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The astronomer, Chauvenet, in his "Manual of Spherical and Practical
Astronomy" mentions two methods of checking the arc angles on a
sextant.  One is to calibrate it against angles measured between
objects on land with a transit, and the second is to measure
interstellar distances.  Frank Reed discussed this recently and George
Huxtable presented algorithms for calculating the angles around
February of 2003.  The interstellar distances need to be corrected for
the effect of refraction.  Several texts present tables of interstellar
distances, but they are simply wrong, as they have not been corrected
for refraction, and other effects.

I've tried measuring interstellar distances and it's difficult, just
like a lunar is more difficult than an horizon shot.  I might get
better when I acquire a more powerful telescope for my sextant (only
have a 2.5x right now).  However, currently I don't think I could
detect errors of less than one minute of arc.

Not having tried it, I have thought an advantage of the terrestrial
angles method, if one has access to a good transit or theodolite, would
be that the sextant could be clamped in a tripod, which could make the
angle determinations much more accurate.  That's not possible with
celestial objects due to the rotation of the earth, unless the sextant
were mounted in what astronomers refer to as an equitorial mount for a
telescope.  A disadvantage of the terrestrial method is that the
objects have to be at least three miles away, to negate parallax in the
sextant, although closer objects also could be used if their distance
could be measured precisely and the parallax error of the sextant were
determined.  Chauvenet also presents a method of measuring the parallax
error of a sextant, using a transit/theodolite.

I have also thought that there must be shops that calibrate transits.
Might they not also calibrate a sextant?

Fred

On Feb 2, 2004, at 5:57 PM, Royer, Doug wrote:

> After reading Joel's post I started to think about this matter.Frank
> started
> a thread(part 1 of 2) on measuring the angles between stars to find
> error.I
> was interested in the subject but didn't persue it further untill part
> 2 is
> written.However,after Joel's post I wonder if one can calibrate the
> instruments' non-adjustable error useing this or some other method.
> Doesn't collumation error have to do with the alignment of the scope
> to the
> frame?I've read in some book also,that to check collumation one places
> a
> grid a certain distance from the sextant and scope and checks for the
> error
> in that way.I'll have to dig around in these books to find if that is
> what
> was being written about.
> I've read in older nav. books a procedure called "interstellar angles"
> or
> some such.There was even a table with angles between stars(such as
> Sirius to
> Spica,Spica to Aldebahran etc.)that spanded close to long angles to
> measure.
> My question is : is this the method one can use to find or calibrate
> the
> non-adjustable error that Fred states?
> As I posted awhile ago I took my sextant into Baker Marine years
> ago(when
> they offered this service)to have the arc error checked(for a healthy
> sum)only to find out it was well in spec < 10" of arc over the entire
> arc of
> 130*.So,because of the cost of this couldn't Fred or any of us use
> this or
> some technique to find the error in some of the older sextants that
> state
> the error on the certification papers inside the sextants' box?
>

   

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