From owner-fitsbits@kochab.cv.nrao.edu Fri Nov 13 16:47:28 1998 X-VM-v5-Data: ([nil nil nil nil nil nil nil nil t] ["13466" "Fri" "13" "November" "1998" "16:36:29" "-0500" "Don Wells" "dwells@NRAO.EDU" "<199811132136.QAA05091@fits.cv.nrao.edu>" "263" "ADASS-FITS-BoF annual report (WCS negotiations)" "^From:" nil nil "11" "1998111321:36:29" "ADASS-FITS-BoF annual report (WCS negotiations)" nil nil nil] nil) Return-Path: Received: from cv3.cv.nrao.edu (cv3.cv.nrao.edu [192.33.115.2]) by fits.cv.nrao.edu (8.8.7/8.8.8/CV-2.2) with ESMTP id QAA05141 for ; Fri, 13 Nov 1998 16:47:28 -0500 Received: from kochab.cv.nrao.edu (majordom@kochab.cv.nrao.edu [192.33.115.108]) by cv3.cv.nrao.edu (8.8.5/8.8.5/CV-2.7) with ESMTP id QAA16025; Fri, 13 Nov 1998 16:47:27 -0500 (EST) Received: (from majordom@localhost) by kochab.cv.nrao.edu (8.8.8/8.8.8/CV-2.2) id QAA01660 for fitsbits-spinner; Fri, 13 Nov 1998 16:43:56 -0500 (EST) Received: from fits.cv.nrao.edu (dwells@fits.cv.nrao.edu [192.33.115.8]) by kochab.cv.nrao.edu (8.8.8/8.8.8/CV-2.2) with ESMTP id QAA01655 for ; Fri, 13 Nov 1998 16:43:53 -0500 (EST) Received: (from dwells@localhost) by fits.cv.nrao.edu (8.8.7/8.8.8/CV-2.2) id QAA05130 for fitsbits@majordomo.cv.nrao.edu; Fri, 13 Nov 1998 16:43:53 -0500 Received: from fits.cv.nrao.edu (dwells@fits.cv.nrao.edu [192.33.115.8]) by kochab.cv.nrao.edu (8.8.8/8.8.8/CV-2.2) with ESMTP id QAA01603 for ; Fri, 13 Nov 1998 16:36:50 -0500 (EST) Received: (from dwells@localhost) by fits.cv.nrao.edu (8.8.7/8.8.8/CV-2.2) id QAA05091; Fri, 13 Nov 1998 16:36:29 -0500 Message-Id: <199811132136.QAA05091@fits.cv.nrao.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Mailer: VM 6.35 under Emacs 20.2.1 Precedence: bulk From: Don Wells Sender: owner-fitsbits@kochab.cv.nrao.edu To: fitsbits@fits.cv.nrao.edu Subject: ADASS-FITS-BoF annual report (WCS negotiations) Date: Fri, 13 Nov 1998 16:36:29 -0500 ADASS-FITS-BoF annual report (WCS negotiations) Don Wells [Chair, IAU FITS Working Group] 1998-11-13 In recent years the FITS BoF [Birds-of-a-Feather] sessions at ADASS meetings have been the primary annual face-to-face meeting for the FITS community, and the agenda of each BoF has acted as an annual report for FITS. I have prepared this memo as a written version of the three viewgraphs which I used at the ADASS'98 FITS BoF, held at Univ. of Illinois late in the afternoon of Monday November 2nd. There were 30-40 people present in the room. 1. The NASA NOST FITS Standard A technical panel under the leadership of Bob Hanisch has been working on this codification of all FITS Agreements through the end of 1997. A draft version was submitted to the community for review in the spring of 1998, and 140 comments were received from the worldwide FITS community. The technical panel has considered each of these comments, and has decided on responses to them. Bob Hanisch was not able to attend ADASS'98, so the secretary of the panel, Bill Pence, read Hanisch's report to the BoF. The panel expects to send out the responses soon, and to have a new version of the Standard ready for review within a few months, perhaps within a few weeks. The ultimate goal of this effort is to produce a definitive version of the FITS Standard, a version which will be approved by the three FITS regional committees and by the IAU FITS Working Group, so that it can replace the FITS papers. Don Wells stated that he would like for this definitive version of the FITS Standard to be published in A&A Supplement. 2. The FITS Y2K (DATE-OBS) Agreement This agreement was adopted by the IAU-FWG in November 1997, one year ago. Don Wells pointed out that full interchange of the new notation is to begin 1999-01-01, only two months from now. He asked who had changed code in response to this agreement, and roughly half of those present raised their hands. 3. IAU Comm5 Task Group [TG] - Designations and OBJECT strings Don Wells reminded the BoF that the IAU TG on Designations has asked that the maximum lengths of OBJECT strings in interchange be increased to at least 26 characters in order to support the IAU standard object designation syntax. The NOST technical panel has addressed this issue and has decided that all string values in FITS headers may be up to 68 characters in length, which will solve the technical aspect of this problem. The real goal is to use object designations which will not be ambiguous when they appear in archival databases; in particular, comments about filter choices, weather, etc., should not be appended to object names in OBJECT strings. Ideally the IAU prefixes and coordinate syntax will be implemented and used in data acquisition systems. Wells urged the BoF to 'help stop namespace pollution!' 