Calibration of GBT data

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8.3.4.4 Calibration of GBT data

Data from the GBT are uncalibrated and come as sets of integrations representing the different phases of a calibration cycle (e.g., on source, calibration on, on source, calibration off, on reference, calibration on; on reference, calibration off). Currently, there are a number of routines emulating the standard GBT calibration (in GBTIDL):

calps - calibrate position switched data
calfs - calibrate frequency switched data
calnod - calibration nod (beam switch) data

All these routines calibrate the spectral data to antenna temperature adopting the GBT calibration method as described in the GBTIDL calibration document available at:

http://wwwlocal.gb.nrao.edu/GBT/DA/gbtidl/gbtidl_calibration.pdf

There are two basic steps:

First, determine system temperature using a noise tube calibrator (sd.dototalpower())

For each integration, the system temperature is calculated from CAL noise on/off data as:

T_sys = T_cal × ---<refcaloff>-----
<refcalon-refcaloff> + Tcal
2

ref which refers to reference data and the spectral data are averaged across the bandpass. Note that the central 80% of the spectra are used for the calculation.

Second, determine antenna temperature (sd.dosigref())

The antenna temperature for each channel is calculated as:

T_a(ν) = T_sys × sig(ν)-ref(ν)
ref(ν)

where sig = 1
2 (sig_calon + sig_caloff), ref = 1
2 (ref_calon + ref_caloff).

Each calibration routine may be used as:

  scans=sd.scantable(’inputdata’,False)         # create a scantable called ’scans’
  calibrated_scans = sd.calps(scans,[scanlist]) # calibrate scantable with position-switched
                                                # scheme

Note: For calps and calnod, the scanlist must be scan pairs in correct order as these routines only do minimal checking.

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