High Frequency Observing (K, Ka, Q)

Current->Revised OSS Guidelines

• Sensitivity
  • SEFD listed at 1400 Jy

• EVLA Frequency Bands and Tunability
  • In general, for all frequency bands except Ka, if the total span of the two independent IFs (defined as the frequency difference between the lower edge of one IF pair and the upper edge of the other) is less than 8.0 GHz, there are no restrictions on the frequency placements of the two IF pairs. For Ka and Q bands (the only two bands where a span greater than 8 GHz is possible), there are special rules:
    • At Ka band, the low frequency edge of the AC IF must be greater than 32.0 GHz. There is no restriction on the BD frequency.
    • At Q band, if the frequency span is greater than 8.0 GHz, the BD frequency must be lower than the AC frequency.

• Calibration and Flux Density Scale
  • Accurate flux densities can be obtained by observing one of 3C286, 3C147, 3C48 or 3C138 during the observing run. Not all of these are suitable for every observing band and configuration – consult the VLA Calibrator Manual for advice. Over the last several years, we have implemented accurate source models directly in AIPS and CASA for much improved calibration of the amplitude scales. Models are available for 3C48, 3C138, 3C147, and 3C286 for L, C, X, Ku, K, and Q bands. At Ka band either of the K or Q band models works reasonably well.

Note: Must update OSS Flux densities of Standard Calibrators

• General Guidelines for Gain Calibration
  • If your target object has sufficient flux density to permit phase self-calibration, there is no need to calibrate more than once hourly at low frequencies (L/S/C bands) or 15 minutes at high frequencies (K/Ka/Q bands) in order to track pointing or other effects that might influence the amplitude scale.

• Rapid Phase Calibration and the Atmospheric Phase Interferometer (API)
  • For some objects, and under suitable weather conditions, the phase calibration can be considerably improved by rapidly switching between the source and calibrator. Source-Calibrator observing cycles as short as 40 seconds can be used. However, observing efficiency declines for very short cycle times, so it is important to balance this loss against a realistic estimate of the possible gain. Experience has shown that cycle times of 100 to 150 seconds at high frequencies have been effective for source-calibrator separations of less than 10 degrees. For the VLA this was known as “fast-switching.” For the EVLA it is just a loop of source-calibrator scans with short scan length. This technique “stops” tropospheric phase variations at an effective baseline length of ∼vat/2 where va is the atmospheric wind velocity aloft (typically 10 to 15 m/sec), and t is the total switching time. It has been demonstrated to result in images of faint sources with diffraction-limited spatial resolution on the longest EVLA baselines. Under average weather conditions, and using a 120 second cycle time, the residual phase at 43 GHz should be reduced to ≤ 30 degrees. Further details can be found in VLA Scientific Memos # 169 and 173. These memos, and other useful information, can be obtained from Reference 12 in Documentation.
  • Note that the fast switching technique will not work in bad weather (such as rain showers, or when there are well-developed convection cells – most notably, thunderstorms). An Atmospheric Phase Interferometer (API) is used to continuously measure the tropospheric contribution to the interferometric phase using an interferometer comprising two 1.5 meter antennas separated by 300 meters, observing an 11.7 GHz beacon from a geostationary satellite. The API data can be used to estimate the required calibration cycle times when using fast switching phase calibration, and in the worst case, to indicate to the observer that high frequency observing may not be possible with current weather conditions. A detailed description of the API, and instructions for accessing its data, may be found at http://www.vla.nrao.edu/astro/guides/api/.

Detailed Guidelines

High Frequency Observing Draft by C. Brogan

Observing Preparation Recommendations

Scheduling

The scheduling of high frequency projects is dependent on the weather conditions. The two quantities currently used to determine if atmospheric conditions are suitable are the wind speed and the API rms phase. The fraction of time that both quantities are satisfactory depends on the time of year and time of day. A good summary of the monthly conditions at the EVLA as a function of LST is at this page: Monthly_Conditions_at_EVLA.

Calibration Strategy

Known RFI

Calibrator Catalog and Selection

Monitoring Observations

Post-processing Guidelines