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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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