[iaufwg] CRPIX clarification

[Arnold Rots]

I can live with the current proposal and am glad that any reference to
the location of the integral value in the pixel was not included.
Although this is unambiguously defined for spatial pixels/bins, such
is not the case for time bins and fixing the integral values to the
center of the bin/pixel would have run afoul of conventions in the
timing world - the TIMEPIXR keyword in the HEASARC conventions.

  - Arnold

William Pence wrote:
> Below, is the latest version of the CRPIXj clarification that 
> incorporates Bill Thompson's revision to the sect. 8.1 paragraph and has 
> "conventional" in italics.  This does not include Steve Allen's 
> suggestion of using "initial pixel" instead of "first pixel" in the 
> definition of CRPIXj;  I'll do so if others think the current wording 
> could be a source of confusion by programmers who might interpret  the 
> "first pixel" to be the pixel following the "zeroth pixel" and hence is 
> really the second pixel in the array.
> 
> On a procedural note, here is how I propose we complete this process of 
> producing the final standard document:
> 
> 1.  IAU FWG members have 1 more week to post suggestions here on the 
> iaufwg email list on how to clarify the use of CRPIXj.
> 
> 2.  On Tuesday, May 27th, I will post an announcement on the FITSBIT 
> email list to say that the IAU-FWG has approved the new FITS standard 
> document along with a proposed clarification on the usage of CRPIXj.
> 
> 3.  We then allow 3 weeks for any further comments or suggestions from 
> the FITSBITS community (which includes the regional FITS committees) on 
> the proposed clarification of CRPIXj.
> 
> 4.  Assuming that the discussion of the CRPIXj clarification on FITSBITS 
> is relatively non-controversial, we will incorporate the resulting final 
> version of the proposed clarification of CRPIXj into the official FITS 
> standard.  The IAU-FWG will only conduct another formal vote on this if 
> a) there is some controversy that must be resolved, or b) if any IAU FWG 
> member specifically requests a format vote.
> 
> Let me know if you have any comments about the process outlined above.
> 
> Here then, is the latest version of the proposed change:
> 
> 1) Insert the following paragraph at the start of Sect. 8.1:
> 
>    Rather than store world coordinates separately for each datum (e.g. as
>    is done for random groups data), the regular lattice structure of a
>    FITS image offers the possibility of defining rules for computing
>    world coordinates at each point.  As stated in Sect. 3.3.2 and
>    depicted in Fig. 3.1, image array data are addressed via {\em integral
>    array indices} that range in value from 1 to NAXISj on axis j.
>    Recognizing that image data values may have an extent, for example an
>    angular separation, spectral channel width or time span, and thus that
>    it may make sense to interpolate between them, these integral array
>    indices may be generalized to floating-point {\em pixel coordinates}.
>    Integral pixel coordinate values coincide with the corresponding array
>    indices, while fractional pixel coordinate values lie between array
>    indices and thus imply interpolation.  Pixel coordinate values are
>    defined at all points within the image lattice and outside it (except
>    along {\em conventional} axes, see Sect. 8.5). They form the basis of
>    the world coordinate formalism in FITS depicted schematically in Fig.
>    8.1.
> 
> 2) Remove the italics from "pixel coordinates" on p75 (they are used in
>     the first mention above).
> 
> 3) Change the definition of CRPIXj on p77 to:
> 
>     CRPIXj - [floating-point, indexed, default: 0.0]
>         Pixel coordinate element of the reference point for axis j,
>         corresponding to $r_j$ in Eq. (8.1).  Note that the reference
>         point may lie outside the image and that the first pixel in the
>         image has pixel coordinates $(1.0, 1.0, \ldots)$.
> 
> 
> -- 
> ____________________________________________________________________
> Dr. William Pence                       William.Pence at nasa.gov
> NASA/GSFC Code 662       HEASARC        +1-301-286-4599 (voice)
> Greenbelt MD 20771                      +1-301-286-1684 (fax)
> 
> 
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> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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