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Fringe stopping prior to cross-correlation allows longer visibility integration times without introducing de-correlation, which in turn allows the size of the correlator output products to be substantially reduced. The new correlator for the MWA, "MWAX", was designed from the outset to include pre-correlation fringe stopping. However, it was not supported in the first operational release and users were required to perform phase corrections post-correlation within Birli. Fringe stopping is currently available now performed on for all observations by request and its use is encouraged. Feedback is especially welcome at this stage in the commissioning processdefault, though it can be disabled on request.
Background
The fringe stopping process involves applying virtual signal delays to compensate for varying path delays between tiles during an observation. The major advantage of this approach to geometric corrections is that it is performed prior to cross-correlation, integration and averaging. Prior to fringe stopping, many of the observation modes available to MWA users involving long integration times or broad frequency averaging would have resulted in unacceptable geometric errors, as downstream processing tools require relatively fine-grained data to perform corrections effectively - especially at longer baselines, where the rate of fringe rotation is higher. Short integration times and fine frequency resolution both have a large impact on the size of the visibility data produced in a given observation, without necessarily improving the scientific usefulness of the data - typically, most of this resolution is averaged away in the final data products. Fringe stopping allows users to make use of long integration times and broad frequency averaging, without sacrificing the quality of the data. Smaller data volumes are easier to manage, store and process, and longer observations that may have produced prohibitively large volumes of data in the past may be feasible with fringe stopping.
We encourage all All MWA users were encouraged to test this new capability for any observations in the first observing semester of 2023. Starting in the second observing semester of 2023, fringe stopping will be became the default mode for all MWA observations. Observations without fringe stopping will continue to be supported on an opt-out basis.
Usage
Fringe stopping is available on an opt-in basis for any observation during the first observing semester of 2023now performed for all observation by default. Users may request opt out of fringe stopping at the time of scheduling, or at any time prior to the start of the observation. There is has been no change to how observation data is delivered and no software changes are were required to use it, apart from that you one should not tell Birli to apply geometric corrections to a fringe-stopping observation. By default, Birli will detect detects whether an observation was made with fringe-stopping, and geometric corrections will not be run unless explicitly specified at the command line or selected in ASVO.
Fringe stopping is available also performed by default for voltage-capture (VCS) observations. In this case, the voltage data files will have whole-sample delays applied to each RF channel and fractional delay metadata will be is populated in the file headers. Voltage data files are have also been upgraded to allow for commensal research users accessing VCS data to selectively enable or disable fringe stopping after-the-fact, irrespective of the configuration requested for the original observation. Over the coming days, this page will be updated with a full schema for the upgraded voltage data files and software for applying/removing fringe stopping corrections.
Implementation
A technical summary of the MWAX fringe stopping implementation will be was published soon as part of the forthcoming MWAX paper, a detailed description of the design and validation will be published shortly thereafterat a later date.
A (very) brief overview: when a fringe-stopping observation is scheduled, the scheduling software generates an additional data table in the metafits file sent to the correlator, containing periodically updated Az/Al values for each tile, reflecting small changes in pointing direction required during the observation period. These updated pointing directions are calculated at 4-second intervals using the astropy library and picked up where the real-time part of the fringe stopping system begins, during UDP voltage capture. For each Az/Al entry, and for each tile, a path length difference is calculated relative to the array centre, and converted to a time delay. The time delay is interpolated between the 4s reference points at a 5ms granularity, corresponding to the FFT block size in the correlator. The delay is split into a whole-sample and fractional-sample component, the latter of which corresponds to a delay shorter than the sampling period of the coarse-channelised voltage data. The whole-sample component is used to shift actual voltage samples in the data forward or back by an integer number of samples. The fractional component is stored in a metadata table in the voltage capture file, and applied by the correlator after the F-stage, in the frequency domain, as a phase rotation.
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