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DIY Counterweight for CPC925

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#1 barbarosa



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Posted 19 September 2011 - 12:46 PM

I am posting this here rather than in the CPC forum because this DIY counterweight kit it might be applicable to other models and brands.

The normal setup for my scope is dew shield made from Reflextix, a Stellarvue 50mm RACI finder scope, a 20mm Meade 5000 SWA EP, or a Samsung SBC-2000 camera. Compared to the Nexstar 8SE the CPC is rock steady, with better go-to accuracy and tracking. One thing that bothered me was that tracking seemed to have the occasional hesitation or jump.

I followed the Starizona instructions to check static and dynamic balance. It shames me to say it, but my CPC 925 was unbalanced.

I looked at the various kits offered and for posts on DIY counterweights. DIY appealed to me, because I do like to tinker, and if a DIY setup failed, the cost was low.

The parts for this kit are common hardware items available on the net or in many hardware or big box stores. In my case, this was Orchard Supply Hardware, Ace Hardware and Rockler Woodworking and Hardware. The total cost is under $25.00

Parts list—
1- 24” “Universal T-Track”
1- Short section of PVC pipe, D=1.5” or larger or a PVC coupler.

2- 1/2” 8x32 pan head or round head screws and washers (to fasten PVC radius block to rear cell)
2- 1/2” 8x32 flat head screws (to fasten PVC to track and track to front cell)
1- 8x32 nut (to fasten PVC to track and track to front cell)
1- 8x32 lock washer (to fasten PVC to track and track to front cell)

2- 5/16" fender washers (1½” OD)
9- 2¾” galvanized washers
9- 1¼” galvanized washers
1- 2½” 1/4x20 hex head bolt.

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PVC Radius Block
To make the radius block cut off 2” or 3” from a 2” PVC pipe. Use a table saw to make a lengthwise cut in the pipe. Place the piece in boiling water to soften it. Use kitchen tongs to remove the piece and to spread the cut wider. Use a steel spatula or other tool to press the piece flatter. PVC has some degree of memory, so a shallow arc rather than a flat piece is the goal.

If the cut offs on the piece were not square, the piece you have flattened won’t have parallel sides Use the table saw to rip the long sides parallel and then cross cut the ends square to the sides. Rip the piece again to the final width, about 1+”

Measure and mark out the centerlines of the PVC strip. Mark for two holes centered on the long axis and 1 ¾” center to center. Use an 11/64” bit to drill the holes

Remove the two roundhead screws at the bottom of the rear cell and use them to attach the PVC strip to the cell.

The T-track has countersunk holes at intervals. Attach the T-track to the front cell using a ½” 8x32 flat head screw. You will need a spacer the approximate thickness of the PVC strip. A hard rubber plumbing washer will work.

The other end of the track will extend past the PVC strip on the rear cell. Center the track on the strip and mark it where it crosses the centerline between the two screws.

Remove the track and still using an 11/64” bit; drill a hole in the track at the mark. Reattach the track with the front screw, center it on the PVC strip and using the hole just drilled mark the strip..

Remove the PVC strip and drill an 11/64” hole at the marked spot. Turn the strip over and counter sink the hole for the flathead screw that will attach the strip to the track. The PVC strip attaches to the track with a ½” 8x32 FH screw, nut and lock washer.

Note: A person with better skill might have marked and drilled all holes at one time. I did not because I knew that at least one mark or one hole would be slightly off. The hole I drilled in the T-track was just off center.

Install the PVC strip and the track, and mark the track for length. Remove the track and cut to length.

Remove the PVC strip and lightly sand it with fine paper. Apply a coat or two of flat black paint. De-burr the ends of the track. Paint or a blue sharpie hides the raw aluminum.

The Weight

I looked around the garage for something to use as a weight. I rejected a jar full of washers for 5/16” bolts and an old spark plug socket and the 12” aluminum disk still needs a purpose. It was time to go to the hardware store.

Galvanized washers in various sizes were in bins at $2.19 per pound. The 2 ¾” OD washers have a 1 ¼” D center hole and 1 ¼” OD washer is a snug fit in that hole.

I left the store with ~2 lbs. of washers (9 large and 9 small) and two 5/16” fender washers (D=1.5”).

The weights consist of a stack of large washers with the smaller washers filling the donut holes. A fender washer at each end and a ¼ bolt down the center keeps the stack together and fastens it to the track.

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Rockler says that their T-track accepts T-bolts, ¼” hex bolts and 5/16” hex bolts. However, the screw in the front cell is perfectly flush with the bottom of the track a ¼” bolt does not clear. I used a SS flathead screw and it did not seat flush. The grade marks on the bolt head did not clear so I ground them off. I took a section of the T-track to the store and none of their 5/16” hex head bolts would fit, so I used 1/4x20 bolts that I had on hand. When I get to it, I’ll pick up a SS 5/16” bolt and grind the head to fit.

#2 bluedandelion


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Posted 19 September 2011 - 12:51 PM

Nice cost effective solution. Thanks for sharing.

Now if only you could find an orange (yellow would have been close enough) track to complement the Celestron colors :)


#3 barbarosa



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Posted 19 September 2011 - 01:12 PM

Alas a custom color was not in keeping with the frugal objective. Rockler only stocks in blue, but T-track is available in red and aluminum and at lower cost. However, the desire for instant gratification overcame my frugality but not to the extent of custom colors.

#4 dobsoscope



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Posted 21 September 2011 - 02:23 PM

What about a DIY solution for a counterweight on the main GEM shaft? Any ideas?

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