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Subject: Sextant Error and Whole Horizon Mirrors
From: Chuck Griffiths (griffiths_chuck@XXX.XXX)
Date: Tue Jul 09 2002 - 14:43:02 EDT
Bruce Stark's recent posting in which he mentions the need for lunarians to pay
close attention to mirror adjustment brings up a problem with errors I've run in
to.
While I've been measuring lunar distances, I've noticed that I have an error in
my sextant that I can't remove or account for. This is easiest to see measuring
interstellar distances because the angular distance doesn't change with time. If
I move the images from one side of my whole horizon mirror to the other it
appears that I have a mirror error. This error manifests itself by making the
measured angle smaller when the images are on the area of the mirror closer to
the frame and larger when the images are farther from the frame.
Just to be perfectly clear I'll try to describe the problem one more time. If
the image of two stars are brought into coincidence in the middle of the field
of view of the horizon mirror (while carefully rocking the sextant to swing the
arcs of course) the images will move apart or closer together if the images are
moved from the center of the field of view to the edge of the field of view.
Watching for this is a challenge as other lunarians will testify. I've had the
best luck reclining in a chair with something to brace my arms on.
I thought, at first, that this was due to the index mirror not being
perpendicular to the frame. I've adjusted my sextant several times using both
the image of the arc and using small cylinders placed on the arc (Celestair
sells a product for this, but I just used something similar). After each
adjustment I removed the resulting side error. But this apparent movement of the
images from one side of the mirror to the other persists. I haven't taken enough
measurements at different angles to say how consistent it is over the whole arc
but it's usually in the neighborhood of about .2 arc minutes. I imagine that the
problem is related to the fact that I adjust the index mirror at one point on
the arc and as the index arm moves the pivot must not be perfectly parallel with
the arc.
I would be very appreciative if other list members with whole horizon mirrors on
their sextants could comment on whether their sextants also demonstrate this
error and whether they were able to remove it by adjusting the index mirror.
Chuck Griffiths
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