A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Greg Rudzinski
Date: 2017 Aug 6, 14:28 -0700
Better to do this experiment with the same sextant. First put in the fridge without scope. Right after it is removed from fridge measure index error with scope attached (condensation on mirrors may delay observations). Let sit in Sun without scope attached ( direct Sun on scope lens for long periods is bad for the lens coatings. A hole might get burned into something also.). Repeat index error measurement with scope attached to compare. There shouldn't be much difference on metal sextants. Plastic sextants (especially black sextants) will show differences in minutes of arc. Best to keep all sextants out of direct sunlight.
I noticed just now testing a plastic Davis MK 15 that the largest observed change occured in side error (at least 4'). Index error changed in both directions 2' depending on what side was exposed to the Sun. Don't expect to come up with some way to compensate for this expansion and contraction effect. I advise removing sextant from box ten minutes or so before checking index error / observations. Always shade from direct sunlight when not making observations.