Source Calibration notes for the Pipeline
Currently (August 2009) the standard GBT IDL calibration procedures use a single (scalar) value for the Noise Diode values to calibrate spectra. We plan to switch to regular use of Reference Source based determination of the calibration values. Simultaneously we plan to begin use of a vector version of the noise diode values. These values must be smoothed and filtered to avoid introduction of additional noise and baseline features.
Ron Maddalena has made a number of experiments in SCAL processing and is writing a detailed description of the process. Ron's web page
contains references to a number of draft documents and presentations on GBT Calibration.
Glen Langston also summarized and tested accurate Source calibration techniques, based on observations of planets. These tests are described in a GBT Memo on Accurate Temperature Calibration
(GBT Memo 232 Langston et al, April 2004).
One of the main issues remaining with completion of the SCAL process is determining the optimum averaging of calibration values, to reduce the noise introduced in the calibration process. The averaging of calibration values must be balanced against excessive smoothing of the band pass spectrum, which introduces spurious features in the spectral baselines.