High Frequency Strategy

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

This document is intended for observers planning VLA observations at high frequencies, specifically Ku-band (12–18 GHz), K-band (18–26.5 GHz), Ka-band (26.5–40 GHz), and Q-band (40–50 GHz). All of these receiver bands share at least some of the same problems and solutions as compared to lower frequency bands (e.g., the need to account for antenna pointing, atmospheric phase coherence and opacity calibration). In particular, the calibration overheads for high frequency observing are typically considerably larger than for lower frequency observations, significantly impacting the overall time request. As described below (and in more detail within the Calibration section), the overheads grow with increasing frequency and maximum baseline length.

Ku-band, being the lowest of these high frequency bands, can, in some cases, be more suitably grouped with lower frequencies, e.g., when observing at or below 15 GHz only. We will point out some of those cases in this document.

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). 

 

Calibration


Antenna Reference Pointing

For high-frequency observations above ~15 GHz, the a priori antenna pointing is generally not accurate enough, and thus pointing calibration is needed.

  • Calibrate the pointing by observing a nearby calibrator in interferometric pointing mode. This calibrator should ideally be within 10° of the sky position of interest, as pointing corrections are needed for the azimuth and elevation near the target. The local pointing corrections can then be applied to subsequent scans.
  • For this, choose a strong point-like calibrator (calibrator code P or S, but never a W, X, or ?) and bright (0.3 Jy/bm or brighter) at X-band using the NRAO default resource in the OPT (X band pointing).
  • Once a pointing calibration is determined, it typically remains valid for about 20° away from the AZ/EL for which the pointing was obtained. This translates to the need of repeating the reference pointing every hour or so during nighttime, and every 30-40 minutes during daytime observing (including sunrise/sunset).
  • Often the flux density and bandpass calibrators need pointing calibrations that differ from the target pointing calibrations as they typically are far away from the target source. These calibrators commonly use pointing scans on themselves.

Note that near zenith (elevation > 80°) source tracking becomes difficult. Therefore, it is recommended to avoid such source elevations during the observation preparation setup.

For more details see the Antenna Reference Pointing Calibration guidelines located in the Calibration section.

Absolute Flux Density Scale 

In most observations, the accuracy of the absolute flux density scale is tied to the final uncertainty of whatever analysis you plan for your science target. Therefore, it is important to observe a known flux density standard with high signal-to-noise. Absolute flux density scale calibration is more difficult at higher frequencies because:

  • all of the standard absolute flux density scale calibrators are significantly weaker at higher frequencies, and;
  • they also range from being slightly to very resolved in the smallest to largest configurations, respectively.

For this reason, it is important to use a model image for the absolute flux density scale calibration in your data reduction. Models are already available in both CASA and AIPS for the standard flux density scale calibrators: 3C286 (J1331+3030), 3C48 (J0137+3309), 3C147 (J0542+498), and 3C138 (J0521+1638). Note that because 3C138 and 3C147 exhibit variability, 3C286 and 3C48 are better for absolute flux density scale calibration at the higher frequencies of the VLA.

To limit the effects of atmospheric extinction, it is advantageous to observe your flux density scale calibrator as close as possible in elevation to your target(s). This will automatically reduce uncertainties that arise from opacity and gain curve corrections. As described below, the phase fluctuates quickly at high frequencies, so in order not to de-correlate the amplitude of your flux density calibrator you will need to do a phase-only calibration that can achieve a S/N of at least 5 on a single baseline (assuming single polarization and the bandwidth of a single spectral window) in a solution interval shorter than the timescale of large phase variations (typically a few seconds at the highest frequencies/longest baselines). The required S/N for a single baseline over the full observing time of the absolute flux density calibrator should be > 20.

Following this advice, you should be able to get to an absolute accuracy of about 10%. If your science requires more detailed understanding of the absolute flux density calibration precision, it is wise to include a brief observation of a second known flux density standard if possible (this may be difficult to schedule in some cases). In this case the second source should also be observed at similar elevation to your target. Note that the VLA Calibration Pipeline will only use the first flux density calibrator seen in the schedule for use across all targets in the schedule.

For more details, refer to the Flux Density Scale Calibration in the Calibration section.

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 more details, refer to the Bandpass and Delay Calibration in the Calibration section, or the Spectral Line section.

