Subsections

• A Note on Blanking

ACT_STATE Binary Table Extension

The spectrometer ACT_STATE  table describes the state of the calibration and switching signals input to the spectrometer at the time of the correlations. The order of the rows of the ACT_STATE  table define the order of the correlations in the DATA column ACT_STATE  axis in the DATA  binary table extension (see below).

An example ACT_STATE  header, plus ASCII listing of typical table data, is given below:

ACT_STATE Binary Tables Extension: Header
-------------------------------------------------------------------------------
XTENSION= 'BINTABLE'           / binary table extension
BITPIX  =                    8 / 8-bit bytes
NAXIS   =                    2 / 2-dimensional binary table
NAXIS1  =                   24 / width of table in bytes
NAXIS2  =                    4 / number of rows in table
PCOUNT  =                    0 / size of special data area
GCOUNT  =                    1 / one data group (required keyword)
TFIELDS =                    6 / number of fields in each row
TTYPE1  = 'ISIGREF '           / label for field   1
TFORM1  = '1J      '           / data format of field: 4-byte INTEGER
TUNIT1  = 'T/F     '           / physical unit of field
TTYPE2  = 'IASIGREF'           / label for field   2
TFORM2  = '1J      '           / data format of field: 4-byte INTEGER
TUNIT2  = 'T/F     '           / physical unit of field
TTYPE3  = 'ICAL    '           / label for field   3
TFORM3  = '1J      '           / data format of field: 4-byte INTEGER
TUNIT3  = 'T/F     '           / physical unit of field
TTYPE4  = 'ESIGREF '           / label for field   4
TFORM4  = '1J      '           / data format of field: 4-byte INTEGER
TUNIT4  = 'T/F     '           / physical unit of field
TTYPE5  = 'EASIGREF'           / label for field   5
TFORM5  = '1J      '           / data format of field: 4-byte INTEGER
TUNIT5  = 'T/F     '           / physical unit of field
TTYPE6  = 'ECAL    '           / label for field   6
TFORM6  = '1J      '           / data format of field: 4-byte INTEGER
TUNIT6  = 'T/F     '           / physical unit of field
EXTNAME = 'ACT_STATE'          / name of this binary table extension
END
-------------------------------------------------------------------------------

ACT_STATE Binary Tables Extension: Data (complete)
-------------------------------------------------------------------------------
      ISIGREF     IASIGREF         ICAL      ESIGREF     EASIGREF         ECAL
           1J           1J           1J           1J           1J           1J
          T/F          T/F          T/F          T/F          T/F          T/F
1           1            0            1            0            0            0
2           0            0            1            0            0            0
3           1            0            0            0            0            0
4           0            0            0            0            0            0
-------------------------------------------------------------------------------

There can be different numbers of active columns in the ACT_STATE tables corresponding to the number of switching signals selected for the bank during a scan. The Keyword NAXIS2 is the number of rows in the table and defines the number of states of the spectrometer during an integration. The number of rows in the ACT_STATE  table is a power of 2, where the power is the number of active columns of the table. For example if there are 3 switching signals used, then there will be 3 columns in the table whose values are changing from row to row resulting in 8 rows.

Note: In the above example, the states occur temporally in the order 4, 2, 3 and 1 as specified in the example STATE table.

The inputs to the spectrometer are arranged into two groups, internal and external inputs. There are 4 internal states and 4 external states. Only 6 of these inputs may be selected for any one scan.

The definitions of the possible ACT_STATE table columns are as follows:

ISIGREF Signal-Reference beam indicator. Value zero indicates signal beam, a non-zero value indicates a reference location.

IASIGREF Internal Advanced Signal/Reference input. This may be used to trigger some devices (especially mechanical ones) to start switching between states in advance of the ISIGREF signal.

ICAL Indicates that the switching CAL noise diode was on during the this state of the spectrometer integration. Value one indicates CAL signal was on. Value zero indicates the CAL signal off.

Identical columns labelled ESIGREF, EASIGREF and ECAL are used for the external switching signals.

A Note on Blanking

The blanking signal is generally referred to as one of the ``switching'' signals. However, the two levels of the blanking signal do not correspond to two different states; rather they determine whether data will be added, or ignored, into the state defined by the other switching signals. Thus the internally (or externally) generated blanking signal will never appear as one of the switching signals used in the ACT_STATE table to define the states.

If the Spectrometer is the Switching Signal Master (as recorded by the STATE table keyword MASTER, then information about the blanking applied during the scan is available from the BLANKTIM column in the STATE table. If some other device is the Master, then the information about blanking must be derived from the FITS file written by that device.