A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Ed Popko
Date: 2019 Oct 22, 12:02 -0700
I have worn glasses all my adult life, mostly to correct mild astigmatisms in both eyes (my corneas not perfectly spherical). I dislike wearing them when using my sextant. The tradeoff for not wearing glasses is a slight bloom to stars, planets and the moon. I have gotten decent results in the past because I average a bunch of shots, but now I want to do better, especially with lunars.
I asked my Optometrist if I could take an old pair of glasses, remove the right lens (the eye I favor when using a sextant) and somehow adapt it to my sextant’s scope. He mentioned I could wear contacts, but I’m not a fan. Yes, you could take an old pair of glasses apart but why not just order a lens to your latest prescription and grind the edge down to fit my sextant’s eye cup. That way the prescription is up to date and you will know the correction’s rotation axis.
Attached are a series of pix showing the whole process. I’ll omit the eye exam, I’m sure you have been-there-done-that. The pixs are numbered in the sequence of the whole process. Pix names explain the content.
Viewing through the retrofitted scope is much improved. The change is not subtle. Stars are almost perfect pinpoints and the blooming is gone. The edges of the sun and moon are nice and sharp too.
There is a subtle point in mounting the corrective lens to the scope. The normal focus adjustment of any scope assumes you have perfect vision. Yes, you can get the image reasonable sharp even with an uncorrected astigmatism (unless it is really bad), but focus adjustments will not eliminate the negative effects of the astigmatism simply because your cornea isn’t perfectly spherical. So, I had to play with the scope’s infinity focus and the axis of the corrective lens to get a super sharp image. Once I had these two coordinated, I fixed their position by taping the eyecup and focus ring to the scope’s body. Neither can change position, the result - corrected infinity focus!