[iaufwg] CRPIXj clarification

[William Pence]

Mark Calabretta wrote:
>>> Can you remind me again of the reasons for removing the text "(e.g. as
>>> is done for random groups data)".
>> A couple people during the FITSBITS discussion had issues with this 
>> phrase, so it is much simpler to just delete it rather than spend time 
> 
> What were their reasons?

Mark,

In hopes of speeding up this process, let's step back a minute to review 
how we got to the current point:

On May 19, the IAUFWG formally approved the new FITS Standard, however 
William Thompson's vote was conditional upon rectifying (and I quote) 
"one small omission having to do with the definition of the keyword 
CRPIXj.  Nowhere does it state that CRPIXj runs from 1 to N, instead of 
from 0 to N-1."   This is the only thing that needs to be resolved to 
finish the long process of updating the FITS Standard.

In response to my private email to you and Eric Greisen on how to 
address this issue, you proposed adding a new introductory paragraph at 
the start of section 8.1, as well as modifying the definition of CRPIXj 
itself to state that the first pixel has a value of 1.

There was some discussion by the IAU FWG of your proposed wording on May 
19 and 20 .  This discussion was then moved to the wider audience on 
FITSBITS where it continued until about June 16.  The net result of 
these wide ranging discussions was to make a few small changes to the 
wording of your original proposal.  In particular, the parenthetical 
clause referring to random groups was dropped after Eric Greisen raised 
an issue with it.  The clause was simply eliminated, rather than spend 
more time debating how to reword it, for several reasons: it is 
irrelevant to the William Thompson's concern about the starting point of 
CRPIXj, it is not essential to the mean of the sentence it is embedded 
in, and because the reference to group parameters within random groups 
is too obscure to be helpful to most readers.

As a result of the above process we now have a proposal on the table (as 
shown below) for the IAU FWG to consider.  Mark, I hope you can accept 
the wording as a reasonable compromise.  I believe the rest of the 
working group is ready to proceed with the vote to approve this proposal.

regards,
Bill

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Proposed change to the wording of the new draft of the FITS Standard:

1) Insert the following paragraph at the start of Sect. 8.1:

   Rather than store world coordinates separately for each datum,
   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]
     Location of the reference point in the image 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)