Specific Resource Details

by Gustaaf Van Moorsel last modified Jan 27, 2017 by Lorant Sjouwerman

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 pointing resource in C band is not recommended). 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. It is important to use these pre-defined setups as they typically use a different frequency, bandwidth and integration time than the other (default) resources.

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. Tipping scans are currently disabled.

Resources at Ka band: There is an issue with specifying the frequency of IF pair AC at Ka band. That is, tuning any part of the AC IF pair band below 32.24 GHz will not result in valid data, regardless whether this is A0C0 or any of A1C1 or A2C2. Only the BD IF pair can be tuned to frequencies below 32.24 GHz; use the BD IF pair instead of AC IF pair when you only need one IF 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 IF pair AC is tuned below this 32.24 GHz it will try to swap the AC IF pair with the BD IF 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 IF pair BD to the resource if you anticipate this might happen, if you still have this freedom in your resource of course.

When observing very close to 32 GHz with both IF pairs, some combinations where frequency coverage of AC and BD are overlapping are not possible. Consult Helpdesk for options, preferably before submitting the proposal.

The very wide bandwidth of the Ka band receiver, from 26.5 to 40 GHz, would suggest that IF pair separations of up to ~13 GHz are possible. Restrictions in the signal path, however, limit this separation to 10 GHz. The OPT web application validation will issue an error if the separation between IF pairs AC and BD is more than 10.5 GHz in sky frequency (with IF pair AC tuned to have the higher frequency centers). 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.

Resources at K and Q band: If you choose the RF signals in the different IF paths to be separated by a large amount, it is possible that the OPT will only let you create a resource where the baseband frequency center(s) in IF pair AC is higher than the baseband frequency center(s) in IF pair BD, similar, but the reverse of the Ka band restriction above.