Very Low Frequency Strategy

by Tony Perreault last modified Jun 16, 2020 by Frank Schinzel

This document is intended for observers planning VLA observations at very low frequencies, and in particular at P-band (230–470 MHz). Polarization observations with P-band are currently offered through Shared-Risk Observing (SRO). The 4-band (54–86 MHz) system is currently available for continuum observations through SRO as well while it is undergoing commissioning. Joint 4-band observations with the Long Wavelength Array in New Mexico (ELWA) are only available through the Resident Shared Risk Observing (RSRO) program as described in the VLA Observational Status Summary (OSS). Observing time to use the ELWA 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 observations at very low frequencies, 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. Figure 6.3.1 shows 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.

At 4-band, narrow-band RFI is generally not a major issue since most high-power analog TV channels have disappeared with only low-power TV stations transmitting below 100 MHz. Note, the presence of a self-generated RFI comb peaking at 60 MHz with a spacing of 5 MHz at antennas with old antenna control units (ACUs). This issue will improve over time with more antennas having the new generation ACUs. The comb can easily be excised, affecting only a small number of channels. Another hard to detect source of RFI, of particular concern in more compact configurations, is produced by arcing on powerlines. This produces primarily bursts of broad-band noise and can significantly affect the sensitivity of antennas near overhead powerlines and create spurious high cross-correlations. While we try to address powerline issues as they arise, the RFI environment at the VLA is still being evaluated on an ongoing basis.

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 very low frequencies 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 or Local Ionospheric Weather from LWA.
  • 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 4/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

There are three different instrument setups available to observe with the VLA at 4 and P-band:

  • P-band setup: 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.
  • 4-band setup: Provides 8 subbands from the B0/D0 IF pair, each 4 MHz wide, to cover the frequency range 54-86 MHz, with a channel resolution of 15.6 kHz. The correlator integration time of this setup is 2 seconds with the 8-bit samplers.
  • 4&P-band setup: This combined the two setups from above to observe 4 and P-band simultaneously.

Requantizer Setup Scans

We strongly recommend using requantizer gain level setup scans in very low frequency 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 4 & 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.  P-band models are available for these sources, or at minimum a subset of, 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 each correlator mode used, even for purely continuum projects. For P-band observations it is necessary to use a calibrator that is > 40 Jy to properly calibrate the bandpass response, this includes continuum and spectral line projects. 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. For calibration of 4-band data it is strictly necessary to observe a bright kJy-scale calibrator to solve for delays and bandpass. The by far single best calibrator is Cygnus A, however if observing at the galactic anticenter, depending on the array configuration and resolved structure, Taurus A or Virgo A might be suitable as well. For more details, refer to the Bandpass and Delay Calibration in the Calibration section, or the Spectral Line section.

Complex Gain (phase and amplitude gain)

Considering the large field of view of very low frequency 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 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.

For information on rapid phase calibration and the Atmospheric Phase Interferometer (API), refer to the VLA OSS. More details regarding cycle times may be found under the Calibration Cycles of the Calibration section.

Polarization

Polarimetric observations are available for P-band only under SRO. The details for polarimetric calibration are currently being commissioned. Guidance on observation setup and calibration using AIPS is documented in EVLA Memo #207.

At 4-band due to the nature of the dipole response, the sensitivity and beam-shapes are vastly different between the two polarizations. Thus, only continuum imaging is currently commissioned below 100 MHz. It is available to perform polarimetric observations at 4-band through RSRO proposals.

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

Instrument Validation

SB Validation

Please submit any questions to the NRAO Helpdesk.