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gbt2class: Generating CLASS binary files from raw GBT data (beta release)

1. Introduction

1.1 CLASS

CLASS (Continuum and Line Analysis Single-dish Software) is a software package for reducing spectroscopic data, and also continuum drifts obtained on a single-dish telescope. CLASS supersedes the older LAS program which was restricted to spectroscopic data.

CLASS is divided in different parts, called ``Languages'', which have somewhat independent functions. Language LAS contains all the general utility functions to handle the data structure, plot the spectra or drifts and calibrate them. Language ANALYSE contains functions to analyse calibrated spectra in more detail. Language GTVL includes commands for basic graphic actions performed by the graphic library used by CLASS ; the same library is used by GreG.

CLASS is installed in Green Bank but training, bug fixes, and other support for CLASS itself is not provided at this time.

1.2 gbt2class

gbt2class is the filler which converts GBT FITS data into the CLASS binary format. It is currently in its first stages of development, and is being tested internally by the software group. Although CLASS itself is not supported in Green Bank, the CLASS filler, gbt2class, is supported. Users are encouraged to make requests to ClassBetaRequests.

2. Processing Steps

• Read GBT FITS files into Python data structures to obtain a complete description of an observation on a scan-by-scan basis.
• If necessary, convert the velocity convention to radio.
• For ACS data, do the necessary FT with appropriate corrections (e.g. Van Vleck) to convert from lag space to a spectrum
• If requested, perform a limited calibration of the data using the same code used by sdfits.
• Write the header and data into the CLASS binary output file.

3. Observing Modes Currently Supported

IMPORTANT: Currently gbt2class supports one observing mode: Total power (OffOn or OnOff) ACS observations. The data are not fully calibrated even in this case. The user must complete calibration of the data inside of CLASS (see below). Progress is being made on filling frequency switched scans, and at this time the data from such scans can be seen in CLASS but the phase description in the header is incomplete.

3.1 Backends

• Spectrometer/ACS: Limited support. Total power scans taken in position switching mode can be filled into a CLASS binary file, but calibration of the data is the responsibility of the user. Frequency switched scans are available with (S-R)/R calibration only. All other spectrometer modes are not yet supported.
• Spectral Processor: Not currently supported. Comments on this issue are encouraged (ClassBetaRequests).
• DCR: Not currently supported. Comments on this issue are encouraged (ClassBetaRequests).

3.2 Calibration

In the default mode ( -mode = cal), ACS scans are filled into CLASS binary files with an individual record entry for each integration, and for each ACS sampler. The Telescope keyword in CLASS is used to identify the record. An example Telescope entry might be "GBT-A9-0", which indicates sampler A9 and the first integration (starting the counting at 0).

Calibration in gbt2class currently shares code with the sdfits program. Calibration of total power data (two phases per integration) is done on an integration basis, and reduces the two phases into a total power bandpass spectrum. (S-R)/R*Tsys and all subsequent calibration must be done by the user in CLASS. For frequency switched data, class does not yet enter header values properly in the CLASS structure but a rudimentary calibration is available, again on an integration basis.

• Scans will be entered into CLASS as separate integrations for each sampler of each bank of the Spectrometer. Each integration is calibrated over all phases.
• The same calibration strategies that are used to produce GBT SDFITS (SdfitsBeta) are also used here.
• In the above mode of calibration, there can be several entries in the CLASS file for one GBT scan. The current labeling scheme uses the Telescope header variable, with the first three characters, 'GBT', followed by '-', the sampler name (bank + port), '-', and then finally the integration index. ( ex: 'GBT-A13-2' ).

A workaround has been developed by visiting Astronomers for this problem using DISH. Too see instructions for how they did this for their observations, see Calibrating with DISH

4. Known Issues

• No beam information written to class binary.
• Frequency switched scans: currently, the FOLD command will not work on a frequency switching scan. For now, one can use the MODIFY FSWITCH command to enter in your phase info for that scan, and then use the FOLD command. (ex: MODIFY SWITCH_MODE f 2 0.0 0.0 1.1 1.1 2.2 2.2 )
• Regardless of which velocity convention was specified when taking data, gbt2class writes all data using the radio velocity convention, making appropriate conversions when necessary. Be sure to check your velocity axis carefully.
• Sampler names used in entry name, rather then more meaningful labels, such as polarization & beam.
• Although data from multiple IFs are filled, only the first IF is properly doppler tracked.
• Nonzero sky frequency offsets are not yet handled. That is, if you ran config_tool with a nonzero g.deltafreq then you must adjust your frequency scale in CLASS appropriately.
• 'Fi' Class header variable (Frequency of Image) not understood properly, left at zero.
• Rest frequency of reference channel is valid as long as there is only one line of interest. In the case of multiple IFs, where the two IFs are centered on two different lines (HI and OH for example), only the line whose rest frequency was entered in GO will be converted correctly. Use the -frequencies option to manually set this during writing of Class binary.
• Most calibration header variables are left at 0.0, even ones we could retrieve from Engineering FITS files (like Tatm). Safest approach for the time being.
• No measures yet of application's speed.
• Possible floating point errors: some quantities (such as rest frequency) show problems (ex: 1655.4081 is written to binary class file as 1655.40815532)
• Frequency info from each spectrum cannont be found in Class, only the rest frequency.
• RA Mapping Offsets should be regarded with caution. We have implemented an RA offset scaled by the secant of the observed position rather than the secant of the map center, but this decision was made arbitrarily and we are not sure what CLASS expects in this field. This issue probably only afffects large maps near the pole.

5. Class Users

If you are a CLASS user and would like to analyze some of your GBT data with CLASS, please let us know and we will add you to our ClassUsersGroup.

6. References

• UsingClass
• CLASS Manual (HTML)

Core Team

-- NicoleRadziwill - 25 Feb 2004