Buy a sextant

[Douglas Denny]

Douglas,

Now that's a handsome photo!  Is that a magnifying glass near the index clamp?  What is the index vernier scale graduated down to?  .2   .4   ,6   .8?  Or are there individual lines for each of the tenths?

Clen

Silly Me !  That's not my Freiberger in the photo;  that's the SNO-T. . . I had forgotten I took the SNO-T to try out whilst on the sailing trip around Ireland.

The Freiberger and SNO-T look almost the same - frame shape and colour, micrometer drum etc, but you are correct, that is indeed a magnifying lens you see on the photo with the SNO-T.  The Carl Zeiss Frieberger has a scale lighting attachment on the index arm near the micrometer drum.

Both have the same micrometer drum markings:  sixty,  one minute of arc divisions on the micrometer drum.

The one minute divisions are very near 2mm apart so it is easily possible to estimate to within 0.2 minute of arc and when practiced it is possible to be reasonably sure of 0.1 minute of arc estimated.

I modified the SNO-T when using it for Lunar distance measurements by sticking a very small piece of clear sheet plastic onto the datum marking next to the drum with vernier markings which I had printed onto the clear plastic.
This gives reliable vernier measurement of 0.1 moa from the micrometer drum.  (See pic).

The Freiberger sextant telescope proved to have considerable prismatic errors inherent in the field of view from one side to the other which gave me confusing results for Lunars.  Hence it is important using all sextants to keep measurements viewed as near as possible in the centre of the field of view.

The SNO-T proved to have less prismatic error and more reliable results in use.  Both however, are very good sextants.

I include a picture of my Box sextant which is a beautifully made precision instrument, which though only about 75mm in diameter is capable of reliable measurements within one minute of arc.

The three pics are: 1)   SNO-T micrometer drum with stuck-on vernier scale I attached
                             2)   Carl Zeiss Jena Freiberger survey sextant (which allows measurements over 200 degrees)
                             3)   Troughten and Simms  Box sextant.

Douglas Denny.