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    Re: SNO-T tests
    From: Bill B
    Date: 2005 Dec 13, 16:37 -0500

    Frank
    
    Trying to get a handle on this and have a few questions and observations.
    
    I assume it is important to use a flat-screen CRT or LCD monitor so distance
    does not change due to curvature of a standard CRT.
    
    I also assume you want the screen perpendicular to the line of sight on both
    the horizontal and vertical to avoid convergence.  And old photographer
    copy-stand trick is placing a small piece of mirror in the center of the
    base, and aligning an single-lens reflex camera so the lens sees itself in
    the mirror.  An assistant could do that with a monitor in the center of a
    monitor.
    
    Is the line if sight in this case considered to be from the scope, rather
    than the index mirror?
    
    When you wrote of distance from the monitor, what point on the sextant is
    used as the reference point?
    
    Confused on, "I wrote a very simple  piece of software that displays two
    vertical white lines, one above the other,  which can be separated at
    regular pixel intervals (I used 40 pixel jumps)."  Does this mean that the
    ends of the line segments do not overlap, so you are butting one line
    segment up to the other when you align them?  Would the target be similar to
    below? (Connect the dots vertically.)
    .
    .
    .    .
         .
         .
    
    Referring to, "Then I compared two runs of these measurements with the
    linear increase that I would predict if the sextant has no micrometer
    error."  How did you predict the values?
    
    In my sextant-as-rangefinder experiments, I found the parallax changed
    (front silvered index mirror) with angle enough to throw things off
    considerably.  I also would expect the distance from the point seen on the
    index mirror to the monitor to change, but not enough to account for more
    that a couple of seconds error over 2 degrees.
    
    BTW, if I can get a handle on the above, it struck me that instead of using
    a monitor I could use graphics/CAD software and produce target and have
    played out at 2400 dpi.
    
    Thanks
    
    Bill
    
    > The setup:
    > I wrote a very simple  piece of software that displays two vertical white
    > lines, one above the other,  which can be separated at regular pixel intervals
    > (I
    > used 40 pixel jumps). I set  this up and ran it on my laptop which I placed
    > at the far end of a room about 25  feet away (so that I could focus the
    > telescope). I placed my sextant on its side  and carefully measured the angle
    > between
    > the upper and lower line, repeating at  each interval. The angles ranged in
    > roughly 5 minute of arc steps from 0 to 2  degrees (two minutes or so for each
    > observation and the walk across the room to  move the line over one step so
    > over three-quarters of an hour of work for each  run). Then I compared two
    > runs
    > of these measurements with the linear increase  that I would predict if the
    > sextant has no micrometer error. Sure enough, there  was a nearly sinusoidal,
    > cyclic difference between the actual observations and  the linear prediction.
    > The amplitude from top to bottom was 0.7 minutes of arc  --exactly the same
    > magnitude as the mysterious errors I had seen previously. I  have not yet had
    > the
    > opportunity to try correcting real observations, of lunar  distances e.g.,
    > with my new micrometer correction table, but I'm optimistic that  this will
    > render this instrument essentially perfect (*after* correcting for arc  error
    > and
    > micrometer error). Lots of fun! I highly recommend trying this.
    >
    > By the way, as a bonus, this is an easy, extremely accurate way to test  for
    > backlash error. With the same sextant, I found zero backlash error every  time
    > I checked for it. It was also interesting how repeatable these measurements
    > turned out to be. Ninety percent of the time the angles between the lines were
    > the same to 0.1 minute of arc accuracy when re-measured, and over 95% were
    > no  more than 0.2 minutes different when re-measured.
    
    
    

       
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