20-Meter Data Analysis with GBT Software


The first trial of the 19-element, cryogenic PAF on the GBT produced a Tsys/eta (eta = aperture efficiency) that is considerably higher than expected from earlier results of the same PAF of the 20-meter telescope. In addition to a very different telescope geometry, the GBT data were recorded and analyzed by a new suite of computer code so we find it necessary to build a software bridge between the 20-meter data and the GBT analysis code so that the two data sets can be compared. This web page documents the code bridge.

Data File Conversion

The original raw ADC data files from the twenty meter used file names such as 3c48xelapr2901off010101.dat and g11x11apr2901off080101.dat and were stored in directories that were named by the recording date, such as 04292011. The GBT file naming scheme combines the recording date, time, and two-digit serial number into the file name, e,g,, 2013_12_23_11h27m31_01.dat. The 20-meter log files were archived, and the data recording time and date from these can be combined to create new file names in the GBT format.

Each data scan creates ten raw data files, one from each of the 10, 4-channel ADC cards. All ten files are stored with the same name but in different directories who's names specify the ADC card numbers. However, only collated data from the 20-meter were archived. Fotunately, the collation format is the same for the 20-meter and GBT so the collated files may be used with only a name change.

The original GBT data were archived on one of two large RAID drives. The individual data files from each of the ten ADC computers are in the directory paths

/export/raid[1|2]/pafdata/[data session name]/paf[01-10]/

e.g., /export/raid1/pafdata/TGBT13B_905_07/paf01

Since the individual files from each ADC computer were not saved for the 20-meter data, these directories are omitted. The collated data from the GBT are in directories with the path name

/export/raid1/[data session name]/AGG

For the twenty-meter data we'll use the directory names

/export/raid2/T20M_042911/AGG and


to reflect the month, day, and year that the data were recorded. The 042911 data are from the 20-meter and the 061411 data are hot/cold load measurements. The 20-meter data contains values for only X-polarization dipoles (ADC channels 1 and 2). For compatibility with GBT data the 20-meter data will be converted to 4-channel format with zeros in the Y-polarization data (ADC channels 3 and 4).

The details of transferring an aggregated data file from the 20-meter archive to the GBT file system and renaming it are handled by the python scripts, twm_log.py, twm_hc_log.py, and copy_agg_files.py. The first two scripts scan the original 20-meter data acquisition logs and retrieve information that is required to generate new files in the raw data format generated by the GBT PAF system. This conversion involves copying the data to new files that use the GBT date/time file naming convention and adding zeroes to the new files at the Y dipole datta locations. This conversion process created three log files (TwentyMeter_01.log, TwentyMeter_CL.log, and TwentyMeter_HL.log) that list the old and new file names for each file along with some telescope pointing and measurement frequecy information, where applicable. The python scripts and log file are on the Green Bank file system at


In this same directory is a script, make_20m_rcvr_fits.py, for generating receiver FITS files for the 20-meter data in the directory


These receiver fits files are then used to generate correlation FITS files in the directory