GBT Calibration Web Page
This page contains information related to the calibration of GBT data. For frequencies less than 60 GHz, the receivers on the GBT have noise-diodes used to track the gain of the system. The effective strength of these noise diodes as measured through the system can drift slowly over time and require updating on the time scale of every 6 months to 2 years to maintain calibration to within 15%. The observatory has been admittedly negligent in tracking the calibration of the noise diodes, and we recommend that users always take their own calibration data for their individual observing programs.
Beginning for semester 18A, the observatory has started to update the noise-diode TCAL tables for all of our receivers. The new values are based on astronomical measurements and are tied to the VLA absolute flux density scale (Perley & Butler 2017, ApJS, 230, 7).
TCAL Calibration Files
The updated TCAL files along with the previous version of the files for each of the receiver bands are provided below. The files are named with the date of the observations used to derive the calibration data. The date for the database update is when the GBT system started using the file for GBT observations.
The "ps" files provided within the above links show the comparison plots of the latest values compared to the previous version (the latest values are plotted with a solid line and the previous values are shown by the dotted line).
Instructions for Using a Different TCAL Calibration File
The calibration information from the TCAL calibration fits files are loaded into the SDFITS file by sdfits. Users could use a different file by replacing the TCAL calibration file under the receiver directory for the raw GBT data and re-running sdfits.
Users who decide to simply scale their results by the ratio of the new/old TCAL values for their frequency of interest are warned that this will not yield the optimal results for the accumulation of data of both polarizations.
It is not recommended to blindly adopt TCAL calibration values that are more than 1 year away from their target observations. Users are encouraged to collect thier own calibration data for their observing programs.
GBT 15A486 Calibration Program
In 2015 and 2016, we observed the standard calibrators to check the calibration of the GBT using the current noise doides calibration values in comparison with the VLA calibration scale (Perley & Butler 2013). The results from this work demonstrated the need to re-measure the noise diodes on the GBT.
GBT Data Archive
GBT archival data can be retrieved via the web-based NRAO/GBT Data Archive Accesss Tool. Recent data from ongoing programs can be found on the GBO disk (e.g., /home/gbtdata and /home/sdfits) and the recent VEGAS data are stored on the lustre file system.
GBT Data Reduction Documentation
If you have questions, contact David Frayer.