Copyright – P. J.
Smith
Goto Sources of information on small lens
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problems and techniques.
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'solution'.
Goto Selection of window glass.
Small lens making has not been a mainstream issue for ATM'S. There have, however, been articles appearing every so often in the usual sources and these contain a wealth of information. While the ATM series 1, 2, and 3 may not be entirely up to date with modern methods of mirror making, they are as good a source of information on making small lenses for the amateur that I know. Other sources concentrate on commercial production. This is not so relevant to amateurs without diamond generating equipment. These other books also tend to be very expensive. ATM 1, 2, and 3 have been reprinted but the material has been rearranged so consider it very relevant which edition you have. The material referred to below will be there somewhere but it may not be in the book number referred to. Since this is probably more accessible many, I would suggest all of this material be obtained and you make photocopies as working material in your workshop.
Some sources of information are listed. The best are indicated with more stars. I have related the number of stars as more relevant to small lens production and eyepiece design than the quality of the book overall.
* Procedures in Experimental Physics - John Strong. Reprinted by Lindsay Publications. Some basic information on lens production. Note that the otherwise excellent illustrations of Ronchigrams are transposed and are thus incorrect.
* Optics and Optical Instruments - B. K. Johnson Introduction to optical theory and instruments. A nice blend of theory and practice. A small, older paperback often found going cheap.
* Lenswork for Amateurs - Orford. Quaint, dated, not entirely accurate book on small lenses. If you see a copy have a look, but don’t go out of your way to get this one. It is more of a collector’s item
** Amateur Telescope Making Book 1 - Ingalls. Contains some
very short articles on making small lenses for eyepieces. Worth reading.
** Amateur Telescope Making Book 2 - Ingalls. Contains some
very short articles on making small lenses for eyepieces. Worth reading.
** Advanced Telescope Making Techniques Vol 2 - Allan Mackintosh. Some
information on making miniature lenses.
****Amateur Telescope Making Book 3 - Ingalls. This contains a brilliant concise description of small lens production by Ferson and Lenart. Written for the very advanced amateur, its methods use basic equipment and are probably more relevant to amateurs today than professionals. If you can only get one reference on small lens production, this is probably the best. Also sound information on eyepiece design by Selby with many prescriptions. It is best to rework the prescriptions with a modern raytracing program.
**** Prism and Lens Making – Twyman. Much detailed information on making small lenses. Being an older book, the methods are more relevant to amateurs. This was for years the bible for optical shops and contains much of the shop wisdom passed down to opticians working for Adam Hilger.
**** Advanced Optical Techniques - Van Heel. Contains an excellent condensed chapter on lens production. Worth obtaining a photocopy of the relevant chapter. The rest of the book is fascinating but of no relevance to eyepiece production.
** Optical Production and Technology – Horne. Some information on making small lenses. Being more modern, this is less relevant to amateurs but it is a well presented, excellent book. Anyone interested in optics manufacture will find it worth reading.
** Optical Workshop Principles – Deve. Basic information about small lens production. Quite dated but interesting. The fact that it is rather old fashioned will make it more relevant to amateurs.
* Master Optical Techniques – De Vany. Not much on small lenses but a lot on prism making.
I simply do not have the time or else I would collect together techniques and write a book on this topic. A comprehensive volume would be quite substantial. Maybe one day!
Many of these books are old. Some are rare, others were once very common. I would suggest doing a search on the web because some may be available quite cheap. One suggested source of information on second hand books is http://www.bookfinder.com .
Never forget that the best teacher
is experience. Small lenses are quick
to make (polish in a few minutes) and do not
involve much expensive materials,
so the best thing is to start making mistakes.
Plastics have been used for lenses for a long time. The results can be excellent. Despite this, the use of optical glass for top quality products has not been seriously challenged.
This is due to various problems such as
Reduced scratch and solvent resistance.
Large temperature effects on optical properties.
Large coefficient of expansion with temperature.
The difficulty of optically bonding to glass.
The ease of obtaining superior polish on glass.
If optical glass is unobtainable then the use of plastics is a serious option and can give excellent results but it is not my preferred substance. Glass is a much nicer material to work accurately.
I find the most serious problem with plastics is the difficulty of bonding to glass. After trying various possible solutions, some more high-tech than others, I have concluded that the most practical is old fashioned Canada Balsam solution. Because of the very different expansion of plastic and glass the bond fails unless temperature variations are reduced to a minimum. Some have lasted 3 years quite satisfactorily if kept away from temperature extremes.
I hope that modern technology can reliably solve this problem because polycarbonate is actually quite attractive from an optical designers point of view.
Shaping plastic.
A radius turning attachment (easily made) is very effective on a lathe for turning the concaves on the polycarbonate. With care a close approximation to a sphere which only needs a little finish grinding can be achieved. The setup for a specific radius takes the longest so turning a few at once makes sense.
