Each row of the IF table describes a complete IF path from source to backend. All complete paths for a scan are entered regardless whether the path terminates on an active backend or not.
XTENSION= 'BINTABLE' / binary table extension BITPIX = 8 / 8-bit bytes NAXIS = 2 / 2-dimensional binary table NAXIS1 = 4370 / width of table in bytes NAXIS2 = 2 / number of rows in table PCOUNT = 0 / size of special data area GCOUNT = 1 / one data group (required keyword) TFIELDS = 25 / number of fields in each row TTYPE1 = 'BACKEND ' / label for field 1 TFORM1 = '32A ' / data format of field: ASCII Character TUNIT1 = 'none ' / physical unit of field TTYPE2 = 'BANK ' / label for field 2 TFORM2 = '2A ' / data format of field: ASCII Character TUNIT2 = 'none ' / physical unit of field TTYPE3 = 'PORT ' / label for field 3 TFORM3 = '1J ' / data format of field: 4-byte INTEGER TUNIT3 = 'none ' / physical unit of field TTYPE4 = 'RECEIVER' / label for field 4 TFORM4 = '32A ' / data format of field: ASCII Character TUNIT4 = 'none ' / physical unit of field TTYPE5 = 'FEED ' / label for field 5 TFORM5 = '1J ' / data format of field: 4-byte INTEGER TUNIT5 = 'none ' / physical unit of field TTYPE6 = 'SRFEED1 ' / label for field 6 TFORM6 = '1J ' / data format of field: 4-byte INTEGER TUNIT6 = 'none ' / physical unit of field TTYPE7 = 'SRFEED2 ' / label for field 7 TFORM7 = '1J ' / data format of field: 4-byte INTEGER TUNIT7 = 'none ' / physical unit of field TTYPE8 = 'RECEPTOR' / label for field 8 TFORM8 = '8A ' / data format of field: ASCII Character TUNIT8 = 'none ' / physical unit of field TTYPE9 = 'LO_CIRCUIT' / label for field 9 TFORM9 = '32A ' / data format of field: ASCII Character TUNIT9 = 'none ' / physical unit of field TTYPE10 = 'LO_COMPONENT' / label for field 10 TFORM10 = '32A ' / data format of field: ASCII Character TUNIT10 = 'none ' / physical unit of field TTYPE11 = 'SIDEBAND' / label for field 11 TFORM11 = '2A ' / data format of field: ASCII Character TUNIT11 = 'none ' / physical unit of field TTYPE12 = 'POLARIZE' / label for field 12 TFORM12 = '2A ' / data format of field: ASCII Character TUNIT12 = 'none ' / physical unit of field TTYPE13 = 'CENTER_IF' / label for field 13 TFORM13 = '1E ' / data format of field: 4-byte REAL TUNIT13 = 'Hz ' / physical unit of field TTYPE14 = 'CENTER_SKY' / label for field 14 TFORM14 = '1E ' / data format of field: 4-byte REAL TUNIT14 = 'Hz ' / physical unit of field TTYPE15 = 'BANDWDTH' / label for field 15 TFORM15 = '1E ' / data format of field: 4-byte REAL TUNIT15 = 'Hz ' / physical unit of field TTYPE16 = 'HIGH_CAL' / label for field 16 TFORM16 = '1J ' / data format of field: 4-byte INTEGER TUNIT16 = 'none ' / physical unit of field TTYPE17 = 'TEST_TONE_IF' / label for field 17 TFORM17 = '1E ' / data format of field: 4-byte REAL TUNIT17 = 'Hz ' / physical unit of field TTYPE18 = 'TEST_TONE_SKY' / label for field 18 TFORM18 = '1E ' / data format of field: 4-byte REAL TUNIT18 = 'Hz ' / physical unit of field TTYPE19 = 'TEST_TONE_CIRCUIT' / label for field 19 TFORM19 = '32A ' / data format of field: ASCII Character TUNIT19 = 'none ' / physical unit of field TTYPE20 = 'TEST_TONE_COMPONENT' / label for field 20 TFORM20 = '32A ' / data format of field: ASCII Character TUNIT20 = 'none ' / physical unit of field TTYPE21 = 'SFF_MULTIPLIER' / label for field 21 TFORM21 = '1D ' / data format of field: 8-byte DOUBLE TUNIT21 = 'none ' / physical unit of field TTYPE22 = 'SFF_SIDEBAND' / label for field 22 TFORM22 = '1D ' / data format of field: 8-byte DOUBLE TUNIT22 = 'none ' / physical unit of field TTYPE23 = 'SFF_OFFSET' / label for field 23 TFORM23 = '1D ' / data format of field: 8-byte DOUBLE TUNIT23 = 'none ' / physical unit of field TTYPE24 = 'TRANSFORM_COUNT' / label for field 24 TFORM24 = '1J ' / data format of field: 4-byte INTEGER TUNIT24 = 'none ' / physical unit of field TTYPE25 = 'TRANSFORMS' / label for field 25 TFORM25 = '4096A:SSTR256/059' / data format of field: ASCII Character TUNIT25 = 'none ' / physical unit of field EXTNAME = 'IF ' / name of this binary table extension COMMENT Sky Frequency Formula: COMMENT sky = SFF_SIDEBAND*IF + SFF_MULTIPLIER*LO1 + SFF_OFFSET COMMENT Signed Sum of the LOs: COMMENT sum = -(SFF_MULTIPLIER*LO1 + SFF_OFFSET)/SFF_SIDEBAND COMMENT BANDWDTH of 0 denotes the bandpass is outside the optimal range COMMENT BACKEND: name of the terminating backend COMMENT BANK: name of the backend's set of inputs