Very Low Frequency Strategy

by Tony Perreault last modified Jun 23, 2017 by Emmanuel Momjian

This document is intended for observers planning VLA continuum observations at very low frequencies, and in particular at P-band (230–470 MHz). Spectral-line and polarization observations with P-band are currently offered only through the Resident Shared Risk Observing (RSRO) program as described in the VLA Observational Status Summary (OSS). Therefore, topics that relate to P-band polarimetry and spectroscopy are not covered in this section. Furthermore, the 4-band (54–86 MHz) system, which is currently available on approximately half of the antennas, is also excluded because it is currently under development. Observing time to use the 4-band may be requested through the RSRO program.

Important Notice:

The very low frequency bands (P and 4) of the VLA provide linear polarization, while all the other bands (L to Q; from 1 to 50 GHz) provide circular polarization. Whereas CASA version 4.6 and later in principle can load those data sets into a single measurement set for processing, it is strongly recommended to put the linearly polarized data in a measurement set of its own, different from the circularly polarized data, and process the 4 and P-band data separately from the other bands. In CASA convert the SDM-BDF to a measurement set, then use split to separate the linearly polarized data. When using BDF2AIPS in AIPS, the data will always be split per band for separate processing.

 

Observing Considerations

While preparing P-band observations, the following issues need to be kept in mind because they can have severe consequences:

Radio Frequency Interference (RFI)

RFI is a major issue at P-band (list of P-band RFI). Of the 256 MHz frequency span that P-band observations may cover, about 30-40% may be affected by RFI. The following images show time vs. frequency of a single short baseline. The frequency range is 224-352 MHz (top) and 352-480 MHz (bottom), and the channel separation is 125 kHz. Hanning smoothing was applied to minimize Gibbs ringing.

 

Solar Activity

While observing at P-band, the following guidelines may be followed while the Sun is quiet:

  • In general, make sure that the Sun is more than 30° away from your sources. See the Avoiding the Sun section for more array configuration dependent details.
  • In D-configuration, the solar disk will likely cause a problem on the short baselines (< 100m) if the Sun is less than 90° away from the observed source.
  • The phases are always calibratable except during solar storms.

When the Sun is active, high solar activity results in emission that can severely affect the data to such a degree as to potentially render your observations useless. Also, such activity causes disturbing ionospheric effects. Therefore, it is imperative to avoid observing at P-band at daytime (including sunrise and sunset) during times of high solar activity, i.e., when solar flares occur and which are more frequent during solar maximum. Solar flares with as much as a million Jy at P-band with narrow angular extents are a source of major interference. These flares are equivalent to bright, unresolved sources with time-varying flux densities that make it very difficult, if not impossible, to remove their effects. As a consequence, the resulting images will be of poor quality and low dynamic range.

Important Note: It is the responsibility of the observer to monitor if and when their sources will be too close to the Sun in addition to the activity of the Sun. If you do not want your SB(s) executed on the array due to a solar flare, which would take a few days to reach Earth after the occurrence of an eruption on the Sun, you should cancel the submission of your SB(s) through the OPT and resubmit them later. Do not rely on the operator to know about conditions relating to solar flares.

Some useful links:

  • Solar activity monitoring: Solar activity and general space weather can be reviewed at the NOAA site. The site provides solar activity forecasts and geophysical (geomagnetic field) activity forecasts along with GOES X-ray flux values.
  • Ionosphere monitoring: Global Ionospheric TEC maps.
  • Near real-time and archival solar activity can be seen from < 100 MHz all-sky observations by the LWA.

Note:The statement to avoid observing at P-band at daytime (including sunrise and sunset) during times of high solar activity should not translate to checking the boxes of AVOID SUNRISE and/or AVOID SUNSET in the OPT, because these will not prohibit observing at daytime during a solar flare event. Also, these options in the OPT are not meant to address or avoid solar activity, but to avoid ionospheric phase fluctuations for low frequency observations and strong temperature gradients for high frequency observations during sunrise and sunset.

Instrument Configuration

Use the default NRAO P-band setup that provides 16 subbands from the A0/C0 IF pair, each 16 MHz wide, to cover the frequency range 224–480 MHz, with a channel resolution of 125 kHz. The correlator integration time of this setup is 2 seconds with the 8-bit samplers.

Requantizer Setup Scans

We strongly recommend using requantizer gain level setup scans in P-band observations. Here we note two important sections on the use of these setup scans in 8-bit observations:

  1. See the 8-bit General Information section under the '8/3-bit Attenuator Settings and Setup Scans' page of the VLA Observing Guide for information on how to trigger the requantizer gain setup scans while using the 8-bit samplers.
  2. See the Special Case II in the '8/3-bit Attenuator Settings and Setup Scans' page of the VLA Observing Guide for the reasons to use the requintizer setup scans in P-band observations, and the current implementations in the Observation Preparation Tool (OPT) to utilize such scans.

 

Calibration

A number of calibrator observations are needed for the P-band observations. These usually include: flux density scale calibrator, complex gain (phase and amplitude gain) calibrator, and a bandpass calibrator to correct for the delays as well as the relative gains of the spectral channels even though the observations are intended for continuum science. It is recommended that the flux density calibrator be observed at least once in an observing run (a scheduling block).

Absolute Flux Density Scale

Accurate flux densities can be obtained by observing one of 3C48, 3C123, 3C138, 3C147, 3C196, 3C286, 3C295, or 3C380. Note that P-band models are not available for these sources in CASA or AIPS, therefore if a given calibrator is resolved (see the VLA calibrator list), make sure to set appropriate uv ranges while executing calibration tasks. For more details on flux density calibration, see the AIPS task SETJY or the CASA task setjy.

Bandpass (and Delay)

For the VLA it is essential to calibrate the spectral response of your chosen correlator mode, even for continuum projects. For P-band observations it is necessary to use a calibrator that is > 40 Jy to properly calibrate the bandpass response. Some of the flux density calibrators noted above can be used for bandpass calibration at P-band. The bandpass calibrator can also be used to calibrate the delays.

Complex Gain (phase and amplitude gain)

Considering the large field of view in P-band observations, it will always be possible to self-calibrate the target phases because there will be a strong source or several strong sources within the field. It is recommended, however, to observe a complex gain calibrator (flux density > 10 Jy) with a gain cal + target + gain cal cycle time of 30 to 60 minutes regardless of the array configuration for initial calibration and system monitoring purposes.

For more details on the above calibrators, refer to the Calibration section.

Before submitting a scheduling block (SB), refer to the Presubmission Checklists:

Instrument Validation

SB Validation

Please submit any questions to the NRAO Helpdesk.