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Download PDF by Stephen F Tonkin: Amateur telescope making

By Stephen F Tonkin

ISBN-10: 1852330007

ISBN-13: 9781852330002

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If one zone's correction lags behind the others, the chances are that overcorrection will occur in another part of the mirror before the original zone is brought into line. In my quest for optical perfection, I made close to a dozen parabolising attempts before achieving the final figure. Some of those attempts resulted in good mirrors that were just not quite good enough. I was after perfection, and although my patience and determination were tested, it was only a matter of time before I arrived at a figure that was as close to perfect as I was likely to produce.

I have to confess to experiencing similar pleasant surprise when I gave it first light. The Objective Lens Friends and relations well know that I am always on the lookout for anything that might be usable as part of a telescope, so I was not overly surprised when my sister-in-law telephoned to say that she had found something in a junk shop. This "something" turned out to be a reconnaissance camera lens dating from World War II. 3; a quick calculation will confirm that the effective aperture is just over 80 mm (3{in).

This is what produces the familiar diffraction spikes around bright stars. One can make this diffraction less apparent by simply curving the vanes. This has the effect of spreading the diffraction out to such an extent that it becomes invisible. The stars lose their spikes and the image takes on a wonderful, refractor-like quality, minus the chromatic aberration. For my 6 in I built a curved spider that consists of a single curving arc. The main design consideration is that the part of the spider that obstructs the mirror should be an even fraction of a 360° arc.

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Amateur telescope making by Stephen F Tonkin

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