Low Frequency Strategy

This document is intended for observers planning VLA observations at low frequencies, specifically L-band (1–2 GHz), S-band (2–4 GHz), C-band (4–8 GHz), and X-band (8–12 GHz). These four receiver bands share at least some of the same problems and solutions, as compared to higher frequency bands. Detailed guidelines for calibration are given in the Calibration section of the VLA observing guide. For an instrumental overview, general performance, and some specifics of receiver band (e.g., sensitivity, etc.) of the VLA, consult the current Observational Status Summary (OSS).

 

Observing Considerations

 

Receiver band changes; switching to and from P-band

The OPT accounts for a default 20 seconds to switch between bands (subreflector rotation and focus) which is typically sufficient. However, when switching to/from P-band and L-band or S-band an additional 15s is needed which currently is not accounted for in the OPT. Even when there is no actual telescope slew motion when the switch involves observing the same source, the Reports tab will only show 20s "slew" overhead. For these band switches please be aware of this extra overhead and allow a minimum of 35 seconds on source for the scan following the receiver band switch. When switching to/from P-band to other bands than L or S, the 20 seconds overhead is adequate.

Radio Frequency Interference (RFI)

RFI is a major issue at L, S, and C-bands and, to a certain extent, at X-band. More information about RFI can be found in the VLA RFI page. Here we note that S-band (2–4 GHz) is subject to very strong RFI from a number of satellites, in particular those providing satellite radio service. A satellite passing through the VLA beam during the initial slew may play havoc with the attenuators. To properly set up S-band observations, please refer to Special Case I in the 8/3-bit Attenuation and Setup Scans page. To set up C- and X-band 3-bit observations near the Clarke belt, please review the Low Frequency Observations in the Presence of Strong RFI section in the 8/3-bit Attenuation and Setup Scans page.

Note: C-band is also impacted by strong RFI caused by microwave links near 6 GHz in the A and B configurations. As a result, 3-bit data obtained with the standard setup are corrupted. We advise observers to use a setup with mixed 3-bit and 8-bit samplers. Refer to the C-band Observations in the Presence of Strong RFI Microwave Links section for more details.

Solar Activity

High solar activity results in emission that can severely affect the data to such a degree as to potentially render the observations useless. Also, such activity causes disturbing ionospheric effects. Therefore, it is imperative to avoid observing at L- and S-bands during daytime (including sunrise and sunset†) at times of high solar activity, i.e., when solar flares occur (which happens more often during solar maximum). Solar flares with as much as a million Jy at L-band with narrow angular extents are a source of major interference. These flares are equivalent to bright unresolved sources with time-varying flux densities making 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.

Even the quiet Sun can pose problems for low-frequency observations, resulting in degraded image quality if the Sun is too close to the science target. The Avoiding the Sun section gives guidelines on how far sources should be from the (quiet) Sun as a function of VLA observing band and configuration.

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 monitoring the activity of the Sun. If you do not want your SB(s) executed on the array because of 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 Space Weather 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 L- and S-bands during daytime (including sunrise and sunset) at 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. 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. 

 

Calibration

A number of calibrator observations are needed for the subsequent data reductions. These usually include: flux density scale calibrator, complex gain (phase and amplitude gain) calibrator, and polarization calibrators (if polarization is part of your science objective). You will also need to observe a bandpass calibrator to correct for the delays as well as the relative gains of the spectral channels, even if the observations are intended for continuum science. It is recommended that the flux density scale calibrator be observed at least once in a scheduling block (SB). The flux density scale calibrator itself, if desired, can be used to correct for the bandpass and antenna based delays, considering that the standard VLA flux density calibrators are strong sources at L, S, C, and X-bands. 

Absolute Flux Density Scale

Observe one of the standard VLA flux density calibrators to achieve absolute gain calibration: 3C286, 3C147, 3C48, 3C138**. The calibrator 3C295 may also be an option, however its use should be limited to L-band (or lower) frequencies and only in C and D array configurations.

** The flux density scale calibrator 3C138 is currently undergoing a flare. From VLA calibration pipeline results, we have noticed that 3C138 is deviating from the model. The amount of this deviation is still being investigated by NRAO staff, but does seem to effect frequencies of 10 GHz and higher. At K and Ka-bands the magnitude of the flare is currently of order 40-50% compared to Perley-Butler 2017 flux scale. If you care about the flux density scale of your observations above 10 GHz, monitoring datasets are publicly available in the archive under project code TCAL0009, from which you may find an updated flux density ratio to use for your data.

Bandpass (and Delay)

For the VLA, it is essential to calibrate the spectral response of each correlator mode used, even for purely continuum projects. The requirements for bandpass calibration, however, are very dependent on your science goals/type of observation. If you are observing spectral lines, ensure you have a strong enough calibrator in order to perform bandpass calibration. For the VLA low frequency bands, this calibrator could be the same source as the flux density scale calibrator. However, this will still depend on the specifics of your observations. 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)

Complex gain calibrators need to be observed both before and after the target observation(s). In the choice of the complex gain calibrator, obviously a calibrator close to your target source will decrease slewing times. However, for the L, S, C and X-bands, we recommend choosing a strong P calibrator farther away, rather than a weak nearby S calibrator. See the VLA calibrator manual for a description of the calibrator codes P and S. It is highly recommended that the complex gain calibrator be within 15° of the target source(s).

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

Cycle Time

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

Information on polarization, including the most commonly used polarization calibrators, can be found in the Polarimetry section. (Please note that avoiding daytime observations is even more critical for polarization observations, especially during peak solar activity, due to ionospheric Faraday rotation.)

 

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

Instrument Validation

SB Validation

 

Please submit any questions to the NRAO Helpdesk.

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