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[fitsbits] Proposed Changes to the FITS Standard
On Wed 2007-08-01T17:51:47 +0200, Thierry Forveille hath writ:
* p92: the discussion at the bottom should indicate that the diurnal Doppler correction is only weakly dependent on position and therefore great accuracy is not required. Depends on the context, and on what's meant by "no great accuracy": it's 1 m/s for 7 arcsec (worst case position on the sky), so for planet searches we need arcsecond precision or so (our best precision is under 1 m/s, and we want some safety margin to keep that particular contribution to the error budget negligible). Perhaps just give the 7" -- 1 m/s equivalence here and let readers make their own decision? And for pulsar timing it's even more relevant to get the position right. But even at the level of seven arcseconds on the surface of the earth there are issues because some (pre-1980) geodetic datums can produce topocentric positions which are off by many hundreds of meters. In Paper III we did not want to get into the specifics of geodetic datums. For that reason we stopped at recommending post-1980 coordinates. Whether that means WGS-84 or ITRF or ETRF or GTRF or any other satellite-centered, VLBI-aligned reference frame is irrelevant at the level of 1 m (and below that level everything is so heavily model dependent that FITS WCS III didn't dare to tread). For the quality and interpretability of any data with such precision nothing can substitute for having read Lindegren and Dravins and being familiar with the work of IAU Comm 52 (and all the efforts of its predecessors RCMAM and others). I'm not much more sure how all that would fit into the standard than I was at the time we were considering Paper III. -- Steve Allen WGS-84 (GPS) UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99855 University of California Voice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m |
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