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[fitsbits] CRPIX clarification
In order to bring this discussion to a conclusion and finalize the new
FITS Standard document, I suggest 2 small modifications to the previous proposal to clarify the use of the CRPIXj keyword. These are in response to the objections raised here by Steve Allen on 27-May-2008: 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. 2. Delete the phrase "in units of pixels" from the first sentence in the definition of CRPIXj as shown he CRPIXj - [floating-point, indexed, default: 0.0] Location of the reference point in the image for axis j \delete{, in units of pixels,} corresponding to $r_j$ in Eq. (8.1). The full text of the proposed change is shown below. I don't think we should get bogged down here in trying to expand this discussion to cover other issues such as Fourier sampling theory and image display conventions. These issues go beyond the scope of what the IAU FWG intended to be clarified when it conditionally approved the new FITS standard last month. Bill ================================================== =========== Proposed changes to the wording of the new FITS Standard document to clarify the use of the CRPIXj keyword. The current version of this document is available at http://fits.gsfc.nasa.gov/fits_draft.html. 1) Insert the following paragraph at the start of Sect. 8.1: Rather than store world coordinates separately for each datum, one may define 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 NASA/GSFC Code 662 HEASARC +1-301-286-4599 (voice) Greenbelt MD 20771 +1-301-286-1684 (fax) |
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