Fine grinding is not very satisfactory in a metal lap because the plastic is softer than the steel so the abrasive embeds in the plastic rather than the steel lap. What would be more satisfactory is to use bonded abrasives. It is possible to have small pieces of very fine wet and dry abrasive paper held by pitch to a lap. Pressing forces it to take the correct shape.
Polishing can be done the same way with a thin cloth pitched to the lap and pressed. Experimenting with polishing agents is worthwhile as some plastic polishing substances work quite well. The usual polishing with Cerium Oxide on pitch does not seem to work as well as on glass.
It is difficult to achieve a perfect polish. Luckily Canada Balsam used to optically glue the plastic to glass minimises this problem.
Centering - a quick and dirty
method.
One of the most important processes is centring and accurate mounting of the elements. This is essential and may put off the enthusiastic amateur.
There is a method of 'centring' small lenses that requires no precision edging. As would be expected, the method is not quite as good as the traditional precision method but it has the advantage of being more accurate as the depth of curve increases.
If two perfectly aligned, perfectly squared tubes are brought together and the assembled lens is lightly clamped between them, the lens elements should take up their 'best' aligned position.
This can be arranged between the headstock and tailstock of a small lathe. Or a small precision bored jig can be made which accepts two tubes. A small weight on the top one will tend to push the lenses into alignment. Or small tubelike fittings can be slipped over a micrometer spindle and anvil.
Some wiggling of the lenses will assist the process of finding the best centred condition..
If an optical cement has been applied first it will act as a lubricant and can be allowed to harden overnight while still clamped. Or UV zapped. Care must be taken not to overtighten if done in a lathe or micrometer.
The interesting thing about this method of centring is that the three elements in a triplet can be aligned and bonded in one setting. It does not matter that the edges of the lenses may not line up - as long as the surfaces are truly aligned.
Finally, if the mounting system in the eyepiece holds the lenses by the outer curved surface and no contact occurs on the edges, the assembled eyepiece lenses should stay aligned.
For those who would discount this technique as too crude, millions of commercial lenses are automatically centred and edged by a similar method. For success, the curves are better if not too shallow. The method must be applied with precision equipment.
An ATM who understands the principle involved can make a success of this method and finish with a satisfactory eyepiece without any traditional centring and edging at all.
While the term 'window' glass has been used freely there may be other good sources of satisfactory common 'crown' glass.
The most obvious property is colour. Viewed edge on, window glass usually has a green - blue tint from impurities. It is tempting to base the selection on this very visible criterion. In reality, the homogeneity of the glass is far more important and one should never assume that water white glass is better. In thin lenses the problems of colour are not as objectionable as one would expect. When seen from the edge, the green colour is a result of light being trapped between plane parallel surfaces over a considerable distance so the edge colouration is accentuated.
Refractive index and dispersion vary with the source of glass. Slight departures in these properties are not as harmful as one would think. The calculated efl. changes but actual performance is not too bad.
Flatness on window glass will vary. A very quick but surprisingly sensitive test is to view very distant streetlights or stars reflected in the surface at a grazing angle of 45 degrees or less. The test can be made much better by using binoculars to view the reflected image from the glass surface. If the image remains round when slightly defocussed both inside and outside of the focus, the surface is flat enough for application in a small diameter lens.
Homogeneity is a problem especially with steeper curves. Viewing through a plane polished sheet usually hides these errors if they are in layers parallel to the surface although viewing distant objects using binoculars through the sheet is more sensitive.
Examination of lenses after they are shaped is a far better test but somewhat frustrating.
Fine grinding the edges and temporarily fixing on a thin piece of glass with salad oil enables edge examination. Immersing unpolished blocks in a fluid such as kerosene is useful although a liquid of refractive index of 1.52 allows better examination. A detailed discussion appears in one of the ATM books.
Working window glass into round blanks is not difficult by scribing, snapping, and nibbling with tile cutting devices found at hardware stores. (Eye protection !!!)
Finally, a source of good glass is old Tank prisms. These seem to turn up in out of the way places. A lot of tanks have been transformed into junk one way or the other. Tank prisms may often be found as scrap if their silvering degrades badly. The optical properties are usually close to K5 crown glass. Unfortunately diamond sawing facilities will be needed unless you know a lapidary enthusiast. The kind found on construction sites for sawing bricks, concrete etc. usually chip glass too much but can always be tried.
Actually, thin diamond saw blades are very cheap. They are more gentle and waste less glass. Some very thin 4 inch Asian blades cost about $10 each and are quite satisfactory. It is easy to construct a very useful saw quite cheaply which will happily handle many cubic yards of glass before wearing out. The main requirement is for a very true running blade and provision for a coolant / lubricant although water will do. If plain water is used the blade must be oiled afterwards to stop rusting. Cooking oil works. Diamond trepanning cutters are also available at Lapidary Suppliers which make production of round blanks from large blocks of glass a pleasure.
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