The Green Bank Telescope

GBT 4mm Receiver Project Book

Chapter 8

Data Processing

Author: David T. Frayer


The 4mm system will support the DCR, spectrometer, GUPPI, and VLB backends. The DCR backend will be used primarily for pointing, focus, and occasional tipping observations. Only one beam of the 4mm system will be used for VLB observations.


The majority of the data taken with the 4mm receiver will be spectral line observations with the spectrometer. The calibration document (pdf) provides the formulae for calibrating the data. For proper calibration, the system needs the measurements made during the calibration sequence, values from several temperature sensors, real-time estimates of the current weather conditions (tau_o and T_{atm}), and system parameters such as telescope efficiencies. The following parameters need to be saved within the sdfits file of every spectral line scan.
  • T_{amb} derived from the ambient-load sensor during the calibration sequence.
  • T_{cold} derived from the cold-load sensor during the calibration sequence.
  • V_{amb} measured during the calibration sequence.
  • V_{cold} measured during the calibration sequence.
  • V_{sky} measured during the calibration sequence.
  • T_{rx} -- receiver temperature calculated by equation 7 for the calibration sequence.
  • T_{sys} -- system temperature calculated from equation 9.
  • g_{avg} -- the average gain across the bandwidth measured from the calibration sequence (equation 11).
  • g_{nu} -- the gain as a function of frequency measured per 1~MHz [TBD] interval across the bandwidth from the calibration sequence (equation 11).
  • A -- airmass using the appropriate relationship as function of elevation.
  • tau_{o} -- zenith opacity from the weather database.
  • T_{atm} -- effective atmospheric temperature from the weather database.
  • The outside temperature near the telescope measured by one of the weather stations or another sensor.
The following set of parameters need to be saved for each session.
  • Y -- Y-factor measured in the lab.
  • A_{geom} -- system parameter for the area of the GBT.
  • eta_l -- system parameter for ohmic losses and rearward spillover.
  • eta_a -- aperture efficiency giving the effective area of the GBT (A_{effective} = eta_{a} A_{geom}).
  • eta_{mb} -- system parameter for the main-beam efficiency appropriate for extended sources larger than the beam (T_{mb} = T_{A}^{*}/eta_{mb}).
The GBTIDL processing system should support the temperature and flux density scales listed below. The velocity scales will be the same as those already supported. The default temperature scale will be T*_A. The relationships between the different temperature scales are given in the calibration document (pdf) .
  • T_A -- Observed antenna temperature
  • T'_A -- Antenna temperature corrected for opacity
  • T*_A -- Antenna temperature from the chopper wheel calibration
  • T_mb -- Main-beam temperature appropriate for extended sources
  • S_nu -- Flux density scale appropriate for point sources


The DCR will primarily be used for pointing, focus, and tipping observations. The processing these data for the 4mm receiver should be similar to other GBT receivers. For the proper calibration of science continuum data, the ancillary temperature sensor measurments, and information, and measured voltages from the clibration sequence would be needed. The calibration techniques would similar as for the spectral line observations.


The VLB data processing is done in Soccoro. The 4mm system will provide a table of T_{sys}, gains, and opacity values as a function of time for the VLB backend.


Although the demand for GUPPI on the 4mm receiver is not clear, it has been recommended that we try to support this backend. The new spectrometer will potentially be similar to GUPPI, so sorting out potential issues with GUPPI may help to expidite the implementation of the new spectrometer.

See the Calibration Document for the GBT 4mm Receiver for additional details (pdf)


The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Last updated 27 October 2010 by Roger Norrod