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

Perry Greenfield perry at stsci.edu
Thu May 3 12:45:19 EDT 2012

On May 3, 2012, at 11:45 AM, David Berry wrote:

> On 3 May 2012 16:39, Wolfgang Kerzendorf <wkerzendorf at gmail.com>  
> wrote:
>> Hi all,
>> This topic has expanded a lot;-)
>> So adressing point 1: Yes, you're technically right. That both of  
>> these things do coordinate transformations. But there are many  
>> coordinate transformations in astronomy and in astropy we don't  
>> have a one-size fits-all mentality about them (in my sense for good  
>> reason). For example, switching between different units, doppler- 
>> shifting spectra , etc... all are technically coordinate  
>> transformations, but we treat them differently and not through one  
>> coordinate transformer.
> Is there a reason for that?

I agree that they are both identical in some respects (and it is worth  
considering a common api for at least a good part of both). To me the  
main difference is that one is standards focused (dealing with well  
known coordinate  systems and the transformations between them,  
whereas wcs is dealing more with instrumental effects and transforming  
from that to world coordinates (which may be one of the standard  
systems, but it may not). In one most of the transforms are simple, as  
is the geometry. In the other they can be quite complex. The former  
usually transforms few distinct coordinates (yes, you can do big  
catalogs too), and the latter transforms large numbers of pixel  
coordinates in many case.

So I'm not surprised that people come from different directions when  
approaching these two issues.


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