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Bill, who knows a lot about photography, wrote a note, from which I quote
its kernel.

| Somehow--as I read it--you confuse cartography with photography. Not like
| you. Reducing your argument to the absurd, it is impossible to take
| photographs of the heavens with a camera lens, a camera mounted to
| telescope, or plates mounted to an observatory telescope and accurately
| determine relationships or angles.  You need to share this with the minds
| behind the biggest best telescope EVER (now under construction) which
will,
| BTW, use a digital recording device. 
|
| The above portion of your argument--as I understand it--falls apart IMHO
| given the following.  Lens distortion a given, if we are dealing with the
| celestial sphere how does a sextant differ from a camera back?  All a
| sextant does is measure the angle between two objects, an object and the
| horizon, the top and bottom of an object (say lighthouse) etc.  Question.
| What is the software imbedded in the sextant that maps the celestial
sphere
| to a plane? Answer. The NA.

That's the aim, all the time, in celestial navigation, to measure angles.
And if we're going to try to measure those angles in terms of pixels across
a camera image, then for that purpose photography suffers from exactly the
same limitations as does cartography (plus a few more). It's simply
impossible to represent angles, in proportion, over a rectangular grid on a
plane surface, which is why any map in an atlas must always be distorted in
some way. In mapping, one way round the problem is to give up trying to map
on to a plane surface, and create a globe instead.

Another way would be to store somewhere a mapping conversion table for all
those coordinates from the rectangular grid in terms of the angular
coordinates they correspond to; which is more than a simple matter of
proportion. Then, knowing those coordinates, you can get the angle in the
sky between A and B, which is the usual aim of celestial navigation. (Note
that here I haven't even referred to lens-distortion,which adds extra
complication in photography, and telescopy.)

In an astronomical telescope, that matter is of crucial importance,
especially for wide-angle survey telescopes of the Schmidt family. I'm not
familiar with the details, but presumably a vital part of the setting up of
such a camera is the detailed mapping between position on the image-grid or
plate, and the celestial angles that are subtended, and the subsequent
rechecking of that conversion for any changes. No doubt there are
listmembers who are much better informed about those mattars than I am.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.



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