The MWA Correlator v2 (aka MWAX Correlator) is still under development. This article summarises the significant differences between the legacy (Ord/Current/v1) correlator and the new MWAX correlator.

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Introduction

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• The associated metafits file for each observation should be considered the source of truth for everything, except things that vary per coarse channel, which will be in the primary HDU of each coarse channel FITS file. 
  • Only the most basic information such as the obsid, projectid, time, correlator mode are repeated in the FITS Primary HDU (purely for convenience).
• The number of antenna can change on observation boundaries, so do not assume 128. The new MWAX correlator is designed to support up to 256 tiles, but will likely start at 128 and grow as next generation receivers are added to the array. E.g. the next increment might be 136T if we added one more receiver connected to 8 tiles.
• Within the FITS image HDUs for visibilities, NAXIS1 represents the fine channels * polarisations * 2 (real/imaginary). NAXIS2 represents the baselines (antennas x (antennas+1))/2
• Polarisations are in XX,XY,YX,YY order
• Antennae are ordered by the "antenna" value as per the ‘BINTABLE’ in the associated metafits file called ‘TILEDATA’. See MWAX Antenna Ordering for more details.
• Baselines are antenna vs antenna, not input vs input (e.g. ant0 vs ant2, not ant0x vs ant2y), with all of the polarisations XX,XY,YX,YY grouped together with that baseline in the visibilities.
• Baselines are in lower regular triangular order. 0-0..0-n, then 1-1..1-n, then 2-2..2-n, etc.
• Coarse channel numbers (CORRCHAN) are always in sky frequency ascending order.
• Real and imaginary data values are 32 bit floats
• There are significantly more correlator modes to support. Rely on FINECHAN and INT_TIME for the correct mode information from the metafits. See: MWAX Correlator Modes (128T)

• The number of coarse channels per observation could change (once we deploy replacement receivers we could choose to use >24, or if we allow astronomers to choose LESS than 24 course channels with the existing receivers, or if an MWAX server is offline), so do not assume 24.

• Instantaneous bandwidth / coarse channel width could change (once we deploy replacement receivers), so do not assume 30.72 MHz (24 x 1.28 MHz).

• Weights are provided in the gpubox files after each visibility HDU containing one timestep/integration. The weights and how they are determined is discussed below:
  
  • Each visibility has a multiplicative weight applied, based on a data occupancy metric that takes account of any input data blocks that are missing due to lost UDP packets or RFI excision (a potential future enhancement).  The centre (DC) ultrafine channel is excluded when averaging and the centre output channel values are re-scaled accordingly.  Note that only 200 Hz of bandwidth is lost in this process, rather than a complete output channel. 
  • As part of the M&C system, the application of weights can be turned on or off per observation based on the science case/needs. With weights not applied, the data will be averaged in the correlator without taking into account the weights. Either way the weights are supplied in each alternate ImgHDU for your information.
• The MWAX correlator will provide options on a per observation to apply geometric and cable delays. When these are provided, they do not need to be performed by downstream tools such as Cotter or the RTS. See MWAX Metafits Changes.
• he MWAX FITS files contain a keyword "CORR_VER" which represents the correlator version number. If this keyword is missing, assuming this is a FITS file from the Ord/Legacy correlator. "2" is the value for the MWAX correlator.

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The weight data is represented by a single-precision floating point value per baseline and polarisation. Below is an example using 4 antenna 4 antenna (ant0, ant1, ant2, ant3) example and 4 polarisations (xx,xy,yx,yy) in one time step / integration (we only have time step per HDU). The ImgHDU data for the weights would look like the following: