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    Re: Artificial horizons
    From: Fred Hebard
    Date: 2003 Jan 3, 22:32 -0500

    Oil works for bright stars.  Blackening the pan (spraying with ultra
    flat black enamel) helps a lot, especially when street or houselights
    get in the line of sight.  I think it also obviates the need for
    darkened oil.  This on the lines of Bruce Bauer's black-painted
    horizon mirror.  I'm wondering whether a thin layer of oil on top of
    water, especially maybe black-dyed water, would have a higher
    The mirror I was using was a standard wall mirror, about 20 x 30
    inches.  The large size was very helpful in identifying targets, as I
    could see whole constellations.   However, the frame prevented
    leveling with a long level, and the surface additionally was
    detectably unflat.  Leveling with thin wedges worked well enough.  My
    fancy new (1948-vintage) Husun has a dark lens that fits on the
    telescope, for determining index error, which considerably reduces
    sun glare from mirrors.  There were double reflections from the top
    and bottom surfaces of the mirror; shades may have helped here.
    When purchasing spirit levels, make sure they read the same when
    pointed in both directions and turned upside down, if capable of
    that.  Only the more expensive do so, in my experience. I can barely
    detect a movement of 14 minutes of arc in my masons level.  Perhaps
    other levels are more sensitive.  I tested the sensitivity by wedging
    with sheets of paper from a thick ream of measured thickness and
    known number.
    Thanks for the help with the Gaussians.  Hope to get back on that shortly.

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