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    Re: Remember your first time?
    From: Fred Hebard
    Date: 2005 Oct 31, 11:49 -0500

    On Oct 31, 2005, at 11:19 AM, Paul Flint wrote:
    > ...How do I use the different colored filters? The orange one seems
    > good for cloudy days. The blue for bright days? Do you use them all
    > at once?
    > Is there a slick way to check index error with an artificial horizon?
    Nice story Paul!
    I'm not sure about all the colors, but I believe you just put filters
    down until the image is quite dark (you don't want any black spots as
    after images!).
    You can check index error by looking directly at the sun, touching the
    two images just like in the horizon.  Touch the lower limb of the
    movable image (through the index mirror) to the upper limb of the fixed
    image (through the horizon mirror) and record the reading.  Then
    reverse, touching the upper limb of the movable image to the lower limb
    of the fixed image, and record the reading.  Half the difference
    between the readings is the index error.  Always adjust in the same
    There's a trick in recording the second reading, when the upper limb of
    the movable image touches the fixed image's lower limb.  If the index
    error is remotely correct, it will be negative.  Record it as a
    positive number, the number of minutes below zero.  This will reverse
    the readings on the scale.  For instance, if the micrometer is between
    24 and 25 and is 1/5th torward 25, that would be 35.8 seconds off the
    arc, not 24.2 seconds.
    The semi-diameter is one fourth of the sum of the readings and provides
    a good check on your technique.  Semi-diameters of the sun are
    catalogued daily in the Nautical Almanac.
    By the way, for artificial horizons, I like to set the two images
    slightly apart and let them converge, then mark the time.
    If you're reading through the window, open it.

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