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[fitsbits] Review of the FITS-IDI convention
Somewhat belatedly on my part, here is an update on the status of the
FITS Interferometry Data Interchange (FITS-IDI) Convention. As you may recall, this convention was submitted to the Registry of FITS conventions (http://fits.gsfc.nasa.gov/fits_registry.html) in May 2009, and has been under public review since then. As described below, Eric Greisen has revised the definition document of the IDI convention in response to the comments from Bill Cotton. I have placed the revised document on the FITS Registry web page, along with a new "Comments and/or Critiques" link to all the comment that have been posted so far on FITSBITS regarding this convention. The public review period will remain open for a few more weeks in case there are any new comments about these revisions. - Bill Pence .................................................. ..... William Pence wrote: Hi Eric, I'm getting back to some unfinished FITS-related business, one of which is to get the FITS-IDI convention (documented at http://fits.gsfc.nasa.gov/fits_registry.html) moved from the "under review" category into the "registered" category. Before doing so, it seems that some sort of response to the comments made by Bill Cotton needs to be provided. Are you planning to revise the document to address his concerns? I attach Bill's text with detailed comments and new PS and PDF versions. The changes during the FITS-IDI comment period are shown in blue, those between the early Flatters document and the present on in red. Cheers, Eric .................................................. ........... I have revised the document, using blue color to represent changes made during the IAU Committee period. Bill's comments are precede by a and my answers follow each with no indentation. I have also corrected more typographical errors. - Sect 2.1. Even after a decade, this really gets me torqued up. The weight was intended to be just that - a relative weight and not a patch for the VLBA's inability to do divisions. This perversion of the definition borders on making it useless. There REALLY HAS to be some way to convey relative weights. A third weighting method was added (p 7) along with a keyword WEIGHTYP in the UV_DATA header. The keyword is optional with default the old meaning for weights (p 16) - sect 4.1.1 6th paragraph. Shouldn't the RA and Dec be specified to be those of the standard equinox and epoch? If the coordinates are given in the UV-DATA header, shouldn't the equinox and epoch also be given?, there may not be a SOURCE table in this case. The EQUINOX keyword is added as an optional keyword in the UV_DATA for single-source data (p 13, 15, 16) - Sect 5.2, ARRAY_GEOMETRY table, Mount type should also have a value defined for an aperature array (as opposed to a steerable device). Added (p 19) - Sect. 6 ANTENNA table. This really should include the antenna diameter as a number of arrays are heterogenous. The ARRAY_GEOMETRY table (p 18, 19) now includes an antenna diameter The ANTENNA table contains a FWHM parameter with one value per band. The polarization calibration parameterization may need to be rethought with the upcoming wide band systems. A single calibration per band may not be enough. Indeed but beyond the scope of this manuscript. - Sect 8.2 SOURCE table. This MUST include the epoch of the position as well as the equinox. The EPOCH keyword has been added (p 23, 24) Eq. 7 is wrong, the subtraction should be a division. Yes (p 23) "Source-specific frequency offset" paragraph: the text should say that these are the frequency offsets of the frequency of the reference pixel and should be added to the "BANDFREQ" value from the FREQUENCY table Sentence added (p 23) "RESTFREQ" paragraph. This should state that this is the rest frequency of the transition in that band. It should also say what to do if there are multiple transitions. Clarifications added incl that there is no way for 1 / band (p 24) "Source positions" paragraph. Does the VLBA really use 2000.0 or 1950.0 for sources with meaningful proper motions? Worse - it uses EPOCH not EQUINOX for EQUINOXes - note added (p 24) Footnote 4.If this is what the VLBA really does then this is REALLY BAD. In AIPS I have given up on apparent position columns and on input I recompute them (not good for moving sources). The EVLA UV-FITS output has apparent positions precessed back to MJAD 0. - Sect 16, BANDPASS table. There is no provision for a parameterized bandpass function. Correct - sentences added to this effect (p 36). My experience is that it is bad to interpolate in the parametrization values and there are way too many parameterizations possible. Should BP tables be written (and only LWA had tried), they should be expanded to the simple form we can all understand and interpolate (in time) as nneded. - Sect 17, CALIBRATION table. The behavior of AIPS is not part of the format definition. However, I do agree with the sentiments you express at the beginning. The use of TSYS_n, TANT_n, SENSITIVITY_n and PHASE_n are undefined and I can't think of a sensible use of them in this context. The text should spell out the usage of REAL_n, IMAG_n, RATE_n and DELAY_n. It's also not clear that this bilinear phase model is adequate for wide bandwidths (may need higher order in delay) and/or high frequencies (may need higher order in time). I do not understand the remark about AIPS nor the remark about bi-linear, neither are mentioned in the text. I have left the description of this table deliberately vague since I do not believe that anyone has even considered implementing it. A number of columns in it make no sense to me in terms of how AIPS does calibration - the Tsys, Tant, Sensitivity, and Phase columns make no sense to me. The latter 2 are also covered in the real and imaginary columns of the gain. A further remark about the uncertainties in this table was added (p 38). Eric Greisen |
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