NavList:

Dustin,

Image 1. No sun filters shading the index mirror would suggest a high altitude moon observation.

               (US Navy MK 3 with split horizon mirror)

Image 2. No sun filters shading the index mirror with index arm at or near zero would indicate an index error check using the horizon.

               (Cassens & Plath with whole horizon mirror)

Image 3. Sun filters shading the index mirror with index arm near zero would indicate a low altitude observation of the Sun.

               (Astra lllB with split horizon mirror)

As long as the bridge glass is flat there shouldn't be a refraction problem. No worse than stacking filters.

Greg Rudzinski

From: Dustin Baenen
Date: 2017 Sep 15, 14:59 -0700

I notice the last two pictures posted show sextants being used from the interior of the ship shooting "sights" through glass windows.  I would think that the glass would create some kind of extra refraction error or perhaps just an abboration leading to reduced accuracy.  I have no evidence to support this hypothosis,  just seemd like "common sense".  I assume these navigators were demonstrating the use of the sextant (the second photo shows the gentleman taking a sight with the index arm right around zero) for the photographer.  Can someone please confirm this thought or set me straght?  Is it ok to sight through a glass window?  On my Davis AH there is two panes of glass that the light passes through before hitting the sextant but because they are at complimentary angles I assumed this cancled out any error the glass might produce.  Im sorry that this was slightly off topic to the thread but it made me wonder.