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Frank,
Do you agree qith my other conclusion from the
"erroe-due-to-colimation" formula
err=-i^2 sin(1')tan(A/2)
I mean that this formula shows that it is impossible to
check the index error for back sights with a quadrant,
at least in sea.

(The method described by George and involving measuring some
fixed angle close to 90 firat by fore sight and then by back
sight should probably work, but does not seem to be of much
practical value in sea.)

On the other hand, this argument does not apply to the
periscope device which George made.
(Contrary toi what I said in my first message on this
topic). Because with this
device,
the sextant index error can be found in the usual
way. The only question is how one adjusts the mirrors
or the prisms of the telescope itself.
They need two checks if I understand correctly:
a) that the angle between the mirrors is exactly 90 degrees,
and
b) that both poeriscope mirrors are perpendicular to the frame.
But this is the question to George, how he does this.

Alex.

   

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