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    Sextant Telescope Collimation
    From: Frank Reed CT
    Date: 2005 Oct 18, 16:38 EDT

    If the  telescope of a sextant is not exactly parallel with the instrument's
    frame, the  instrument will measure large angles to be too large. The error is
    proportional  to the square of the telescope's tilt and proportional to the
    tangent of half of  the measured angle. I've been testing a group of sextants
    in the past two weeks,  and all of them had significant telescope collimation
    problems that could easily  yield errors of one to ten minutes of arc. So how
    do we test it, and how do we  fix it?
    To test for telescope collimation problems, measure a large angle  --ninety
    degrees or more. Any star-to-star distance will work for this, but they  should
    be distinguishable (one considerably brighter for example). A terrestial
    angle will work, too. Bring the two objects together in perfect contact near the
    center of the telescope's field of view. Then rotate the sextant so that the
    two  stars move across the field of view towards and away from the
    instrument's  frame. The stars will separate slightly. If the telscope is properly
    collimated,  they will separate symmetrically. That is, you will find that the stars
    are a  couple of minutes apart when the stars are on the right side of the
    field of  view, in contact at the center of the field of view, and a couple of
    minutes  apart again when they are on the left side of the field of view. If
    the test  reveals that the telescope is not correctly collimated, then it should
    be  adjusted.
    To collimate the sextant telescope, you'll need a large room or  hall twenty
    feet or more in length. You'll also need a small straight telescope,  like a
    finder scope for a larger telescope or an old-fashioned sextant  telescope, or
    alternatively a laser level (there are sufficiently accurate  levels available
    in hardware stores for $10 to $15). You'll also probably need a  couple of
    blocks about half an inch high and as identical as possible to support  the
    leveling scope.
    Set the sextant on its side on a table at one end of  the room. Swing the
    index arm as far out of the way as possible. Place the  blocks on the sextant's
    arc (or frame if it's flat enough) and then set the  leveling telescope or
    laser level on the blocks. Now turn on the laser or look  through the leveling
    telescope. Assuming this telescope has crosshairs in the  field of view, you can
    now mark a spot on the wall at the far end of the room  that will be your
    reference point for collimating the sextant's telescope. If  you're using a laser
    level, you've already got your reference point. Take a look  at your sexant's
    telescope and estimate how much farther it is away from your  sextant's frame
    than your leveling scope or laser level. Let's suppose it's half  an inch. Now
    look through the sextant's scope at the far wall. The center of the  field of
    view should be a spot that is that same half an inch higher than the  the
    reference point. But since we already know that the telescope is not  correctly
    collimated, it's probably aimed a considerable distance above or below  the
    correct level. If you find it difficult to locate the center of the field of
    view, draw some horizontal lines on the wall and count off. Now adjust the
    telescope's aim with the screws or knobs for this purpose on the sextant until  the
    center of the field of view is exactly aimed half an inch (or whatever
    height difference you find) above the reference point from the laser or leveling
    telescope. You should try to get this alignment to the nearest inch or so if
    the  far wall is twenty feet away. If you can get the alignment accurate to this
     level, the error in measured angles will be less than 0.1 arcminutes even at
     measured angles of 120 degrees. That is, a tilt of one inch in twenty feet
    (20  ft 4 inches, to be precise) is essentially perfect for all sextant angles.
     Generally, the error, dh, in the measured angle, h, from telescope tilt, T,
    is  dh=T^2 * tan(h/2). Or, if x is the aiming error measured on the wall and D
    is  the distance to the wall, then the error in minutes or arc is
    It turns out that this is a relatively easy  procedure, and it's well worth
    trying. Many sextants include small screws or  knobs that allow the telescope's
    inclination to be adjusted. But if your sextant  doesn't, you'll need to
    improvise some sort of "shimming" system.
    42.0N 87.7W, or 41.4N 72.1W.

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