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Bifocals for Astronomers
Old age is catching up with me. Just like everyone who is this side of 40, I find that my eyes no longer focus over their full range. I have to wear different glasses to read than to see distant objects.
By Michael Covington - 11/25/2004
Most of the time, I wear "gradient" or "progressive" bifocals, which focus at a range of different distances depending on whether I look high or low. But these are no good for astronomy. They rely on my pupils being small, so they only work in bright light. Out under the stars, nothing is perfectly sharp at any distance!
Conventional 2-part bifocals are much better, but they don't mate well with telescope eyepieces. The bottom part of the field is blurred or doubled.
For me, going without glasses isn't an option. Nor is wearing glasses that are corrected for distance vision only. That's what I do when driving, and the stars look beautiful, but I can't read the display on the control box of my LX200 telescope.
The solution? Astro-bifocals (see picture).
I had a special pair of bifocals made with the near-vision segment placed extra low, just for viewing the control box or a small star map. By putting it 12 mm below my pupils, I've secured enough room to get a good view through eyepieces with apparent fields as large as 66 degrees.
Special lenses like this cost extra, but they're worth it.
Why skimp on the optical instrument that you use all the time?
They're not just ordinary bifocal lenses set lower in the frame; they're specially ground to put the optical centers of both segments in the right place.
Any good optician should understand the specifications and be able to fulfill them, since similar glasses are often used by golfers and other sportsmen.
-Michael A. Covington