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Control and Performance
At the top of the page, a panel (not shown for VLBI projects) shows array and telescope configuration information. The configuration information (all calculated assuming the sky-frame equivalent of the Representative Frequency for the first source in the Science Goal) are:
- Antenna Beamsize: The HPBW of both the 12- and 7-m dishes. This roughly sets the field of view of a single telescope pointing. Sources larger than this, or multiple sources spread over a larger area, must be mosaiced together using multiple pointings.
- Number of antennas: The number of atennas assumed for each of the arrays. As the 7-m and TP Array times are actually based on the times required for the 12-m configuration, only the number of 12-m antennas has any practical effect on the OT's operation. These numbers are set by default in the ALMA Sensitivity Calculator.
- Longest baseline: The maximum baselines available for the most compact and extended 12-m Array configurations, as well as the ACA 7-m Array. The baseline lengths are a function of receiver band as the larger configurations cannot be observed at higher frequencies. All baseline lengths are unprojected i.e. they are not a function of source declination. Projection effects are though taken into account when calculating the synthesized beamsize.
- Synthesized beamsize: The synthesized beam sizes for the corresponding maximum baselines. Since Cycle-3, this number is dependent on the source declination, or the average declination if there is more than one source. The best angular resolution is possible when observing a source at the same declination as the latitude of the observatory (
degrees). As the magnitude of the declination difference increases, the angular resolution deteriorates for a given configuration.
- Shortest Baseline: The minimum baselines available for the most compact and extended 12-m Array configurations, as well as the ACA 7-m Array. The baseline lengths are a function of receiver band as the larger configurations cannot be observed at higher frequencies. All baseline lengths are unprojected i.e. they are not a function of source declination. Projection effects are though taken into account when calculating the maximum recoverable scale.
- Maximum recoverable scale: The largest structures that it is possible to image with the most compact and extended configurations. Like the angular resolution, this is now also declination dependent.
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:
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- 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:
- Single: A user may request a single value of angular resolution, although in practice a value within approximately
20 per cent of this will be delivered.
- Range: If the science could be done using one of several nominal configurations, this can be indicated by entering a range. In principle this increases the chances of a project being observed as it gives the observatory more flexibility when scheduling. Note that if the possible configurations include a mixture of long-baseline and smaller configurations, the long-baseline possibilities will be disregarded as the observing overheads are much higher.
- Any: By selecting this the user indicates that the SBs could be observed on any of the available configurations. As with a specific range of values, long-baseline configurations will not be considered by the OT or by the Scheduler.
- Standalone ACA: Instead of entering an angular resolution, the stand-alone ACA is selected using this option. No 12-m configuration will be selected, but the TP array might be added depending on the requested LAS.
- 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 Standalone 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.
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- 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. Potentially, 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. There are restrictions on this though including, for example, that the very largest configurations cannot be combined with any other configuration. Note that the LAS is not the same as the field of view - a source might contain separate components, each with a different angular scale. If the source size is very small indeed, a value of zero is acceptable. This is not shown for VLBI observations.
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- 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.
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- 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 angular resolution of the largest requested angular resolution.
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- 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. ``RepresentativeWindowResolution'' is the usual default and will use the narrowest spectral resolution (including Hanning smoothing and spectral averaging) of the spectral window chosen as the sensitivity driver for the observations. 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. ``FinestResolution'' refers to the smallest spectral resolution of any of the defined spectral windows. This is not shown for VLBI observations.
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- 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 Solar and VLBI observations, the user must enter their own time estimate, including calibration.
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- 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
-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.
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- Are the observations time-constrained?: Clicking this reveals an interface for setting various scheduling constraints for the observations. Currently there are two types of time constraint.
- 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.
- It is also possible to specify the times at which the observations should take place, including relative to other observations e.g. monitoring. Unless the 12-m/7-m simultaneous mode has been selected, this is not allowed if more than a single 12-m configuration is required. Two options are available:
- 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.
- Also included is a free-format text box that can be used to enter relevant information.
- If time-constrained observing is selected, it must be thoroughly justified in the Technical Justification node.
Next: Technical Justification
Up: Phase 1 and Phase
Previous: Calibration Setup
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The ALMA OT Team, 2017 Dec 18