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PERSPEX LENSES AND MIRRORS

J. WALL
30 Bow Arrow Lane, Dartford, Kent
1977, 88, 1 Journal British Astronomical Association

This paper deals with an investigation into the feasibility of making lenses and mirrors in Perspex.

A block of Perspex, 300 mm square by 25 mm thick, was purchased, and from this, two disks were cut, each 150 mm in diameter. The disks were then machined true in a lathe, and one disk given a convexity, with a radius of curvature of 457 mm for a focal length of 932.7 mm. (The refractive index of Perspex is 1.49.) The other disk was then machined to a concave radius of 457 mm, to fit the first disk. The two disks now comprised a lens and mirror combination, which could be ground together in the classical method used in normal mirror making.

With this first attempt, all the normal procedures were used in grinding; that is, the first grade of carborundum was 80, followed by grades 150, 220, 320, 400, 500, and finishing up with grade 600. Grinding was found to be much slower than for glass, Perspex seemingly having a much higher abrasion resistance. Work was carried out on a grinding machine, which emulated the mirror-maker's three motions, and several hours were required on the grade 80 carborundum to bring the two curves into approximate contact. All other grades were given two hours each, and to make sure, the grade 600 grinding was carried on for four hours.

The back surface of the lens was to remain flat, but the original moulded surface was not flat enough to leave as it was. At first, an attempt to fine grind the flat surface with grade 600 carborundum was carried out, but the polished surface would not frost over, and the grinding action would not begin. Finally, the flat surface was re-ground, starting off with grade 80 carborundum, followed by the normal sequence. Polishing the Perspex posed a problem for a while, as either cloth or felt polishers would be too coarse for precision working. Pitch polishers are no good, since the pitch welds instantly to the Perspex, even when wet, so that a new polishing lap material had to be invented. After a certain amount of experimentation, the author discovered that Plasticine was nearly the perfect answer, for this material does not stick to the Perspex and, on trial, a polish was achieved on the Perspex in a satisfyingly short period, the polish being even, and free from scratches and sleeks.

Full polish on all surfaces was carried out on the machine; several hours being required for each surface. Ordinary polishing rouge and soapy water was used. Examination of the polished surfaces soon revealed one shortcoming of the grinding method—the embedding of grade 80 grains into the surface of the Perspex. These had been knocked out during subsequent grinding operations, but huge pits had been left, nearly as deep as the full diameter of the grade 80 grains. The answer to this was to start the grinding operations with a finer grade, and this was done, using grade 120 on the second try, the embedding problem was thus cured.

Figuring of the lens only was carried out, as it was necessary to test the optical transparency of this medium. The lens was set up on an optical bench, and figured by collimation against an optical flat, using the knife edge test. A monochromatic light source was used to illuminate the pinhole (Foucault's test being used).
Figuring Perspex is tricky, because the material tends to take on a roughness which can be seen under test, and is it difficult to get the smoothness of figure achieved with glass. It is obvious that this medium (Perspex) has to be mastered by much practice, and development of personal techniques.

The lens was finally tested by setting it up in a frame, and observing the image quality. Sodium vapour lamps at varying distances along a nearby motorway proved useful for this purpose, as they provided a monochromatic light source, and thus gave sharp images. No significant defects showed up in the transmission properties of the Perspex when various objects were examined under different powers; it seems, then, that Perspex has very good light transmission properties, there being no evident changes of refractive index, or areas of bad transmission that one would get, for instance, with plate glass. The medium was also tested visually for micro-ripple or granulation, but none could be seen. The mirror was not completely polished, and work was discontinued due to the embedding pits left over from the grade 80 carborundum.

A second set of optics were finally made in conjunction with another optical project; these comprised a lens of 1.143 m focal length and 150 mm aperture, and a mirror of 355 mm focal length, having an aperture of 127 mm; the focal ratio being f/2.8.

The mirror was ground and polished as before, but starting with grade 120 carborundum this time; no embedding pits occurred, and a beautiful polish was obtained. The mirror had been perforated with a central hole 25 mm in diameter, and a Perspex plug cemented in with paraffin wax. The mirror was figured to a paraboloid using null test techniques, and a figure of around 1/8-wave was achieved with no more difficulty than with glass, using Plasticine polishers.

A modification to the Plasticine lap evolved during the making of this mirror. The hot weather, during which the mirror was being made, caused the Plasticine to become soft to the point where the lap surface would disintegrate, but this trouble was cured by pressing a single sheet of kitchen paper towel on to the surface, and using this as a surface reinforcement. The paper has good wear resistance, and will last for a long time before it has to be replaced. However, a rougher polishing action is obtained with paper, and the pure Plasticine surface must be resorted to for final figuring.

After the mirror was figured, the plug was removed, and the hole carefully bored out in the lathe to a slightly larger diameter (for reasons beyond the scope of this paper); the mirror was then immediately returned to the test stand. The lip of the hole had taken on the appearance of a volcano, local heating causing a severe turned up edge, but this effect vanished within a few minutes, leaving no change in the overall figure. The mirror was then silvered, using the weak silvering method [Journal, 75 (4), 277 (1965)]. Perspex silvers readily, and the only precautions that must be taken are that no destructive solvents, such as nitric acid or trichlorethylene, are used to degrease the surface. It was found that after a good wash in detergent solution (washing up liquid), a good clean with industrial methylated spirit was quite sufficient treatment prior to applying the silvering solution.
The mirror was tested out, after mounting in a test rig, and using it as a telescope; the images proved to be quite good, and proved quite satisfactorily that mirrors can be made from Perspex.

The lens was ground and polished on both surfaces in the same way as the mirror. Again, there were no embedding pits from the grade 120 stage. Nevertheless, a more stringent purpose for the lens meant that a higher degree of accuracy in the figuring would be required, the lens being integrated into a compound optical system where the dispersion of the simple lens would be corrected out by secondary elements in the train. The residual aberrations were corrected by local figuring of the plane surface of the lens, and as white light was used in the testing stage, the lens was thus put through a more severe testing and evaluation process than before. Again, the Perspex medium proved to have good trans-mission properties, free from changes of refractive index, striations, or any other aberrations that can be found in inferior glass.

The figuring proved about as difficult as for glass, and the polishing away of zonal defects took about the same amount of time, despite the softer texture of Perspex. Under the knife edge test, the surfaces of the lens appeared rough with circular polishing zones, and it was very difficult to get the smoothness of figure that one may achieve with glass; a great deal more work and practice with this medium will be required to find just the right technique. However, Perspex takes a brilliant and almost water clear polish, far more so than can be achieved on glass. And, surprisingly, this surface is quite resistant to abrasion and indifferent handling, although it is much softer than glass.

One may say in conclusion that Perspex seems to have great prospects as an alternative to glass for the manufacture of lenses in particular, thus opening up a new pathway in telescope making. Also the medium looks good as a mirror material, and with the non-availability of large pieces of glass, the amateur might find Perspex a good and cheap material from which to make a large mirror, say in the 600 mm aperture range. The more adventurous telescope maker may find the more exciting field of refractor design open to him, if he cares to experiment with a medium that has the clarity and beauty of the best optical glass. One final note: Perspex can be obtained in thicknesses up to 127 mm, and regular thickness of slab Perspex can be bought over the counter with a piece 25 mm x 300 mm square costing around 0; this is much cheaper than a slab of optical crown this size.


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