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ska-sa / katpoint

Coordinate library for the MeerKAT project
BSD 3-Clause "New" or "Revised" License
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Improve UVW coordinate calculations #46

Closed bmerry closed 5 years ago

bmerry commented 5 years ago

See the comments added to the code for detail about why this approach improves things. As a side benefit, the special case handling for the poles can be removed. This will change the results at the poles, in that orientation will be determined by the given RA.

The golden RA/DEC in the unit tests changed. These are just a paste of what is coming out, rather than independently verified results.

bmerry commented 5 years ago

@mauch FYI.

bmerry commented 5 years ago

DiFX also introduces an l offset target to fix the u vector independently of v (instead of insisting that they are orthogonal by using a cross product). I wonder how big that effect is...

That's a good point. A related issue is scaling - we just normalise the vector we get back, but a unit vector in geocentric coordinates won't be a unit long in topocentric coordinates. But I think that locally the effects scale with β² rather than β (v/c ~= 1e-4) so are too small to worry about.