NavList:

Robert Swartz you asked:  For the life of me, I can't figure out why B&L put horizon viewing optics on an aircraft bubble sextant.  Any answers?

Robert
Airships, maritime reconnaissance aircraft, fleet shadowers, and the like operated at quite low altitudes.  Accurate dip tables were available up to similar heights.  Why bother with a bubble and its acceleration, Coriolis, and rhumb line steering errors if you’ve a natural horizon available.  The limit is normally when the natural horizon is so far away it’s no longer clearly visible.  I’ve also seen claims that the A8-A could also be used to measure lunar distances although with several aircrew watches onboard that seems an unnecessary extravagance. The RAF/Hughes alternative was to produce a small maritime sextant for use in flying boats, often advertised as a flying boat or lifeboat sextant.

Regarding calibration, bereft of collimator I find Frank’s GPS Anti-Spoof app very useful for checking index error although unless you know your stars and have 360 degree traverse you can’t calibrate thoughout the entire 0-90 Hs band that way.  With patience you can follow the Sun throughout the day to calibrate the most commonly needed Hughes 10-degree stop settings. DaveP