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Mark,

The 7x35mm scope gains little using the whole horizon compared to the substantial gains in the field of view when using the 4x40mm scope or sight tube. I find the whole horizon paired with a sight tube great for Sun observations when going to weather in a small craft. The split horizon mirror paired with a 4x40mm scope is the better option for twilight observations. The main benefit of the 7x35mm scope is to better see the distant horizon from the bridge of a ship with a secondary benefit of being able to determine a more accurate index error.

If only one sextant is to be had then a 4x40mm scope with a split mirror covers the most bases. If there is room for two sextants then having one with a whole horizon mirror and the other with a split horizon mirror would be ideal.

Greg Rudzinski

From: Mark Coady
Date: 2015 Dec 26, 01:10 -0800

When I first learned on various sextants, I took to the split mirror design based on the advice of the time about low light/contrast level sights, etc.

When I got my Astra III it came with the standard 3.5 x 40 scope.  This presents the classic split view.  This summer, I purchased the 7 x 35 prism scope to assist with Lunars.

Results were excellent, but I found the switch back to a whole horizon view created by the optics disconcerting at first. I kind of liked the classic split mirror simplicity. I understand this visual change to a complete view to be the result of images superimposed within the scope by the prism arrangment and focal length..

Does the prism scope full view remove the benefit of the split mirror on low contrast marginal sights? 

I understand the light gathering ability of a telescope increases with the size of the aperture, but that percieved visual brightness is quartered by doubling magnification.  This would intially make me think the split still put less in the way of the light and it may be a help.

Did  I miss something, if we already have a whole horizon view with the prism scope....... and the whole mirror used, is there any benefit.