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    Re: Instrument Error
    From: Bill B
    Date: 2005 Apr 29, 15:48 -0500

    > Fred wrote re:sun/moon lunars:
    > Were you always looking at the sun through the clear part of the
    > horizon mirror or at it's reflected image?  I generally aim the
    > telescope at the lower body, as it's easier to hold the sextant.  If
    > you're reversing the body at which the telescope is aimed between early
    > and late afternoon, that might account for the difference.
    Yes, I was reversing. Early pm shots with moon in glass and Sun in mirror.
    Late pm shots with Sun in glass and moon in mirror.
    > If reversing, part or all of it might be shades.
    That thought did cross my mind, but I perceive little if any difference
    between IE checks using the Sun's limbs (3 shades on index and horizon
    mirror) or a star (no shades).  In my case blaming the shades at this
    juncture would be the same as a poor craftsman blaming his tools.  I imagine
    I should do a sanity check when both bodies are at approx. the same
    elevation by doing a few observations and then reversing.  Given my lack of
    prowess with instrument that test might well be garbage in, garbage out.
    > And >
    > Whether reversing or not, the gap in late afternoon might be caused by
    > not aiming straight and not centering the horizon body on the optical
    > axis.  Those sorts of errors always cause the distance to be too great.
    Hard to know, it is tough because of brightness difference to kiss the
    discs.  Irradiation *should* cause me to undershoot.  I did notice in
    tripod-mounted IE checks using the Sun if I got the discs in the center and
    kissing, and left the settings alone for several minutes the tangency did
    drift by several tenths of an arc minute as the images moved right and down.
    (Sun past meredian passage of course.)
    > The data also suggest more practice is needed, as you know.
    You think? :-)  Probably need to stop the marathon sessions (up to 19
    observations in one sitting) as well and relax.  As I noted earlier, when I
    popped out and took one cut standing, went back to work, and repeated in
    10-15 minutes, 5 out of 6 observations were within 0.2'.
    Thanks for your insights.

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