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Isonomia wrote:
> Whilst it is easy to calibrate a sextant's zero reading, is there any
> way to calibrate any other readings?
>   
You should start with aligning the telescope with the frame:
Look through the horizon mirror through the telescope to the horizon (so 
by reversing the sextant) while you hold the sextant horizontal.  Now 
the horizon should show as a continuous line running exactly through the 
middle of the telescope and exactly aligned with the upper edge of the 
mirror (the silvered part that is).

Check if the index mirror is perpendicular to the frame. For this you 
have to look from 'above' the instrument to the divided arc and try to 
see the arc and its reflected image at the same time. When properly 
aligned both arcs should be aligned properly as in attached picture.

Then if the index mirror is perpendicular to the frame. The horizon 
mirror can be checked by looking at a distant object with the index arm 
set at exactly 0 degrees (which is checked at the next step and 
therefore is an iterative action). If you hold the sextant vertical and 
look at an object with distinct vertical lines you should see the direct 
and reflected image nicely on top of each other (properly aligned 
left-right). If not, the horizon mirror needs adjustment. You can of 
course perform this on the horizon as well by holding the sextant 
horizontally.

Then calibrate for index error as you already know. If any large errors 
are found and corrected for you should repeat the last two steps.

Nicolàs





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