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Hi David,
This is relatively straightforward to solve using basic trigonometry. The hard part is drawing the appropriate 2D plane diagrams of a 3D sphere. I think what you're after is this:
NewSpan := 2 * atan(tan(EquatorialSpan/2)/cos(Declination))
where:
* atan is the inverse tangent function,
* EquatorialSpan/Declination/NewSpan are all in the same units specified by your atan/tan/cos functions (either degrees or radians),
* Declination is between 0 and 90 degrees (inclusive),
* EquatorialSpan is between 0 and 180 degrees (inclusive),
* your NewSpan value will be between 0 and 180 degrees (inclusive).
With wide field of fields, you'll also have to take into account atmospheric refraction. Very narrow fields of view should be "close enough".
Hope this helps!
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