A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2016 Mar 28, 19:08 -0700
Mark, I think I know what you're talking about. When you swing the index shades down, and then you look at the lines of sight from the side, you discover that the shades are actually in the way, blocking maybe a quarter of the objective of the telescope. It seems like the design is deliberately stealing aperture from our sextants!
But consider: what is the value of large aperture on a sextant telescope? It's not magnification necessarily. Have you seen the very narrow 12x scopes that were common on old sextants? The aperature is typically about half to two-thirds of an inch -- really quite small. Large aperture scopes on sextants only became dominant after the Second World War. So what changed? What were navigators doing that benefited from a big lense?
For an experimental perspective, try making an aperture mask. Take a piece of circular black cardstock (or something like that) cut to match the aperture of your sextant scope. Then cut a small circle in that. Attach it to your telescope and see how things have changed. Also try making a couple of masks with different size holes and different locations for the holes. For example, if the hole in the mask is well off-center, does that matter?
Although it just seems so wrong to design the instrument so that the shades block a fraction of the aperture, it turns out it doesn't change much. At night, when you might want the maximum value from all that aperture, you don't use the shades. In daylight, stopping down the aperture is essentially harmless.