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Robert,

This doesn't answer your final question, but one thing I noticed
after much not very systematic study was that index and side error,
especially side error, change depending on whether you are looking
straight through the optical axis or are off center.  That may be an
additional factor contributing to the IC change you experience,
although I don't think it would account for a change of 0.7
arcminutes.  Alex avoids some of this source of error because he has
sighting wires in his scope.

Fred

On May 6, 2007, at 11:38 AM, Robert Eno wrote:

> But have experienced this phenomenon in a variety of metal sextants
> including Cassens and Plath and Frieberger, so I do not believe
> that changes
> in IC are all that unusual. Granted, sometimes the determination of
> index
> error can be somewhat objective, however as I indicated in my e-
> mail, what
> is actually happening here is that side error creeps into the
> sextant and
> with it, a change in the index error. I am not exactly looking for
> solutions
> to this problem. I have long accepted that this is the way things
> work. My
> main question to the group is how do others manage it?  Thus far, a
> few
> people have agreed that averaging out the before and after IC seems
> to be
> the way to go. Having said this, I can tell you that while the
> frame of my
> sextant is solid bronze, the frame of the index mirror is
> constructed of
> aluminium. Could this be the reason for the change in IC over time?



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