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Greg, you wrote:
"An unorthodox method that I use for adjusting and determining index error is 
to tweak the side error adjustment just enough to split a star into two 
images with the sextant set at zero (with a bright star I even use a weak 
shade to present a point of light instead of an asterisk of light). With the 
two stars side by side I seem to do a better job of zeroing my index error."

Bill B., if I remember correctly, suggested using fainter stars for this, 
which strikes me as equivalent to your use of shades to eliminate the flares 
in the image. I've tried this, and I agree that it's effective. And yes, I've 
also found, and others seem to agree, that leaving a little so-called "side 
error" is helpful with these sorts of observations. Just as you say, it's 
easier to align the images side by side than to superimpose them.

And you wrote:
"A fussy navigator would probably tweak the side error back but I leave mine 
off so that I can periodically recheck index error. Question: What kind of 
errors get generated on high altitude observations if side error is a few 
minutes out?"

Yeah, you're right about the 'fussy navigator' here, and I think this is 
mostly because so many navigation manuals insist on it. They emphasize that 
one should eliminate "side error" with the same insistence as the other 
adjustable errors. But this is really over-kill. If the side-by-side gap as 
seen through the sextant is k minutes of arc, and the observed angle is d, 
then the error in minutes of arc in any angular measurement is given by
  error=(1/6876)*k^2/tan(d).
So suppose k is 5' which is probably more than you need for the trick you 
describe above. Then if d is 30', the error is about 0.4 minutes of arc. 
That's the error you would find in measuring the diameter of the Sun or Moon 
using the sextant with that large side-by-side gap. But suppose that d is ten 
times bigger, 5 degrees. Then the error would be just about ten times 
smaller, only 0.04', negligible, and dropping roughly in inverse proportion 
to the measured angle. So there are really no practical observations affected 
by "side error" at all. When this has come up before, I think I said that 
checking IC by the Sun's diameter on and off the arc would be influenced by 
this error, but it's not true. The error cancels out. So don't fuss over that 
side gap. It doesn't matter.

-FER
PS: The error formula above if everyhing is done in pure angles (in "radians") 
would be error = (1/2)*k^2/tan(d). Dividing by 3438 turns one factor of k 
into a pure angle and leaves the other in minutes of arc so the result is in 
minutes of arc. 



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