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In an example in the solstice thread I used an index error of 0.5' off the arc. Here is how I determined this value:

When I measure IE using the touching limb method I get results from near zero to 2' on the arc. The last measurement was 2' on.

A few days ago I took 13 PM sights. I calculated the intercept for each of them with AP=GPS. I set up a spreadsheet and adjusted IE to give a minimum average intercept.

The results are:

• IE=0           Mean   0.2 A   Std Dev  1.1
• IE=2' on     Mean   1.2 A   Std Dev  1.1
• IE=0.5' off   Mean    0'      Std Dev  1.1    (if a 3.3' intercept is dropped the  std dev becomes 0.5)

See the attached file.

Is this method valid? For example each intercept has a different azimuth so I am not sure if the intercepts can be averaged.

Can I define Personal Equation + IE = 0.5' off  and used this value in sight reductions? It gave a perfect result for the morning solstice observation  (-;