NavList:

Before you take-off you'll have to calibrate the refraction error of the front window of the aircraft on the ground (unless you're going-up in a WWII bomber or transport w/an astrodome). These were calibreted at the factory and a table of Perspex (plexiglass) calibrations can be found in older Air Almanacs. One way to do this is to shoot Polaris from the airplane while it is on the ground and taking into account your other errors, compare your calculated Latitude reading w/ the posted airfield Latitude.

All this is unnecessary if you use a periscopic sextant.
(BTW I have one w/mount in fully functioning condition for sale).
Email me at jujutsu@hotmail.com)
Robert Swartz
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