Preliminary Things before starting a Refractor.

I strongly recommend that any would-be lens maker should carefully study chapters 3, 4 & 6 of "Telescope Optics, Evaluation and Design" by H. Rutten and M. van Venroiij (Willmann-Bell, 1988) before proceeding to the actual construction of an achromat. If you haven't yet bought the book, I would also recommend that you get the software that comes with the book to run through the designs that you may want to do to get the best radii for the glasses that you have obtained. It is very important that you should have a clear grasp of what you are undertaking and how a basic telescope lens works, because lenses are more complicated and perform more functions than paraboloidal mirrors. And one of the most important things is to grasp the nature of chromatic aberration and how a lens design seeks to control it. Mirrors do not suffer from this defect, but all refractive lenses do this in varying degrees. If your lens is to succeed, it must correctly control the chromatic aberration as well as other aberrations.

Another highly useful book is "The Adjustment and Testing of Telescope Objectives," by H. Dennis Taylor, first published in 1891 and recently reissued by Willmann-Bell Publishers. This short treatise is packed with practical information for anyone who wants to test a telescope, especially an achromatic refractor. It too should be studied carefully by makers of achromats. You can save yourself much confusion and frustration if you gain a clear grasp of what your lens should show and how it should and does work, when it is correctly constructed. I will assume that you have read at least the first of these books and are familiar with aberrations and the basic makeup of an achromat. I will also assume that you can get a copy of Texerau and are familiar with it.

One reason why ATMs shy away from building refractors is that generally some different special equipment is needed than what is needed to make a Newtonian reflector. The best method for making lenses involves the use of 4 tools which mirror makers don't need:

  • a good spherometer capable of reading to 0.0001" or to 0.001mm;
  • a "wedge" tester reading to at least 0.001" or 0.025mm;
  • an autcollimation flat, used to simplify figuring the lens;
  • a narrow passband dielectric filter used to isolate the e- or d- line of the spectrum during figuring.
  • A knife edge or Ronchi tester is generally also needed, but mirror makers usually have one of these already. I will assume that you have access to these tools, although only the wedge tester is an absolute necessity and can easily be improvised at low cost. Resourceful ATMs can find ways to overcome the lack of the other tools, and toward the end of this webpage, I'll suggest alternatives to using them.

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