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Norm,

When doing limb to limb Sun index error measurements using sextant filters I find that at least two horizon filters are needed. When a follow-up observation of the Sun is made the horizon mirror shades are removed. As sky conditions change different index mirror filter combinations are used. These changes cause inconsistencies. The go around is to use a filter on the objective lens of the scope. As for keeping the body centered in the scope field of view well most sextant scopes do poorly (distortion and fuzz) when using edges of the optics. Field flattening would be a nice feature to have in a sextant scope.

Greg Rudzinski

From: Norm Goldblatt
Date: 2015 Dec 21, 22:36 -0800

Centering the scope shouldn't effect anything except that view through horizon mirror might be blocked somewhat on one side, only effecting the brightness of one image and possibly some aberrations, which should be small in a good scope.  Even the index error shouldn't be affected.  If not really bad, either lateral decentering or angular decentering should not affect the readings or index error.  As long as the filters aren't changed, wedge should be accounted for in the index error.

Someone correct me if I'm wrong. Might be some practical problems, but that's how I see it from the 'theoretical 'angle'.