DATA Binary Table Extension

The DATA table contains a (large) DATA column containing correlations from one bank of the spectrometer plus coordinate values to calibrate the correlations.

An example extension header, and a subset of the data is given below:

DATA Binary Tables Extension: Header
-------------------------------------------------------------------------------
TENSION= 'BINTABLE'           / binary table extension
BITPIX  =                    8 / 8-bit bytes
NAXIS   =                    2 / 2-dimensional binary table
NAXIS1  =               262160 / width of table in bytes
NAXIS2  =                   11 / number of rows in table
PCOUNT  =                    0 / size of special data area
GCOUNT  =                    1 / one data group (required keyword)
TFIELDS =                    3 / number of fields in each row
TTYPE1  = 'DMJD    '           / label for field   1
TFORM1  = '1D      '           / data format of field: 8-byte DOUBLE
TUNIT1  = 'd       '           / physical unit of field
TTYPE2  = 'INTEGRAT'           / label for field   2
TFORM2  = '16D     '           / data format of field: 8-byte DOUBLE
TUNIT2  = 'sec     '           / physical unit of field
TDIM2   = '(2,8)   '           / Structure of 3D item
TDESC2  = 'SAMPLER,ACT_STATE'  / definition of axes
TTYPE3  = 'DATA    '           / label for field   3
TFORM3  = '65536E  '           / data format of field: 4-byte REAL
TUNIT3  = 'correl  '           / physical unit of field
TDIM3   = '(32768,2,8)'        / Structure of 3D item
TDESC3  = 'LAG,SAMPLER,ACT_STATE'  / definition of axes
CRPIX1  =                1E+00 / Reference Pixel of Lag data
CRVAL1  =                0E+00 / Reference Value of Ref. Pixel
CDELT1  =                0E+00 / Time offset between Lags
HBTLNGTH=          1.31072E-03 / Length of a memory cycle (heartbeat) in seconds
HBTPERSW=                  764 / Number of heartbeats per switching period
SWPERINT=                    5 / Number of switching periods per integration
UTCSTART= 8.79800000000000E+03 / Actual start time in seconds since midnight
UTDSTART=                52192 / Actual start time in MJD
EXTNAME = 'DATA    '           / name of this binary table extension
END
-------------------------------------------------------------------------------

DATA Binary Tables Extension: Data (1 row)
-------------------------------------------------------------------------------
ROW         DMJD            INTEGRAT       DATA(63336)
1    5.4575432654E+04  5.465702024000E+00  1.2000E+05
-------------------------------------------------------------------------------

In this version of the Spectrometer FITS file definition, the data may only be LAGS (correlations). Susequent versions may allow SPECTRA to be stored in the table; this will required additional header keywords and/or table columns to define the data processing performed.

The DATA table contains rows describing each correlation obtained during the scan. Both the INTEGRAT and DATA columns contain multi-dimensional entries, with variable size axes. Within one scan all correlations must have the same number of lags. The INTEGRAT column is column 2, and the structure of this 2 dimensional element is specified by the keyword TDIM2. The DATA column is column 3, and the structure of this 3 dimensional element is specified by the keyword TDIM3. For example, in a typical polarization observation with 256 lags, a switched noise cal, but no beam switching then DATA has shape TDIM3=(256,4,2). The INTEGRAT column for this observation would have TDIM2=(4,2).

The table FITS keywords are described below:

CRPIX1 Reference lag in the DATA column. This is an index in the Lag data.

CRVAL1 Lag value for the reference lag in the data column. (usually 0)

CDELT1 Increment between samples in the DATA column.

HBTLNGTH Length in seconds of the fundamental memory cycle, i.e., the heartbeat, of the device which is synchronized to the 1 PPS.

HBTPERSW Number of heartbeats or memory cycles per switching period.

SWPERINT Number of switching periods per integration. Note that the number of integrations (or rows in the DATA table) per scan is given by NAXIS2.

UTCSTART and UTDSTART Exact start time of the scan, should differ from DATE-OBS only for use in pulsar mode.

The columns of the table are defined as follows:

DMJD Column 1 of the table contains the date and time of the start of the integration (double modified Julian day including fractional day).

INTEGRAT Column 2 of the table contains the data integration times in each of the states. The total number of elements in this column entry is $1 \times N_{sampler} \times N_{actstate}$.

DATA Third column of the table is a three dimensional array of autocorrelation lags data (float). The size of the array is $N_{lags} \times N_{sampler} \times N_{actstate}$.

TDIM2 The dimensions of the INTEGRAT column entry are described by this keyword. The first dimension always has only one element.

TDIM3 The dimensions of the DATA column entry are described by this keyword.

TDESC2 Type of data along the axes of the 2 dimensional table element in the second column.

TDESC3 Type of data along the axes of the 3 dimensional table element in the third column.

Note that $N_{sampler}$ is the number of rows in the SAMPLER table and $N_{actstate}$ is the number of rows in the ACT_STATE table. The NUMSAMP keyword in the SAMPLER table is not necessarily the same as $N_{sampler}$ .

The purpose of the three keywords CRPIX1, CRVAL1 and CDELT1 is to describe the relationship of the autocorrelation data to the input IF signals. The autocorrelations could be provided in different orders (largest lag first or smallest lag first), and these keywords document this. Also note that autocorrelation data is taken from the IF 50 to 100 MHz band for the low speed samplers and IF band 800 to 1600 MHz for the high speed samplers.

The lag time for each lag in the DATA column is calculated according to the following formula:

$$Coordinate(N) = CRVAL1 + ((N - CRPIX1) \times CDELT1)$$

where N is the lag number.