Homemade C8 Sliding Counterweight

C8 Sliding Counterweight Project

This webpage documents construction of a C8 sliding counterweight. The project goals were to build an inexpensive sliding counterweight system that used the original Celestron C8 counterweights.  Typical prices for a sliding SCT counterweight kit are $80-$150. This project cost about $12 and the project was completed in a single weekend.                                              

Celestron C8 Counterweights

Adding ccd cameras, guide scopes, filters, large eyepieces, etc. produces extra torque that unbalances a telescope and can give drive motor and tracking problems. Celestron supplied the orange tube C8 with a 3-3/4 pound set of six counterweights that could be used separately or threaded together into a stack (below left photo). There are screw holes for mounting accessories on both the front and rear cells. The rear cell accessory holes are located on the bottom (side by side, separated by 1-3/4") and are threaded 8-32. The front accessory holes (top and bottom) are located on center (12-1/4" from the rear accessory holes) and are threaded 10-24. The counterweights screwed into the 10-24 threaded front accessory holes on the top and bottom of the C8 corrector plate ring (below center and right photos). An alternative approach is to mount a rail with a sliding counterweight. Several advantages to a sliding counterweight are quick adjustment without the need for removing or adding weights, and the corrector plate ring is free of large weights that would interfere with a dew heater or dew shield.


Counterweight Holder

The easiest way to attach the C8 counterweight would have been to construct a sliding metal plate and tap it to screw directly into the C8 counterweight. Because I only have metric tools, I couldn't tap a screw hole to match the thread on the C8 counterweight. I could have cut a metric thread into the counterweight, but I did not want to permanently alter the C8 or any of its components. The solution was to attach the counterweight to a wood disk with scraps of aluminum (below left photo). The threaded C8 counterweight screw fits into a hole in the bottom of the wood disk and is held firmly in place by the aluminum angle strips. The counterweight holder slides on an 8 mm solid aluminum rod, that can be seen in the background. The rod slides through holes in the aluminum strips (not shown). The front hole is for a 6 mm nylon locking screw. The completed counterweight holder is shown in the below right photo. I epoxied a M6 threaded sleeve into the locking screw hole. Note that the counterweight threaded hole (bottom) is accessible for attaching additional counterweights.


Counterweight Sliding Rod

The below photos show the completed counterweight system. The counterweight assembly slides along an 8 mm diameter solid aluminum rod. M4 screws attach each end of the aluminum rod to steel angle irons, which screw into the C8 accessory holes. The aluminum rod front end attaches directly to a single angle iron, but the back end attaches to a steel plate bolted between two angle irons with M6 bolts.


C8 Imaging Set-Up with Sliding Counterweight

The below photos show my imaging set-up: C8 with piggyback C90 guide scope, Celestar 8 mount with declination motor and Shoestring Astronomy GPUSB (not shown), Telrad finder, Meade DSI Pro ccd camera with RGB filter set, and SPC900NC guide camera (not shown). This extra equipment adds a lot of weight to the C8, but the sliding counterweight nicely balances the system.