Homemade Guidescope Rail with C8 Radius Blocks

Celestron C8 Guidescope Rail Project

This webpage documents construction of guidescope rail with Celestron C8 radius blocks. The project goal was to build an inexpensive guidescope rail  that mounted into the original Celestron C8 accessory mounting holes. Typical prices for a C8 guidescope rail with radius blocks are $30 for the radius blocks and another $30 (or more) for the guidescope rail. This project cost less than $20 and the project was completed in a single weekend. Below are photos of the finished guidescope rail and radius blocks prior to mounting the guidescope rings and with an Orion ShortTube 80 refractor. Due to recent instances of my equipment photos being posted without my permission on eBay (used in adds for C8's, C90's, and SCT accessories), I have had to watermark my equipment photos.    

Celestron C8 Accessory Mounting Hole Threads

There are screw holes for mounting accessories on both the front and rear C8 cells (top and bottom). The two rear cell accessory holes are symmetrical about the central axis (side by side, separated by 1-3/4") and are threaded 8-32. An 8-32 thread is a #8 gauge screw (4.166 mm thread diameter) with 32 threads per inch. The front accessory holes are located on center (12-1/4" from the rear accessory holes) and are threaded 10-24. A 10-24 thread is a #10 gauge screw (4.826 mm thread diameter) with 24 threads per inch.

C8 Radius Blocks from 2 mm Aluminum Strips

The radius blocks are spacers, cut to the C8 front and rear cell curvatures. These blocks mount into the C8 accessory holes and attach to the guidescope rail. I used the aluminum strip bending machine described on my homemade Large Aluminum Worm Wheels Webpage to bend a 2 mm thick aluminum strip to the same curvature as the C8 front cell (below photo).

Each radius block is constructed from 4 aluminum strips that are held together by epoxy and the C8 accessory screws (below left photo). I test fit the radius block pieces onto the C8 front cell (below right photo).


The front radius block was very simple to construct. I just spread epoxy between the 4 curved aluminum strips, clamped them together, and then drilled a hole on center for the 10-24 threaded screw. The rear radius block was more difficult to construct, because it required two off center holes that extended radially through the block. I found that the easiest solution was to make a cardboard template to transfer the rear cell hole pattern onto the bottom aluminum strip (below left and right photos).


I fabricated a simple jig to hold the aluminum strip at the correct angle in my drill press. The jig was a wood dowel, cut to fit the C8 cell curvature. The jig ensured that the mounting holes were drilled at the proper angle with respect to the curved metal plate (below left photo). I drilled the first plate and test fitted it onto the C8. Next I placed the second plate on top of the first plate, marked where to drill and repeated the process. With each successive plate, I test fit the curved plate stack and made any necessary adjustments with a small file. I epoxied the curved plates together and clamped them to dry overnight (below right photo). I wrapped the 8-32 screws with Teflon tape to prevent them from being glued into the rear radius block.


Guidescope Rail Attachment

The guidescope rail is a square aluminum bar (below left photo). The guidescope rail attaches to a countersunk screw in the rear cell radius block. The front radius block attaches to the guidescope rail with the 10-24 threaded front accessory screw. It is extremely important to dimension the screws to the proper length, so that they don't extend too far into the front and rear cells; this could actually damage the primary mirror, etc.  I measured the original C8 accessory screws (below right photo) and found that they extended 5.9 mm into the C8 front and rear cells. I ordered longer bolts and cut them so they would extend through the radius blocks and 5.9 mm into the C8 front and rear cells. I added a layer of epoxy between the guidescope rail and radius blocks to improve the stability. I cut clear polycarbonate caps to close the ends and cover the 10-24 threaded screw access hole.


Below are photos of the guidescope rail and radius blocks on the C8 rear and front cells (left and right photos, respectively). The clear polycarbonate end caps allow easy inspection of the mounting hardware.


Completed C8 Guidescope Rail

Below are photos of the finished guidescope rail and radius blocks prior to mounting the guidescope rings.


Guidescope Rings

I purchased an Orion ShortTube 80 refractor and the corresponding 105 mm guidescope rings. I fabricated two small wood blocks to fit inside the guidescope rail (below left photo); the wood blocks allow the guide ring mounting screws to be tightened without deforming the square aluminum guide rail. The below right photo shows the fully assembled guidescope rail. 

C8 with Orion ShortTube 80 Refractor Guidescope

The below photos show the completed guidescope rail with an Orion ShortTube 80 refractor mounted onto the C8. I mounted the guidescope so that it was forward on the OTA to compensate for the added weight from the imaging flip mirror and DSLR camera.