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    Sextant Telescope Collimation
    From: Frank Reed CT
    Date: 2005 Oct 18, 19:52 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
    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
    be distinguishable (one considerably  brighter for example). A terrestial
    angle will work, too. Bring the two  objects together in perfect contact near
    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
    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
    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
    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,
    alternatively a laser level (there are sufficiently accurate  levels
    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
    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
    telescope.  Assuming this telescope has crosshairs in the  field of view, you
    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
    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.
    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
    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
    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
    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
    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
    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
    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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