COMMENT PORT: index of the backend's input COMMENT RECEIVER: name of the receiver of origin COMMENT FEED: index of receiver RF entry point (0 indicates none) COMMENT SRFEED1: index of first FEED of a sig/ref pair COMMENT SRFEED2: index of second FEED of a sig/ref pair COMMENT RECEPTOR: name of the receiver's detector COMMENT LO_CIRCUIT: circuit producing the tracking frequency COMMENT LO_COMPONENT: component producing the tracking frequency COMMENT SIDEBAND: resulting sideband: upper or lower COMMENT POLARIZE: resulting polarization COMMENT CENTER_IF: approximate physical center frequency COMMENT CENTER_SKY: approximate center frequency on the sky COMMENT BANDWDTH: approximate resulting bandwidth COMMENT HIGH_CAL: 1 indicates a high calibrator was used COMMENT TEST_TONE_IF: approximate physical test tone frequency, if any COMMENT TEST_TONE_SKY: approximate test tone frequency on the sky, if any COMMENT TEST_TONE_CIRCUIT: circuit producing the test tone, if any COMMENT TEST_TONE_COMPONENT: component producing the test tone, if any COMMENT SFF_MULTIPLIER: Sky Frequency Formula multiplier coefficient COMMENT SFF_SIDEBAND: Sky Frequency Formula sideband coefficient COMMENT SFF_OFFSET: Sky Frequency Formula offset coefficient COMMENT TRANSFORM_COUNT: number of transform COMMENT TRANSFORMS: matrix of transform descriptions (frequencies in MHz) END
The first three columns (BACKEND, BANK, and PORT) specify the termination point of a signal path. The same values are used as identifiers in the backend files respectively as INSTRUME, BANK and PORT.
The next seven columns (RECEIVER, FEED, SRFEED1, SRFEED2, RECEPTOR, LO_CIRCUIT and LO_COMPONENT) specify the origin of a signal path. The value of RECEIVER is the same as for INSTRUME in the primary HDU of the calibration FITS files, and the keywords FEED and RECEPTOR are used in the RX_CAL_INFO binary tables of the calibration FITS files. If FEED is part of a sig/ref pair, i.e., it may be used as part of nodding or beamswitching, then the indices of the two FEEDs are given by SRFEED1 and SRFEED2, otherwise they are set to 0. The keywords LO_CIRCUIT and LO_COMPONENT identify the oscillator used for Doppler tracking and/or frequency-switching.
The following six columns (SIDEBAND, POLARIZE, CENTER_IF, BANDWDTH, and HIGH_CAL) describe the signal itself. The values for SIDEBAND can be upper (`U') or lower (`L'). The values for POLARIZE can be unknown (`U'), linear X (`X'), linear Y (`Y'), right circular (`R'), or left circular (`L'). The values of CENTER_IF and CENTER_SKY are the center frequencies of the nominal band pass of the signal for the IF itself and the corresponding sky frequency. The value of BANDWDTH is the nominal band pass. These values are nominal because ideal filters are used in the emulation usually at the 2 dB or half-power points. If the bandpass is outside the nominal limits of the system, then the BANDWDTH is set to zero to denote the configuration is marginal. The value of the column HIGH_CAL indicates whether the high calibration noise tube was fired during cal phases. Some receivers allow the user to select the calibration level.
The columns TEST_TONE_IF, TEST_TONE_SKY, TEST_TONE_CIRCUIT, TEST_TONE_COMPONENT describe the center frequencies and source of an optional test tone that may be added to the signal path at the receiver.
The next three columns (SFF_MULTIPLIER, SFF_SIDEBAND, and SFF_OFFSET) are coefficients represented as doubles and are used to express the relationship between an IF frequency and its associated sky frequency using either the Sky Frequency Formula or the Signed Sum of the LOs equations. The other independent variable in the equation is the tracking LO frequency (LO1FREQ) which may vary between switching signal phases and is specified in the LO1 Device FITS file (see GBT Software Project Note 006). There may also be an additional frequency offset that varies with switching phase (STATE) that is added to the sky frequency formula. The value of this offset is given by the FREQOFF column in the STATE table of the LO1 Device FITS file (see GBT Software Project Note 006).
The final two columns (TRANSFORM_COUNT and TRANSFORMS) represent a terse textual description of all the circuits/components the signal passed through and their individual effects. This description is not needed for analysis, but is useful for post mortems since it is a thorough description of the IFManager's emulation.