A new project was started this quarter to impliment improved calibration methods for GBT observing and data reduction. The project team includes Toney Minter, Jim Braatz, Ron Maddalena, Bob Garwood and Paul Marganian. The project will impliment vector calibration as its first priority.

Typical calibration involves assuming a constant value for the noise diode brightness temperature (Tcal) and the system temperature (Tsys) when calibrating data. With the GBT's sensitivity and wide bandwidths, the assumption that a scalar value for Tcal and Tsys can be used for the entire bandpass is no longer valid. This is the source of a significant fraction of the GBT baselines. Test observations with frequency dependent Tcal and Tsys vectors are shown in Figure 1. On-Off observations of 3C 123 were performed so that frequency dependent Tcal values could be measured against this flux calibrator. The "vector Tcal" values were then used to calibrate On-Off observations towards NGC 7027. The NGC 7027 "vector calibrated" data are shown as the green line in Figure 1 while the expected flux (from Ott, et al., 1994, A&A, 284, 331) is shown as the black line. Two separate bandpasses of 800 MHz bandwidth with 400 MHz of overlap are shown. As can be seen the NGC 7027 data from the two bandpasses align exceedingly well and the measured flux is consistent with the Ott, et al. flux. This strongly suggests that vector calibration will be able to produce absolute calibration of source fluxes.

-- ToneyMinter - 06 Dec 2007 * NGC7027-3C123Scal-absoluteCalibration.ps: NGC7027-3C123Scal-absoluteCalibration.ps
Topic revision: r1 - 2007-12-06, ToneyMinter
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