By Stephen F Tonkin
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Additional info for Amateur telescope making
In the end, final testing showed the resulting mirror to be excellent - around l/29th wave, although such figures can not be taken at face value. However, at the very least, I was confident that the mirror was far superior to anything I could reasonably expect to obtain from a commercial vendor. Secondary Considerations One of the main disadvantages of the Newtonian reflector arises from the fact that it is an obstructed system; the secondary mirror blocks a small percentage of the incoming light.
The prism is epoxied to the head of an M3 bolt. The bolt protrudes through a hole in the tube and, after adjusting the prism by twiddling with the bolt, I fixed the prism in place with liberal amounts of hot-melt glue. I used hot-melt because it was the only adhesive I had to hand apart from a very small amount of epoxy - I was pleasantly surprised that it did not crack the glass! 1, overleaf). I found a cog to serve as a pinion in my junk box. I made the rack from two brass bolts, whose thread fits the teeth of the pinion, by decapitating them and filing them flat along one side.
25 in (135 mm). Take this number and divide it by the telescope's focal ratio. 58 in (15 mm). A little experimentation quickly reveals the importance of a low-profile focuser. Indeed, it is virtually impossible to keep the secondary reasonably small if a standard rack-and-pinion focuser is used. Such a focuser often stands more than 4 inches tall and would require a secondary mirror nearly twice as large. 58 in across really work? Yes, but there is a catch. With such a small secondary, only the onaxis rays will be deflected to the focal plane.
Amateur telescope making by Stephen F Tonkin