NavList:

On February 18 I finally had an opportunity to make series of sextant observations. Sky was clear almost all the day (it is very rare case here in my region, because the sky is typically overcast for weeks or even months in winter). So I was happy for the chance to make some lunar observations. Lunar distance from the Sun was about 112°. I made 6 lunar sets (each containig 3-5 individual observations). For 5 sets I used SNO-T sextant with 7x power inverting telescope. One set I made with 3.5x power Galilean scope (after checking the results I found that the final error with 3.5x scope differed only 0.1’ from the error made with 7x scope).

Besides I took an opportunity not only to check telescope’s colimation error, but also to measure the error arrising from offset of the Sun/Moon contact point from the center of the field of view. I wanted to compare real observation values with calculated ones.  

For telescope experiments I used 7x inverting scope with crosswires.  So, I checked telescope’s parallelism to the plane of the sextant. It was slightly off, and I had to adjust it a little bit. After that I made the Sun/Moon images to be in contact on one of the vertical wire. Then I very quickly took a look at the micrometer and memorised reading; and as fast as I could I moved sextant so that the Sun and the Moon were in the center of field of view. The two images were overlaping significantly. I turned micrometer untill they were just kissing and wrote down the micrometer reading. The difference between two recorded readings (mean value of 10 individual observations) was 1.2’.

The telescope’s wires in this instrument are arranged in 1° 50’ distances. So the left and right wires are situated 55’ from the center of the field of view. If we calculate the theoretical value for 112° sextant reading, it will be 1.3’ (this means that my observation error was only 0.1’).

I mentioned that I tried to do theses measurements as fast as could. It is important because the lunar distance is changing enough fast. Roughly—about 0.5’  in one minute of time. Any delay is inappropriate here.

I decided to draw field of view of my sextant’s inverting telescope scaled correctly, to show how the Sun/Moon images look if they are in different offset distances from the center (see attached pdf file) and showed corresponding errors. I added also pictures of the field of view without crosswires. For each offset value I added a small table with an error values for sextant readings at 30°, 60°, 90°, 110°, 120°, 130°. (I included also 130° because some sextants allow readings more than 120°.)

Conclusions:

1) If the offset from the center is within a range of +/- 20’, the error will be no more than 0.2’ for sextant angles up to 120°.

2) If the offset from the center is about 1°, the error will be significant even for sextant angle 60° (error 0.5’).

3) If the offset from the center is more than 1.5°, the error will be very significant even for low sextant angles (not fractions of minutes of arc, but minutes of arc).

4) If accuracy is an issue, sextant’s telescope should be allways well adjusted and contact of two images should be made as close to the central zone of the field of view as possible (vertical offset is not important usually). If telescope’s mounting is stacionary and it is not parallel to the plane of sextant, one can prepare a simple correction table and use it in cases when maximum accuracy is required.   

 I know these accuarcy issues are not of great interest, but maybe some lunar enthusiasts will find this information usefull.  

 Modris Fersters