[AstroPy] API question: Instantiating of time/coord and similar

Christoph Deil Christoph.Deil at mpi-hd.mpg.de
Wed May 2 20:44:57 EDT 2012

On May 2, 2012, at 11:56 PM, James Turner wrote:

>> Obviously from my point of view there is 20 years of
>> experience in coordinate handling inside the pyast library and it
>> would be great to leverage that some how. I assume there is a goal of
>> only relying on numpy as the only non-pure python library so pyast
>> would not be acceptable but you could design a system that used AST
>> internally and switched it out at a later date when you wanted to go
>> pure python.
> That sounds pretty sensible to me and good input for those of us
> working with co-ordinates in Python. Perhaps a year ago, I and a
> couple of other people (including Perry) separately recognized some
> important concepts for handling co-ordinates that are missing from
> FITS but that will be needed in Python software, such as combining
> mappings as I think Tim is describing. After a little discussion,
> Paul Hirst pointed me to AST, which had already incorporated the
> same ideas N years previously :-). However, there is indeed some
> reluctance to pull another C library into the equation, for multiple
> good reasons. Although there's bound to be some variation on how to
> approach things, the problems are sufficiently non-trivial that a
> working model and/or prior experience seem valuable things to have...
> Not sure where things stand regarding the GPL etc. though? I think
> the decision was to use BSD in AstroPy. (If you reply just to this
> question I'd suggest changing the subject.)

I have been using the celestial module in the Kapteyn package and liked it a lot:


kapteyn.celestial is 3500 lines of well-documented python + numpy code and has a BSD license.
Some other parts of kapteyn are Cython-wrapped C-code.

Probably it would be better to base astropy.coordinates on PyAST or kapteyn or some other package instead of starting from scratch?
Does anyone have experience with both packages and can comment on their API strengths / weaknesses and performance (i.e. speed and precision)?
(I have CC'ed the kapteyn developers in case they are not aware of this discussion.)

One more question:
Is it planned to also have Alt-Az to RA-Dec coordinate transformations in astropy?
If we want such "observer coordinates" as well,  http://phn.github.com/pytpm/ might be a good starting point.


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