A 10-inch Split Ring Newtonian Reflector

I built this split-ring 10-inch f/6 reflector back in 1980. It is still one of my favorite Newtonian telescopes. The top end of the scope, the octagonal box, was made as small as possible in order to use a low profile focuser. A 10-inch f/6 mirror normally requires about a 2-inch secondary mirror (measuring the minor axis). An f/5 scope requires a 2.5" secondary, about a 25% obstruction. Mine is 1.5 inches, a 15% obstruction (by diameter). Theoretically, a 12% obstruction would cause a reflector to perform like a 10-inch apochromatic refractor.

The scope breaks into three pieces for portability. The section in my right hand is hinged and can be folded flat. The central split ring rides on a pair of roller-skate wheels. The bottom tube was a piece of schedule 40 PVC pipe that I found at a construction site. It was a short cutoff that they had thrown into a dumpster. When you are an amateur telescope builder, you are always on the lookout for pieces of large diameter tubing.

I built the octagonal box to get the focuser as close to the secondary mirror as possible, in order to use a small flat and minimize the secondary obstruction. This time I used a 1.5-inch flat, with no image cut-off. The curved spider vanes were an attempt to defeat the bright diffraction spikes that are caused by straight vanes. The diffraction spikes are still there but you won't see them because they are spread out over the whole field and less objectionable. The absence of glare makes some detail easier to see on bright planets. It also results in a more pleasing image, with a darker background. This seems to help with the overall contrast.

Recently, I replaced the main bearing with a sleeve and rod turned from delrin, a plastic slicker than nylon but not as slick as teflon. A light touch of baby powder makes the perfect lubricant. The scope turns as smoothly as you could want.

In SurfCam, I drew an indexed circle with numbers, projected the circle onto a surface, and machined it with a .020" cutter, all in virtual reality. The program shows the machining on screen in 3-D. You see everything but the flying bits of dust. The file was then posted to the Arrow 500 which did the actual cutting in a sheet of white plastic .080" thick. Not exactly low-tech anymore, but then it's been 16 years since I started building this scope. There is no reason to replace a good scope when you can keep upgrading it.

The declination circles (one on each side of the scope) were drawn and cut as described above. The PVC tube, supported by three small roller bearings, rotates inside the box.