Low Frequency Strategy

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

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

 

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.

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.

The quiet Sun may also pose problems for L-band observations, particularly in the short configurations (D and C). This would result in degraded image quality if the Sun is too close to the science target to within a few degrees. In the extended configurations (B and A), one can expect the effects of the quiet Sun to be reduced. The Avoiding the Sun section gives guidelines on how far sources should be from the (quiet) Sun as a function of observing band at the VLA.

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 they 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 frequencies or lower and only in C and D array configurations.

Bandpass (and Delay)

Needed for both continuum and spectral line science. Could be the same source as the flux density scale calibrator.

Complex Gain (phase and amplitude gain)

Complex gain calibrators need to be observed before and after each target observation. 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

The interval between the observations of a complex gain calibrator in a given scheduling block depends on the configuration of the array and the science goal (i.e., a detection versus a self-cal experiment). Rough guidelines for the typical time between complex gain calibrator observations are given in the table below. Note that with the long cycle times, it is recommended to make sure that the individual target scans do not exceed 10 minutes, e.g., by creating multiple, successive 10 minute scans on a single target (e.g., in a loop) to avoid loss of a lot of data in the unfortunate case when an entire scan might become corrupted.

Table 6.2.1: Low Frequency Cycle Times in Minutes(gain cal + Target + gain cal)
Array configurationABCD
Time on source(s) (in minutes) 10–15 20–25 30–40 50–60

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.