[AstroPy] External packages in astropy

Olе Streicher astropy@liska.ath...
Thu Jun 21 03:06:39 CDT 2012


David Berry <d.berry@jach.hawaii.edu> writes:
> Why is SIP being singled out? What about ZPX/TNX, and the newly
> labelled TPV ? These are all different attempts to solve the same
> problem of polynomial distortions to basic projections, and they are
> all in wide spread use. And there are many other commonly used
> FITS-WCS conventions that are not handled by wcslib, because wcslib is
> the reference implementation of standard FITS-WCS.

I see no contradiction here. One rule of thumb is "be lazy in your input
and be lazy in your output" -- so a wcslib implementation that would
handle SIP and other extensions could still be a reference
implementation. As a compromise, one could use an option "STRICT_WCS" to
build or use a lintian like wcslib. IMO the practical use of this is
quite limited anyway.

I would see just the problem of maintenance these extensions. Just
curious: Are there any patches ready for wcslib to correctly handle
them? 

However, this all should be discussed with Mark Calabretta...

BTW, why are there so many extensions that all seem to handle the same
problem? 

> Astropy needs to solve this issue. Relying on wcslib alone will result
> in there being loads of FITS file with WCS that cannot be read. This
> is the reason that DS9 has switched recently from using
> wcslib+wcstools to handle WCS to using the AST library. AST (and
> therefore pyast) has a more pragmatic approach to handling common
> FITS-WCS conventions and includes support for SIP, TNX, ZPZ, TPV.

.... with the danger to make the current segmentation and confusion
permanent.

AST uses its own, private, incarnation of wcstools files which have some
extensions that are not in wcstools yet. DS9 however includes another
copy of wcstools (resp. wcssubs), which does not have these
extensions. This sounds really fragile to me. Additionally, if someone
finds a bug in wcstools: how does the fix go through to ds9 finally? I
am afraid that it *will* be lost in one of its many ways to the ds9
binary. 

Best regards

Ole


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