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Portholes and Plateglass

Occasionally, we feel called upon to justify the fact that we grind portholes of plate glass and not Pyrex.

Portholes and plate have been rejected by our assailants usually on one of four counts.

1)      Plate glass mirrors are said to be harder to figure because of thermal instability of the soft glass. This is unquestionably true to a small extent. The glass is more easily distorted by the temperature of the water, by the heat generated during polishing, and by the heat from the hands. However, these problems can mostly be obviated by doing most of the polishing upside-down, i.e. with the tool on top (so that the hands do not heat the back of the mirror blank) and by using only lukewarm water on the mirror face in fine-grinding and polishing. Furthermore, these problems show up only as small difficulties, and only in the figuring of the mirror curve, and have no bearing on the final performance of the mirror.

 

2)      Porthole mirrors are said to be thermally unstable in performance due to the distortion of the curve by the more rapid cooling of the front surface on exposure to the night sky. Although theoretically this would be true, our experience is that in almost any situation in which this problem would arise, the turbulence in the tube will be so great as to mask it. If the telescope is kept in a warm place before being exposed to the cool sky, air currents set up in the tube by this foolishness mask all optical defects (except the diffraction pattern of the spider mount) for most of an hour.

 

3)      Porthole glasses and other plate glass blanks are said to be too thin and therefore subject to bending in the telescope. This is no problem at all if the mirror is properly mounted. We have an 18” telescope in Sacramento only 1 1/4” thick, which on a good night shows clear black sky between each pair of stars in Epsilon Lyrae. It hangs on a strap and rests against a nine point suspension system on the tailgate. The suspension system is made of thin cardboard, Masonite, and 1/4” plywood. Also, thin blanks reach thermal equilibrium more quickly than the thicker Pyrex blanks, both in polishing an in use.

 

4)      Heat-treated portholes are said to gradually change shape on long standing, due to the release of internal strains set up in the heat treatment. We have yet to see this sort of distortion in any of our mirrors. If the glass has been lying around for twenty or forty years since its manufacture (as most of our portholes have), it is doubtful if the distortion due to strain release will be detectable within the next 100 years. However, if a mirror is dropped and broken, even slightly, the strain release becomes immediately apparent, and the mirror may require refiguring.

Finally, we have this to say to our assailants: Let’s handle first problems first! The first problem is , “Shall we or shall we not make our own telescope?” Many of our assailants, through fear, have answered this question in the negative. We have answered it in the affirmative.

The second problem is “What shall be the aperture of our mirror? Many of our timorous assailants have settled for very small mirrors, 6” or under, for fear that a larger one would be too costly or too difficult. We prefer the cheaper glass in larger sizes. A 12” porthole will wipe out a Pyrex 8”.

The third problem is the turbulence in the tube, which is best handled by using a ventilated tube, by keeping the telescope at near outdoor temperatures, and by turning the tube over from time to time.

All the rest of our optical problems are minuscule compared to the aforementioned four.

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