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Does anyone know how to check perpendicularity
of the index glass on a pocket sextant?

I recently bought one on e-bay, and now playing with it.
(The result of my observations are in next message).
It is an old Troughton and Simms, with silver scale and veriner
reading to 1'. It really fits in a pocket, and has
a miniatiure 4x telescope (the smallest telescope I've ever seen:-)
The workmanship is superb, it is just a pleasure to handle.
It has 2 index filters, but unfortunately no horizon filter.
Originally it had an eyepiece filter, but it was lost.
(This makes it impossible to determine IC from Sun with the
usual method, as well as to use the Davis art horizon,
unless it is equipped with a dark glass).
It has the full set of adjustment screws,
but I don't understand how to check perpendicularity
of the horizon mirror (the usual method does not work
for the obvious reasons: the reflection of the arc cannot be seen
through the index mirror).

The horizon mirror was not originally parellel to the index mirror
(side error) but this I easily adjusted. The index error was 0
when I received it, and it is still 0 even after the adjustment.

The reason I bought it is its small size,
so I can easily travel with it in my pocket, and no problem
with storage on a small boat arises.
On the other hand, it was my hope that I can achieve a reasonable
precision (say comparable with a good plastic sextant),
sufficient for "practical purposes".
This toy is much smaller and lighter and more plesant to
handle than a plastic sextant. BTW, the price is comparable too.
Anyway, I
believe
the statements
of George that high precision cannot be achieved
(and is not needed) on a small boat
under average conditons anyway:-)

The results of the first test are in the next message.

Alex

   

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