In October of 1989 a meeting of interested representatives of many of the world's radio-astronomical observatories was held in Green Bank, West Virginia, to consider the creation of a standard data transport and interchange format for single dish (i.e. non-synthesis) radio astronomy data. Single dish radio astronomy data typically consists of a data array and a number of associated values (the header) which are necessary to describe the data and the state of the telescope and its environment. This meeting resulted in an agreement which held that FITS binary tables (Cotton, Tody, & Pence 1995; known as ``3-D'' tables in 1989) should be used to hold single dish data. A convention for storing the data matrix and its coordinate system was described. A set of required (CORE) and recognized but not required (SHARED) column names were also agreed to.
In 1992, prototype readers and writers were constructed at NRAO and IRAM. The NRAO prototype has been distributed since then as part of the UniPOPS package. In 1996, the Parkes HI Multibeam Survey (Saveley-Smith et al. 1996) adopted AIPS++ as their reduction package and the convention described here as their data archive format. This convention was revised at that time in light of FITS developments over the previous seven years and Parkes Multibeam and AIPS++ needs. The resulting standard is known as single dish FITS or SDFITS. The primary difference between the current standard and that proposed in 1989 is in the the storage of the data matrix and its associated coordinate axes.
Space limitations do not permit a full and complete description of this convention here. Interested readers should contact the AIPS++ project for a more complete description.
The single dish header information is stored in the columns in the FITS binary table. Each row in the table contains a data matrix and its associated header information.
A multi-dimensional DATA column is required. For fixed-shape columns, the TDIMn convention described in Cotton, Tody, & Pence (1995) is adopted to describe the shape of the array stored in that column. In this case, the product of all elements of TDIMn must be equal to TFORMn.
For variable-shaped columns, a modified TDIMn convention is used. In this case, a column having the name TDIMn where n is the DATA column number is used to store the shape of the DATA column in each row. The product of all of the elements in any cell of TDIMn must be less than or equal to TFORMn for the DATA column. The table ``heap'' (`` P'' data type, variable length array descriptor) can be used to store the DATA values in which case the product of the elements of TDIMn for that row must be less than or equal to the number of elements stored in the heap for that row. Any unused elements of the DATA array should be set to either IEEE NaN (not a number) for floating point arrays or to the value of the TNULLn keyword for integer data arrays.
This convention on storing multi-dimensional arrays in a table may be used on any column in an SDFITS table.
The axes in the DATA column are described using columns (which may be virtual) containing standard FITS image axis descriptions (including the WCS convention described in Calabretta & Greisen 2000). The following three axes are required: frequency-like (``FREQ'', ``VELO'', ``FELO''), longitude-like (``RA'', ``GLON'', ``ELAN'') and latitude-like (``DEC'', ``GLAT'', ``ELAT''). The following are recognized optional axes: ``TIME'' (the time since ``DATE-OBS''), ``STOKES'', ``BEAM'' (a beam ID number) and ``RECEIVER'' (a receiver ID number).
The axis descriptions may also apply to any other column having a TMATXn keyword with a value of True.
... EXTNAME = 'SINGLE DISH' / Single Dish convention / DATA column and coordinate system TTYPE1 = 'DATA ' / DATA column TFORM1 = '226E ' TUNIT1 = 'DEGREES ' TTYPE2 = 'TDIM1 ' / TDIM for DATA column (column 1) TFORM2 = '12A ' TTYPE3 = 'CTYPE1 ' / Velocity axis TFORM3 = '8A ' TTYPE4 = 'CRPIX1 ' / Velocity axis reference pixel TFORM4 = '1I ' TTYPE5 = 'CRVAL1 ' / Velocity at reference pixel TFORM5 = '1D ' TUNIT5 = 'M/SEC ' TTYPE6 = 'CDELT1 ' / Velocity increment TFORM6 = '1D ' TUNIT6 = 'M/SEC ' TTYPE7 = 'CTYPE2 ' / Longitude-like axis TFORM7 = '8A ' CRPIX2 = 1.0 / Longitude reference pixel TTYPE8 = 'CRVAL2 ' / Value at longitude ref. pixel TFORM8 = '1D ' TUNIT8 = 'DEGREES ' ... / CORE columns TELESCOP= 'NRAO12M ' / Designation of telescope BANDWID = 22070316.0 / Total bandwidth (Hz) TTYPE12 = 'OBJECT ' / Observed object name TFORM12 = '12A ' TTYPE13 = 'DATE-OBS' / Date of observation YYYY-MM-DD TFORM13 = '10A ' ... / SHARED columns OBSERVER= 'GARWOOD ' / Name of observer TTYPE16 = 'SCAN ' / Scan number TFORM16 = '1E ' TTYPE17 = 'RESTFREQ' / Rest frequency at ref. pixel TFORM17 = '1D ' TUNIT17 = 'HZ ' TTYPE18 = 'TCAL ' / Calibration temperature TFORM18 = '1D ' TUNIT18 = 'KELVINS ' ... / All other columns TTYPE25 = 'GLON-ARC' TFORM25 = '1D ' TUNIT25 = 'DEGREES ' TTYPE26 = 'GLAT-ARC' TFORM26 = '1D ' TUNIT26 = 'DEGREES '
Calabretta, M., & Greisen, E. 2000, this volume, 571
Cotton, W. D., Tody, D., & Pence, W. D. 1995 A&AS, 133, 159
Staveley-Smith, L., Wilson W. E., Bird T. S., Disney M. J., Ekers R. D., Freeman K. C., Haynes R. F., Sinclair M. W., Vaile R. V., Webster R. L., & Wright A. E. 1996 PASA, 13, 243