Additional Resources

The previous subsections on resources were dealing with resources defined to do the scientific astronomical observations you proposed for. However, to get the most out of your data, it sometimes is helpful to add some specialized scans to the SB in order to optimize the observations or to aid in the calibration of the instrument. Typical for high frequency (higher than ~ 15 GHz) are pointing scans and tipping scans. The observing mode for such scans (pointing or tipping) is selected at the scan level in the OPT. For pointing scans, typically one would use resources that are different from your scientific observation resources, e.g., a different bandwidth, correlator setting, or even a different observing band. We have added some of these resources to the NRAO defaults catalog, available to the OPT at the scan level or to copy/paste to your personal resource catalog.

Pointing scans are used to improve telescope pointing accuracy which increases the sensitivity of the observations. As the instantaneous telescope pointing is only accurate to several arcseconds, this error may become a considerable fraction of the primary beam at high frequencies. Solving for this error is done using primary pointing scans on a strong source at X band, after which a secondary pointing may be performed at the observing frequency if deemed useful. The actual pointing action is selected as Interferometric Pointing under scan mode in the scan details (see OPT), which may use the resources named Pointing presented in the Pointing setups resource group in the NRAO defaults resource catalog.

Tipping scans are used to obtain a measurement of the atmospheric opacity at high frequencies, which allows for an estimate of the loss of sensitivity due to absorption of emission from the source of interest by the atmosphere. The actual telescope tipping action is selected as Tipping under scan mode in the scan details (see OPT). Because you typically want to do tipping scans at your observing frequency you would either use resources from the NRAO defaults catalog or you would reuse your own resource at the frequency you want; no new resources are needed.

Resources at 4, P, and U band: Currently there are no receivers at 4, P and U band (75 MHz, 300 MHz and 15 GHz) on the array. For U band we recommend to use the lower part of the K band receivers (around 19 GHz) instead. This should have been proposed for at the proposal stage. No observations at 4 and P band are anticipated for some time.

Resources at S band: S band is currently deployed on the array. It is likely that if you proposed for observations at S band, not all antennas will be outfitted with S band receivers. At this moment it is not possible to use the other antennas in a different subarray. Commissioning of subarrays with WIDAR is not planned for some time.

Resources at X band: Until the end of the EVLA construction, around late 2012, the VLA type X band receivers will be kept on the array. This means that there is limited X band tuning, only from about 8.0 to 8.8 GHz. Consult the Observational Status Summary or the NRAO eNews web pages for the latest news on this receiver upgrade.

Resources at K band: There is an issue with specifying the frequency of output pair AC at Ka band. That is, tuning any part of the AC output pair band below 32.24 GHz will not result in valid data. Only the BD output pair can be tuned to frequencies below 32.24 GHz; use the BD output pair instead of AC output pair when you only need one output pair for your resource with a frequency tuning below 32.24 GHz. If the OPT web application validation detects that any part of the bandwidth of output pair AC is tuned below this 32.24 GHz it will try to swap the AC output pair with the BD output pair. If this is not possible, it will issue an error (in red font) in the interface feedback strip if this frequency is specified as a fixed sky frequency. It will issue a warning (blue font) for rest frequencies, as the particular tuning depends on the details of observing date, telescope pointing direction and source velocity definitions. Note that a rest frequency above 32.24 GHz may shift to below 32.24 GHz once it is assigned to a scan in the OPT. This should give you an error in the OPT; you should be aware of this possibility and pay attention to this. However, it is better to assign output pair BD to the resource if you anticipate this might happen, if you still have this freedom in your resource of course.

The very wide bandwidth of the Ka band receiver, from 26.5 to 40 GHz, would suggest that output pair separations of up to ~ 13 GHz are possible. Restrictions in the signal path, however, limit this separation to 10.5 GHz. The OPT web application validation will issue an error if the separation between output pairs AC and BD is more than 10.5 GHz in sky frequency (with output pair AC tuned the higher frequency one). A separation of more than 10.5 GHz in rest frequency will result in a warning as, e.g., highly red-shifted lines may end up with less separation when the actual sky frequencies are calculated.