[ATM] Every Achromat Ever
Paul A. Valleli
valleli at rcn.com
Mon Sep 27 12:24:52 JST 2010
There have been a series of professional papers in Applied Optics, published by OSA. Some go back 30 years or so and show the relationship between number of color crossings at the optical axis and glass types by doublets or triplets.
If you delete triplets to achieve an apochromat, it greatly reduces the field of choices.
As you know, color increases as F/No. decreases. Without the cost factor, we see apo's down to F/5.6 but many have six or more elements. More commonly, we see limits at F/8 or F/10.
This is commonly done with BK7 and Fluorite. Single crystal fluorite cracks very easily and breaks under thermal stress.
Synthetic, polycrystalline material is far more durable and therefore, useful. The Hoya FKao3 and related fluorite glasses are expensive and drop out of your concern.
The Stellafane Refractor Interest Group studied this area of concern a few years ago and came to the conclusion that a color-free refractor had to be F/15 to use cheap glasses.
The first choice was Schott K5 and SF1. Next was BK7 and F2.
Following that was K5 and F2. Finally, BK7 and KZFS4.
Each pair has to be tweaked, depending on how much Spherical, Coma, or Astigmatism can be tolerated, and over how much field of view.
Usually, an ED glass will be chosen if it is very important to lower the F/No. That is a simple cost vs. FOV tradeoff.
Chinese optical designers made that trade to come up with the 150mm F/8 lens that is available for several hundred dollars from Celestron, Orion, Meade, Galileo, Vixen, etc. Well trained opticians, working at a low labor rate makes the low cost possible. Experienced users will know that there is a small residual magenta flare that can be eliminated with a Minus-Violet filter.
Buy a reflector if you can't tolerate color !
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