Complex Gain

Phase

Phase fluctuations are caused by variations in the amount of precipitable water vapor (PWV) in the troposphere, as a function of time and position on the sky. This variation acts as an additional source of phase noise when observing at high frequencies.Phase calibration at high frequencies comes down to these questions:

  • Can I use self-calibration? If your target sources strong (generally 0.1 Jy over a 1 GHz frequency band, although you might be able to use somewhat weaker sources) then you can apply self-calibration to the source, and it is sufficient to observe the calibrator every 30 minutes at high frequencies.
      • Note that the slight phase shift induced by self-calibration will shift the source position in your final image. Normally this is not a serious issue. But if you need high astrometric accuracy, then self-calibration may not be a viable option.
      • if your source contains a strong maser, you can use the maser itself for self-calibration.
  • If you cannot use self-calibration:
    • How close the calibrator should be to the target? At these high frequencies, choose a calibrator as close as possible to your target source even if it is weaker compared to other calibrators farther away.
    • what is the cycle time needed to track the phases (so one can remove the variations)? This depends on the frequency and the array configuration. See the Cycle Time subsection below.

Amplitude

Variations in amplitude tend to happen on much larger time scales than phase (minutes rather than seconds). This is because, unlike phase which varies due to turbulence in the troposphere, amplitude is mostly dependent on variations in the integrated precipitable water vapor (PWV) column (i.e., atmospheric opacity). PWV changes with elevation—you look through more water column at low elevation than at high elevation—and time, as clouds with varying water content move across the array. The phase calibrator observations are typically adequate to also calibrate the amplitude variations. Since amplitude is less variable than phase, if you have a weak phase calibrator you may want to average several phase calibrator scans to obtain adequate S/N for an accurate amplitude solution. This is rarely a problem unless the weather is changing very rapidly—in which case the overall calibration is likely to be poor regardless of what you do about amplitude.

In borderline cases, where you might be able to recover some data taken during poor(ish) weather conditions, take care in post processing and be aware that:

      • Decorrelation can cause baseline-dependent amplitude variations;
      • Antennas with unrefined positions (e.g., directly after an antenna move to a new antenna pad) will cause increased problems in patchy cloud conditions because, if they are looking through different water columns, the amplitude corrections will not be well-determined. In this case it may be best to not include these antennas in the data reduction.

For more details, refer to the Complex Gain Calibration in the Calibration section.

Cycle Time

If the variations in the troposphere move across the array at about 10 m/s, then these variations will move 1.2 km in about 2 minutes. The D-configuration maximum baseline is only about 1 km in length, so the screen moves completely across the array in less than 2 minutes. Any changes in phase due to this drift will not be tracked, so a cycle time shorter than 2 minutes in D-configuration will not track the troposphere phase variations at all. Cycle times shorter than 2 minutes in the C-configuration, with a maximum baseline of 3.4 km, should provide some improvement although it may be marginal. For the B- and A-configurations, faster cycle times should provide a means of tracking phase variations due to the troposphere, but it will only correct the phase to the stability one would obtain on ~1 km baselines. A cycling time of faster than 2 minutes in the D- and C-configurations probably wastes available observing time since the troposphere phase changes cannot be tracked. For more information, refer to VLA memo #169 and VLA memo #173.

For cycle times faster than 2 minutes, which might be used in the B- and A-configurations, one is usually limited by the relatively slow slew speeds of the VLA antennas (20 degrees/minute in elevation and 40 degrees/minute in azimuth) and how close a complex gain calibrator is to the target field. Some settling time is also required. It is important to make sure that the cycle time is not so short that it results in no time on source or on calibrator. Additionally, faster cycle times may be needed at low elevations, but cycle times for low elevations have not been investigated sufficiently to give recommendations.

Given these limits, we provide the following table based on empirical experience for the cycle times at high frequencies:

Table 6.1.1: High Frequency Cycle Times in Minutes (gain cal + Target + gain cal )
BandD-ConfigC-ConfigB-ConfigA-Config
Ku (12 - 18 GHz) 20 15 7 5
K (18 - 26.5 GHz) 12 10 4 3
Ka (26.5 - 40 GHz) 9 7 3 2.2
Q (40 - 50 GHz) 6 5 2 1.5

For more 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 guidelines.

 

An Observing Strategy Consideration

Are you doing multiple high frequencies or a mix of high and low frequencies? If so, start the SB with the highest frequency first and progress to the lower frequencies through the SB. Not only are the weather conditions at the start of the observation better than the specified constraints (weather conditions may deteriorate after the start), it also allows for slewing to the pointing source during the start-up time. This start-up time includes all of the initial setup scans as well as at least 2.5 minutes on source in the interferometric pointing mode.

 

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

Instrument Validation

SB Validation


Please submit any questions to the NRAO Helpdesk.