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[fitsbits] CRPIX clarification



 
 
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Old June 16th 08, 09:52 PM posted to sci.astro.fits
William Pence[_2_]
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Default [fitsbits] CRPIX clarification

Taking Rob's suggestion, the proposed changes to the new draft FITS
Standard are as follows:

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 $(1.0, 1.0, \ldots)$.


If anyone objects to the use of the phrase "regular lattice
structure" in that first sentence (Steve Allen?) then I suggest the way
to handle this would be to add a footnote to this phrase, which
elaborates or constrains the meaning within this FITS context.
Perhaps something to the effect:

"In the general case, one should not assume that the points in a
FITS array are regularly spaced, or that the axes
of the array necessarily have any physical units."

If any one feels this is necessary, then please suggest a wording for
such a footnote.

Bill Pence
--

Rob Seaman wrote:
Perhaps simply excise "(e.g. as is done for random groups data)"? The
less said about random groups, the better :-)

Rob
--

On Jun 16, 2008, at 7:20 AM, Eric Greisen wrote:

Mark Calabretta wrote:
On Thu 2008/06/12 14:13:44 -0400, William Pence wrote
in a message to: FITSBITS

1. Change the first sentence in section 8.1 from

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.

to

Rather than store world coordinates separately for each datum, one
may define rules for computing world coordinates at each point.

This change is not acceptable to me.

The underlying basis of the world coordinate formalism is that the
data has a regular structure, unlike random groups for example.
Metrics don't come into it, at least not at this level.

Regards, Mark


I did not like the former wording since random groups does have a
regular lattice with WCS under the same rules as images along with the
random parameters which need not have rules.

Eric Greisen

--
__________________________________________________ __________________
Dr. William Pence
NASA/GSFC Code 662 HEASARC +1-301-286-4599 (voice)
Greenbelt MD 20771 +1-301-286-1684 (fax)


 




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