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SEXTANT TELESCOPE COLLIMATION

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
dh=3438*(x/D)^2*tan(h/2).

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.

-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars

   

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