Desired Performance

Based on the configuration information, the user should then enter desired performance parameters in the bottom panel. These will determine which configurations are required (including ACA) and how much time is required. The following fields are included: Desired Angular Resolution: A number of options are available to request the desired angular resolution or synthesized beam size i.e. the finest detail that will be visible in the map of the source. These are:
 Any value that is entered must lie in the range provided by the available configurations i.e. given in the Configuration Information panel. Unless the Stand-alone ACA was requested, the OT will select one or more 12-m configurations according to the various restrictions mentioned above. If any of these require additional (smaller) array configurations (including the 7-m and TP Array) to achieve the source LAS, only those combinations that require the smallest number of configurations will be considered. An angular resolution is never entered for VLBI proposals. Largest Angular Structure in source: The angular extent of the largest components in the source that need to be imaged. If this parameter (also sometimes referred to as the Largest Angular Scale - LAS) is smaller than the max recoverable scale of the selected 12-m configuration, smaller configurations must be added, if possible. For example, multiple 12-m configurations, the ACA 7-m and Total Power Arrays might be necessary to reliably image the indicated size of the source structure. Various restrictions exist including that the very largest 12-m configuration cannot be combined with anything else. If the source size is very small, a value of zero can be used. The OT will also issue a validation error if the LAS is larger than the field of view as defined by the arrangement of pointings or the dimensions of a rectangle in the Field Setup. This parameter is not shown for VLBI observations. Desired sensitivity: The sensitivity goal for each source. If a rectangle has been defined, this value should be what is desired in the final mosaic i.e. including beam overlaps. For “Individual Pointing(s)”, it is the value per pointing, even if a custom mosaic has been defined. For a continuum observation, this should be the sensitivity desired in the final continuum map. For a line observation, the sensitivity should be that desired in a single channel map, where one usually wants to have several channels across each line profile. If the angular-resolution choice was “Any”, the sensitivity must be entered in flux units. This is not shown for VLBI or solar observations. Equivalent sensitivity: If the requested sensitivity were in flux units then the OT will also display the equivalent in temperature units, and vice versa. Things are more complicated if a range is entered. A desired sensitivity in flux units will be converted to the equivalent temperature for the angular resolutions of the smallest and largest nominal configurations within the range. If the user-requested value is in kelvins, however, the flux-equivalent is first shown for the largest possible nominal configuration. As this flux sensitivity is what the observatory will deliver regardless of what configuration is actually scheduled (the time estimates are always based on the flux sensitivity) it is then converted into a temperature-equivalent using the coarsest requested angular resolution. Bandwidth used for Sensitivity: Specify which bandwidth should be used to calculate the time required. This is used in conjunction with the desired sensitivity and the Representative Frequency. A number of shortcuts are possible or a user-defined value can be entered.The effective channel width of the representative window (“RepWindowEffectiveChannelWidth”) is the default and will use the narrowest effective channel width (including Hanning smoothing and spectral averaging) of the spectral window chosen as the sensitivity driver for the observations. Please refer to the Spectral Setup section of the Technical Handbook for a definition of the effective channel width.This is usually a good choice for observations of spectral lines. If additional smoothing is envisaged post-observation, then a user-defined value can be entered - velocity units are preferred. For continuum observations where the spectral line interface has been used, the entered value should be the total non-overlapping bandwidth that is free from significant line emission.The “AggregateBandWidth” option will calculate the total non-overlapping bandwidth automatically and is the only option for Single Continuum setups. “FinestEffectiveChannelWidth” refers to the smallest effective channel width of any of the defined spectral windows.This is not shown for VLBI observations. Time Estimate: Request an estimate of how long it will take to achieve the scientific objectives of the Science Goal. This brings up a dialogue that contains a lot of information about the times required and parameters used in their derivation. The times reported include calibration and overheads (hardware and software) for each array required. For VLBI observations, the user must enter their own time estimate, including calibration, for the whole science goal. If the Sun is being observed, only the on-source time should be entered. Override OT's sensitivity-based time estimate?: If complicated and bright sources are being observed, the sensitivity target might not be reached due to limited $uv$-coverage. Alternatively, a user might not be interested in sensitivity, but rather in observing a source for a fixed amount of time in order to follow source variability. In either case, the sensitivity-based time estimate can be overridden here, but this must be rigorously justified. The time entered must be equal to the total time required for all the arrays, including calibration and overheads. This option is probably only useful for experts and it is not expected that it will be used very much. This is not shown for VLBI or solar observations. Simultaneous 12-m and ACA observations: If the observations require a single 12-m configuration and the 7-m Array only, it is possible to request that these be observed simultaneously. This is thought to be most useful for the planetary community and will result in the 7-m time being set to the same as that of the 12-m Array. Please note that configurations C-7, C-8, C-9, and C-10 are however excluded from simultaneous observing, as they cannot be paired with ACA. Are the observations time-constrained?: Clicking this reveals an interface for setting various scheduling constraints for the observations. If time-constrained observing is selected, it must be justified in the Technical Justification node. Currently there are two types of time constraint:
  • Single visit: Multiple time windows can be entered, during one of which the SG should be executed. It is possible to import these windows from an external ASCII file. Each visit should go on a separate row and each row should contain two comma-separated values of format YYYY-MM-DDThh:mm:ss e.g.
    2015-03-19T01:30:00, 2015-03-19T01:30:00
    2015-03-21T01:30:00, 2015-03-21T01:30:00
    2015-03-23T01:30:00, 2015-03-23T01:30:00
    2015-03-25T01:30:00, 2015-03-25T01:30:00
    
  • Multiple visits: Multiple dates (either absolute or relative to one of the epochs) at each of which the SG should be executed.

The ALMA OT Team, 2024 Mar 01