By Stephen F Tonkin
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Additional info for Amateur telescope making
In a telescope, the "noise" consists of stray light that somehow makes its way to the image plane, usually because of inadequate baffling. Given that proper baffling is quite easy to accomplish, and that the rewards can be simply astounding, it is surprising how little attention is paid to this in telescope construction. In a nutshell, the point of baffling is to ensure that only light which has fallen directly on the primary mirror makes its way to the image plane. To accomplish this, stray light must be suppressed or, better, completely eliminated.
In those earlier projects, the optical tube assembly was completed in advance before figuring. However, a bad case of laziness prevented me from making the optical tube assembly for this project in advance. Astigmatism is something that I have avoided in the past with other projects, but I had never attempted a thin, large mirror until this project. Astigmatism is avoidable in thin mirrors and I will redouble my efforts to eliminate it in the future. In hindsight, I believe the astigmatism was detected in the Ross Null test used in the Ronchi mode.
Having said that, there's no reason why anyone with determination cannot produce such a mirror the first time out. Perseverance and patience, rather than skill and aptitude, are the crucial ingredients for successful mirror making. The procedure for making a long-focus primary is basically the same as for any mirror, but there are a few significant differences to be aware of. Remember, the goal here is not simply a working mirror, but one suitable for use in a high-performance planetary telescope.