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    Re: Astra IIIb: First Impressions
    From: Frank Reed
    Date: 2014 Mar 11, 19:05 -0700

    Sean, you wrote:
    "As for the index error, I was very careful to read the off arc number correctly, subtracting the indicated minutes value from 60."

    Well, so much for that theory! :) Since you're doing that part right, there may be something else going on. It certainly looks like you've got a net systematic error remaining, and the IC is the most likely culprit. Then again, the apparent systematic error in a set of only five observations could be random chance in a moderately noisy set of observations.

    And you added:
    "I have since attempted to adjust the index error out. I will be taking some more sights soon to see if that improves my results a little. "

    Of course, once you adjust, it changes everything. So you should at least expect a different systematic error. As for adjusting it out, as you know (I am sure), there's no strong reason to eliminate it. But many people do prefer to keep it under one minute of arc. Both for practical observations and also for teaching purposes, it's nice to be able to read the angle and then be done with it. It saves a step when you're not interested in those fractions of a minute of arc.

    I've describe many methods over the years for getting a better IC. Among that various options, the best I've found is simple magnification. Set your sextant on its side on a table pointed at a distant sharply-defined, vertical mark, like a radio tower or a mast. Then remove the standard sextant telescope. Place a small spotting scope (something with a magnification in the range from 25x to 40x) in line with the normal position of the sextant. Then align the direct and reflected images of the distant vertical mark as usual. You should find that you can get the IC within +/-0.1' over and over again.

    You wrote:
    "And finally, regarding the use of an exclamation point as both decimal and minute indicator: I had never seen this until I came here"

    I'm not surprised. I do remember some relative newcomer asking some NavList regular about those "odd exclamation marks" within the past year or two. Maybe that was you. There's jargon that's part of the culture --that we need for practical reasons, and then there are just weird old habits. I contend that the exclamation marks fall in the latter category.


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