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Robin,

The difference of 0.9' between spotting scope zero and x7 scope zero is high ? Reading the vernier off the arc incorrectly perhaps ? Try a straight visual of the micrometer drum when off the arc as a check. Read from 30' the additional minutes with an estimate of tenths of a minute by eye on the micrometer drum (no vernier). Both Sun images should be well shaded and of equal brightness. Remember to keep the Sun focused sharp. Pressing the eye to the occular repeatedly can change focus. I pencil mark the sharp focus setting. Be sure to take fingers off the micrometer before calling the mark. Some micrometer drums move when the fingers are released.

Greg Rudzinski

From: Robin Stuart
Date: 2015 Dec 22, 12:14 -0800

Thanks to all for the feedback. The measurements I got using the solar method on 20 December after setting the I.E. to zero using the spotting scope technique were

On Arc  Off Arc

32.0’     26.8’

32.6’     26.2’

32.0’     26.2’

31.8’     26.6’

31.8’     26.2’

31.8’     26.2’

32.0’     26.2’

31.8’     26.0’

31.8’     26.0’

31.8’     26.2’

These values average to

On Arc  Off Arc

31.9’     26.3’

leading to an I.C. of +0.9’ and a measured solar diameter of 32.8’ compared to the actual value of 32.5’. The standard deviation of the on and off arc values are each 0.25’. This is a bit bigger than the 0.14’ that Mark suggests should be expected.

After the measurements were made the zeroing was again checked with the spotting scope technique and found to not have changed. I did not try moving off zero and back as described by Gary to check reproducibility in the spotting scope.

The sextant is an Astra Pro, which has a bronze arc, with a 7x scope.

The measurements were performed alternately on and off the arc but I did not take care to approach contact from the same direction each time so a backlash errors may be present. The Astra has its Vernier scale marked in 0.2’ intervals and I made no effort to estimate readings to greater accuracy. Arc error in the sextant is unlikely have an effect as everything is being measured so close to zero.

I must say I was disappointed with these results when I got them and would be interested in hearing how they compare with others. There are a couple of obvious outliers but removing them doesn’t help. Given the relative sizes of the standard deviation and it does seem that a systematic bias has been introduced. A prismatic effect in the filters would do it but I would be surprised if it were that large.

 More tinkering is in order but it looks like it will be a few days before we see the Sun again here.

 Robin Stuart

Valhalla, NY