4. WCS [World Coordinate System] negotiations Don Wells reminded the BoF that there were a number of WCS issues outstanding after the ADASS'97 [Sonthofen] BoF, even though significant progress was made in negotiations during ADASS'97. Some of the differences of opinion had prevented conclusion of negotiations for several years. Wells reported that in March 1998 he had created a small ad hoc task group [TG] to work on the outstanding technical issues. This TG was expanded to about 15 people in July 1998, and much progress was made, with the result that Wells reported to the BoF that a working consensus now exists. The final major steps of the negotiations occurred during the 24 hours preceding the ADASS'98 BoF. The WCS Agreement which the TG is producing will be the most complex FITS agreement to date; we have been negotiating it for more than a decade. The items below are not the complete agreement, but only the set of important features which were discussed at the BoF. a) Split G&C into two papers The TG will recommend that the Greisen & Calabretta draft WCS paper be split into two papers. The first paper will have a title something like 'Generalized Representations of World Coordinate Systems in FITS', and will specify meta-rules for FITS WCS conventions. The second paper will specify the WCS conventions for celestial coordinates. The meta-rules are needed because the TG expects that eventually there will be a third paper for the spectroscopic case and maybe another paper on time axes. b) Linear transformation to use CDi_j keywords The TG will recommend that the IRAF CDi_j rotation matrix keywords be adopted instead of the PCiiijjj keywords which were suggested in previous drafts of the Greisen & Calabretta paper. This means that CDELTi and CROTAi will not be used. It also means that the WCS agreement will have backwards compatibility with several existing data archives. c) Multiple-WCS support The TG will recommend that optional additional sets of WCS keywords in FITS headers be distinguished from the default set of WCS keywords and from each other by appending a single alphabetic character [A-Z] to all of the WCS keywords. Examples of such keywords could be CRPIX1B, CD2_1C, CRVAL3D and CTYPE2A. Note that the default set of WCS keywords in these cases would still be CRPIX1, CD2_1, CRVAL3 and CTYPE2. Datasystems which do not support the multiple WCS notation will need to regard WCS keywords with trailing non-numeric characters as unrecognized keywords. Discussion of this idea during the BoF exposed new examples of usage which added to the cases which have convinced the TG that multiple-WCS capability will be valuable. The optional appended version code character will restrict the basic WCS keywords to seven characters. Therefore, the TG will recommend that WCS axis numbers be in the range 1-99 (i.e., axes 100-999 will not be supported). d) New distorted-projection capability to be added The TG will recommend that a set of distortion terms analogous to those used in the DSS [Digital Sky Survey] be added to the basic TAN and ARC projections in order to support a variety of optical imaging systems which produce distorted geometries without requiring re-gridding of the vast quantities of data being produced. The goal is to agree on a set of terms which will represent the geometric distortions of all existing optical cameras. The set will include radial terms, analogous to those in the 'ZPN' projection described in the Greisen & Calabretta draft. The majority of imaging systems will be supported by these radial terms. (The 'radial' terms used in the DSS are X(X^2+Y^2), Y(X^2+Y^2), X(X^2+Y^2)^2 and Y(X^2+Y^2)^2; these implement an elliptically-symmetric radial distortion.) The non-radial terms, such as XY^2 and X^2Y, will be particularly useful for re-imaging cameras like HST's WFPC-1 and WFPC-2. Several TG members believe that such cameras have two 'center-points' for their axes, the tangent point of the main telescope and the centers of the field-flattening lenses of the individual cameras; probably two keywords will be defined to specify a center point offset for the polynomial terms (the main telescope's TAN geometry center point will be specified by CRPIXi). Several TG members intend to collect information about existing re-imaging cameras to verify this conjecture. Work is going on to formulate models that describe the small systematic distortions seen in Schmidt astrometry, relative to the ARC projection, and this important special case may justify explicit support in the FITS standard. Whether or not the TG recommends that terms for such models be added to the set of distortion terms, some version of the pixel correction images described in Appendix A of the Greisen & Calabretta draft will probably still be specified to describe the residual random distortion field for the highest precision, widest field applications. The TG expects that most optical cameras will be represented satisfactorily by the polynomial terms alone. The TG will recommend that all WCS parameter values be conveyed by keywords PVi_k (PVk_i?), where i is the index of the parameter and k is the index of the axis. This notation will replace the PROJPi keywords suggested in the Greisen & Calabretta draft. The axis index for WCS keywords will be restricted to 99 or less; note that if the axis index k is 9 or less (the usual case), the parameter index i in PVi_k may range up to 999 without exceeding the seven character limit set by the plan to use an optional appended character for multiple-WCS support. e) WCS discussions and decisions during the BoF session Don Wells encouraged discussion of items (a), (b), (c) and (d). In the course of the discussion, it became clear that items (a), (b) and (c) had firm support from those present at the BoF. The following issues were discussed: - Wells asked whether the distortion projection should be given some new name to distinguish it from TAN and/or ARC; some TG members have worried that the complicated polynomial would confuse newcomers to FITS WCS who need to know that the TAN projection is appropriate for most optical imagery. It became clear that there was a consensus favoring adding the new terms to both the TAN and the ARC projections rather than creating a new projection. Although the leading radial term of the polynomial is sufficient to represent the difference between TAN and ARC (the DSS uses TAN rather than ARC, as you would expect for a Schmidt camera!), maintaining the distinction has a tutorial advantage. - A number of people at the BoF argued that the order of the indices in the PV keyword should be swapped, to PVk_i; presumably the TG will consider this modification. - Wells raised the issue of whether all parameters of a projection should be restricted to the 'Dec-like' axis (as it was with CROTAk), or alternatively should be associated in some agreed-upon fashion with both axes of a celestial projection. Some TG members have preferred to retain the former convention, but there was a clear consensus at the BoF that parameters should be associated with both axes. This convention will have the advantage that analogous polynomial terms associated with the two axes will have the same indices (one of the drafts considered by the TG had the indices of the second axis offset by 20). - Doug Tody suggested that axis_k=0 be used for specifing general parameters that apply to the entire WCS of a multi-WCS group; the TG will need to consider the implications of this detail of the rules. - Tody also raised the issue of whether the datatype of a PV keyword should be restricted to numerical, or defined by the WCS function type to which the parameter is assigned (this case is unusual in that the PV keywords are not fully defined, rather their usage is defined by each WCS). - Some people at the BoF regretted the demise of CROTAi and, especially, CDELTi, since they are so easy to use and feature in many existing files. They are suggesting that CDELTi and CROTAi could function as functional synonyms of CDi_j. (WCS reading code will have to support CDELTi and CROTAi forever because there are many existing files that use these keywords. The question is whether they are to be "deprecated" or will be recognized as "official synonyms".) Upon a motion by Doug Mink, an informal vote was taken to express general support for the work of the TG as described above, and the show of hands in support was unanimous. At this point in the session Don Wells told Eric Greisen that he and Mark Calabretta now had the working consensus which they had specified as a precondition before undertaking the next round of revision of the WCS draft paper. f) WCStools paper in ADASS'98 session Paper T6.2 ('WCSTools: an Image Astrometry Toolkit') was presented by Doug Mink in the afternoon session on Tuesday, the day after the FITS BoF session. The final sentence of the abstract is: 'The proposed FITS WCS standard is being tracked, and interim formats are being supported.' During the questions after Doug Mink spoke, Don Wells exhorted the community to include proper WCS notations in the headers of all optical imagery. Mink's WCSTools software is available at: http://tdc-www.harvard.edu/software/wcstools/. g) New members for the WCS TG? Don Wells told the BoF that he is prepared to add new members to the ad hoc WCS task group, but that such people should understand that they are expected to contribute to the solutions to FITS WCS problems. Wells is especially interested in adding people with knowledge of the geometry of re-imaging cameras, or of Schmidt camera geometry or general knowledge of precision astrometry, in order to complete the design of the distortion correction terms for the TAN and ARC projections. 5. Another Issue discussed at the BoF One person (the Chair does not recall who it was) reported having encountered a FITS tape which was not blocked in accordance with the FITS Blocking Agreement (Section 4.6 in the NOST 100-1.2 [1998-04-02] version of the FITS Standards). The people present at the BoF deplored this, of course. [NOTE: the text of this report has been reviewed by the WCS TG, and corrections/additions suggested by TG members have been made.] -- Donald C. Wells Associate Scientist dwells@nrao.edu http://www.cv.nrao.edu/~dwells National Radio Astronomy Observatory +1-804-296-0277 520 Edgemont Road, Charlottesville, Virginia 22903-2475 USA