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At 04:02 28/05/2008, you wrote:
>
>
>I know we are all a bunch of old salts here, but has anyone used a
>theodolite for a celestial site?  I know in theory, they are far more
>accurate, and I was wondering just how accurate a fix one can get. Would
>it be accurate enough for the Mars project?

Yes, I have. The accuracy of the fix depends on a number of factors. I have
a good second order theodolite which will directly in seconds of arc. The
scale cursor is suspended by a pendulum, which keeps the theodolite
"upright" to a third of second of arc. But this instrument is effectively
limited by its 30 power scope, which improves the 1 minute of arc naked eye
resolving power, to about 2 seconds of arc - in good light with good
contrast. At night, with the star moving rapidly across the field of view
and the cross hairs dimly illuminated, it is a struggle to get single sight
altitudes much better than about 3 seconds of arc in my experience. Doing
better seems to rely on the use of 'impersonal' observation methods, for
example a timed camera on the eyepiece and a graduated reticule.

Of course, even with this level of accuracy, one starts to run into
problems as to which geoid you are using for your coordinate system. For
example. If you stand on the centre line of the transit telescope in
Flamstead House at Greenwich with a GPS receiver, you will find that it
gives a WGS84 longitude of a little over 5 seconds of arc to the West. So,
having found a fix good to 1 second of arc according to the geoid assumed
for celestial navigation, you would need to transform this to the
terrestrial reference frame being used by the map on which you were trying
to find my position.

So yes, using a theodolite would probably be accurate enough for the Mars
project. There seems to be quite a lot of work being done to estimate a
Mars geoid - if that isn't an oxymoronism :-).

Geoffrey Kolbe


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