NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextant optics
From: Paul Hirose
Date: 2004 May 5, 13:50 -0700
From: Paul Hirose
Date: 2004 May 5, 13:50 -0700
George Huxtable wrote: > > 3. Only that part of the exit pencil that can pass through the pupil is > collected by the eye to form an image. Any excess light goes to waste. That's true. However, the wasted portion of the exit pupil gives your eye latitude to move off-center while remaining fully illuminated. That should be beneficial on a pitching deck. With an oversized exit pupil, I believe the portion of objective lens that's active depends on your eye's position. If it's exactly centered in the exit pupil, the center of the objective is being used. Say your pupil size is 5 mm and the scope is 4x40. Then the center 20 mm of the objective is active. If you move your eye off-center in the exit pupil, the active area of the objective moves too. This could be used to vary the relative illumination of the horizon and body. That's what intuition tells me, anyway. I own only bubble sextants, so I can't test the theory. There are times when an undersized exit pupil can help. Some astronomy buffs report sharper stars with 10x50 binoculars compared to 7x50. The smaller exit pupil avoids the outer area of the eye's lens, where aberrations tend to be worst. Also, as we age our maximum pupil opening tends to decrease. Many people in middle age can attain only 5 mm in darkness, though in their youth they could get 7 mm. Some years ago I measured my maximum pupil size. The procedure involves poking a pair of pinholes in a piece of metal foil and holding it to your eye while viewing a suitable target at night. It must be bright enough to be visible, but dim enough to keep your pupil wide open. By trial and error you find the widest pinhole spacing that fits your pupil. I think the test can also be done by determining the widest opaque strip you can see around on both sides. Same principle as the pinholes, really.