John





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Copyright ©2010 Michael L. Weasner / etx@me.com
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