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PSRCHIVE user documentation: pac
1.0 PurposeThe Pulsar Archive Calibration program, pac, performs both polarimetric and flux calibration of pulsar observations. Calibration information is provided by calibrator archives, which may contain either observations of an artificial reference source or a table of model parameters that describe the instrumental response (such as that produced by pcm). To simplify the task of matching calibrators with observations, pac uses a database and selection criteria.
1.1 Supported Models
Currently, pac can perform polarimetric calibration using a number of different models of the instrumental response:
2.1 Creating the calibrator databaseUse of the calibrator database not only simplifies the process of matching calibrator archives with pulsar observations, it also reduces the amount of time spent re-loading calibrator archives in order to access the header information.
Before creating the database, all calibrator archives (including flux calibrator observations and solutions) should be gathered into one directory. Within this one directory, the archives may be sorted into sub-directories; pac will search the entire directory tree below the specified base directory.
If a single observation of the reference source extends across multiple archive files, these should be concatenated into a single file using psradd. It is not necessary to tscrunch the integrations together, as estimates from multiple integrations will be used to form a weighted mean. However, if the phases of the reference source waveforms are aligned, it is better to tscrunch and produce a single integration with high signal-to-noise ratio.
After gathering the calibrator archives, run pac -wp $MYCALS, where $MYCALS is the path to the base of the directory tree that contains the calibrator observations. By default, pac will search for calibrator archives with the following file extensions: cf, pcal, fcal, pfit, and fluxcal. If the calibrator archives have different file extensions, these can be specified using the -u .myext command-line option; the extension may be any value except for txt. The dot preceding the extension name is optional.
When completed, pac will output the database in the text file, $MYCALS/database.txt. The amount of time required to run will depend upon the number and types of calibrator archives in the directory.
Example: Specify ExtensionAll of the calibrator archives have been gathered into the directory /psr/data/calibrators, this is the current working directory, and all of the files have the extension, .fb:
pac -w -u fb pac: Generating new calibrator database pac: Writing database summary file to /psr/data/calibrators/database.txt pac: Finished all files
Example: Specify FilesAll of the calibrator archives have been gathered into sub-directories of /psr/data/calibrators, this is the current working directory, and all of the files are listed in cals.ls:
pac -W cals.ls pac: Generating new calibrator database pac: Writing database summary file to /psr/data/calibrators/database.txt pac: Finished all files
2.2 Calibrating pulsar observationsBy default, pac will attempt to calibrate both the polarization and flux density of each pulsar archive supplied as a filename on the command line. In order to disable flux calibration, or to choose the applied polarimetric calibration model, use the following command line options:
-P Calibrate polarisations only -S Use the complete Reception model -s Use the Polar ModelThe
Matching CriteriaCalibration information is matched to each pulsar observation using the following criteria.
-c Do not try to match sky coordinates -I Do not try to match instruments -T Do not try to match times -F Do not try to match frequencies -b Do not try to match bandwidths -o Allow opposite sidebandsWhen multiple matches are found, the calibrator archive with an epoch closest to the time of the pulsar observation will be selected. If a match is not found for polarimetric calibration, an error message will be printed and pac will not produce any output for this file. If a match is not found for flux calibration, a message will be printed, the output file will have only its polarization calibrated, and the output filename will have the letter 'P' appended to it.
OutputFor each input pulsar archive, a calibrated archive with a similar filename will be output using the same file format as the input archive. The extension of the output filename will be replaced with calib. Currently, no check is made to ensure that the input filename does not also have this extension. To change the output filename extension, use the -e ext command line option.
If the signal-to-noise ratio appears to be correlated with the absolute gain of the instrument, then it may be useful to compensate for any artificial amplification introduced by absolute gain calibration. Otherwise, sections of data with poor signal-to-noise ratio will contribute amplified noise to the total integrated profile. To normalize the profile weight by the absolute gain, use the -G command line option. This option should be used with great care and reservation.
3.1 Known ConfigurationThe PSRFITS file format includes a number of parameters that describe the known configurations of both the front and backends. For a complete description, please refer to the PSRCHIVE/PSRFITS Basis documentation.
3.2 Polarimetric CalibrationPolarimetric transformations are represented by Jones matrices. Each calibration model produces a Jones matrix for each observed frequency channel (or sub-band).
3.3 Flux CalibrationTo perform absolute flux calibration, pac requires flux calibration solutions of the form produced by the fluxcal program.
4.0 Testing and examplesNo test outputs are yet available.
4.1 Choice of calibration modelTo use the
5.0 Known bugs and features that require implementation
Appendix A: Calibrator observationsAll of the pac calibration models require observations of a noise diode that is somehow coupled to the receptors, called a CAL, which ideally presents a 100% linearly polarized reference source.
Observations of the CAL should contain both on and off states, which can be achieved by modulating the noise diode signal with a square wave (typically using a 50% duty cycle) and folding the signal at the modulation period. That is, the mean polarization profile of the CAL should be formed by integrating the polarization statistics (e.g. Stokes parameters) as a function of modulation phase and radio frequency. The CAL should be observed with the same instrumental configuration as used to observe the pulsar (including centre frequency, bandwidth, frequency resolution, etc.).
The frequency of the square wave should be a couple of orders of magnitude less than the smallest bandwidth that you plan to calibrate. For example, if you plan to retain a frequency resolution of 125 kHz, then consider a maximum modulation frequency of 512 Hz (11.123 Hz is used at Parkes). Record for as long as is necessary to achieve a sufficiently strong integrated signal in each of the frequency channels.
A.1 Polarimetric calibrator observationsThe calibrator reference source should be observed directly before or after the pulsar observation. At Parkes, this is usually done while the antenna is pointing at least a full beam width off source; however, this isn't really necessary if the pulsar is weak. For this observation, psredit -c type filename should return PolnCal.
A.2 Flux calibrator observationsApproximately once per day, and for each instrumental configuration (e.g. centre frequency, bandwidth, receiver, etc.), a set of flux calibrator observations should be made. These observations are used to determine the flux of the calibrator reference source with respect to a "standard candle." At Parkes, the radio galaxy 3C 218, or Hydra A, is most often used. Two sets of observations should be made: