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Gary adds:

When reassembling the A-10 the easiest way to align the index prism is
to use the sextant like a normal marine sextant. Set the counter to
zero. Install the index prism and the fixed prism but don't tighten up
the screw holding the index prism shaft, just hold the prism in the
frame with your finger. Don't install the plates that close the inside
of the the A-10 frame as they limit your ability to push the index
prism
out of engagement with the worm.
Go outside and sight on a clear object in the distance, the farther
the
better, by looking around the left edge of the fixed prism. Then move
the index prism up or down by moving it slightly to the left to
disengage it from the worm so that it can be moved up or down. Let it
drop into the worm when you have the same object lined up in the prism
with the object as viewed around the lefthand edge of the fixed prism,
then tighten up the screw to hold the index shaft in.

When reassembling the index prism into the frame put grease not oil on
the tapered shaft because the tapered shaft applies a great deal of
pressure on the mating surface and oil is not strong enough to supply
enough lubrication. If you use oil you will end up with a horrendous
amount of backlash, say about a 30' difference when you bring a star
up
or down..

The trickiest thing about installing the index prism is installing the
spring that goes around the index prism pivot to apply a constant
pressure on the gear teeth against the worm to minimize backlash. You
need a small forceps that lock to grasp the end of the spring and
manoeuvre the end of the spring into the underside of the index prism,
this is a frustrating exercise due to the very limited room for
working
but necessary. You might be a blt to use a piece of fishing line for
this but I haven't tried it.

gl

On Nov 27, 7:08�pm, bruce hamilton  wrote:
> All times GMT 27 Nov 2008
>
> A-10 Sextant with 31.5' index error off the arc (horizon reading is lees
> than zero in case if am mixed up)
> Sextant has no other calibration. I frequently get LOP's within 2 miles
> of my AP so I am happy. It has backlash so I try not to change
> directions. �I suspect some other error as stars with higher altitudes
> tend to give me bigger errors.
> I am actually quite surprised that I am getting a fix that is close
> enough to find the airport (or Bermuda) if I had do. I have an A-12 and
> Mark IX, but I only drag out one an evening.
>
> Thanks
>
> I am presently using CelestNav 3.0 for the palm pilot to work out the
> sites. I would appreciate any comments on the program. I like it for
> everything but the fix calculation, but I think 3.0 is only a Beta.
>
> AP 49d �16.1 N �123d 7.1 W
>
> Pollux � 06:19:33 � 27d �59.3'
> Vega � �06:16:31 � 13d � �0.3'
> Betelg � 06:10:53 � 28d �44.3'
> Altair � �03:56:11 � 22d �37.0'
> Vega � �03:54:45 � 32d �06.9
> Capella 03:52:46 � 39d �35.7'
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