MHO_ManualChannelDelayCorrection

Purpose

MHO_ManualChannelDelayCorrection applies user-specified, per-channel delay corrections to visibility data. For each channel matching a configured label, the operator computes a frequency-dependent phase correction phasor at every spectral point within that channel and multiplies the visibility data accordingly. The delay is specified in nanoseconds and produces a linear phase ramp across the channel’s spectral samples.

Control File Trigger

  • Keywords: delay_offs, delay_offs_x, delay_offs_y, delay_offs_r, delay_offs_l

  • Category: calibration

  • Priority: 3.5

The suffix-free keyword (delay_offs) applies to all polarizations; suffixed variants (_x, _y, _r, _l) restrict the correction to the named polarization.

Parameters for delay_offs variants

Parameter

Type

Description

channel_names

string

Comma-separated channel labels to which the correction applies.

pc_delays

list_real

Delay offset in nanoseconds, one per channel name.

Input Data

This operator acts on the visibility_type container.

Algorithm

MHO_ManualChannelDelayCorrection has no Initialize method; all work occurs in ExecuteInPlace.

The builder (MHO_ManualChannelDelayCorrectionBuilder) parses the polarization from the keyword name, constructs a channel-label-to-delay map from the channel_names and pc_delays parameters, and retrieves target station identifiers.

Execution (``ExecuteInPlace``):

  1. Iterate over the reference (index 0) and remote (index 1) stations.

  2. For each station, check applicability via IsApplicable (same station-identity matching as MHO_LSBOffset).

  3. If applicable, retrieve the polarization-product axis, channel axis, and frequency axis.

  4. For each polarization product:

    1. Check polarization match via PolMatch: if the operator’s polarization is ? (wildcard), all products match; otherwise, the character at the station’s index in the pol-product label must equal the configured polarization (case-insensitive).

    2. For each (channel-label, delay) pair in the configured map:

      1. Iterate over all channels in the visibility container. For each channel, retrieve the channel_label and compare via LabelMatch: if the configured label contains no + or -, the given label’s +/- suffixes (used for DSB halves) are stripped before comparison.

      2. On label match, retrieve the channel’s bandwidth tag. If absent, log an error and skip.

      3. Store the delay value (in nanoseconds) as metadata on the channel axis under the key ref_delayoff_<pol> or rem_delayoff_<pol> (depending on station index).

      4. Compute the effective sample period assuming Nyquist sampling:

        \[t_{\rm eff} = \frac{1}{2 \cdot B \cdot 10^6}\]

        where \(B\) is the bandwidth in MHz.

      5. For each spectral point sp = 0 .. N_sp-1:

        1. Retrieve the frequency offset from the frequency axis: Delta f = f(sp) * 10^6 (in Hz).

        2. Compute the primary phase term:

          \[\theta = -2\pi \cdot \Delta f \cdot \tau \cdot 10^{-9}\]

          where \(\tau\) is the delay in nanoseconds.

        3. Compute the geodetic phase-shift correction:

          \[\phi_{\rm shift} = -\frac{\pi}{2} \cdot \frac{\tau \cdot 10^{-9}}{t_{\rm eff}}\]

        4. Scale by a spectral-point-dependent factor:

          \[\phi_{\rm shift} \gets \phi_{\rm shift} \cdot \frac{-(2 N_{\rm sp} - 2)}{2 N_{\rm sp}}\]

        5. Accumulate: theta <- theta + phi_shift.

        6. Construct the phasor: Phi = exp(i * theta).

        7. For the remote station (st_idx = 1), conjugate the phasor: Phi_rem = conj(Phi).

        8. Multiply the visibility slice at (pol-product, channel, all APs, spectral point) by Phi.

Effect on Data

For each matching station and polarization, the operator applies a frequency-dependent phase rotation at every spectral point within each matching channel. The phase rotation is a linear ramp in frequency determined by the user-specified delay (in nanoseconds), with an additional geodetic phase-shift correction term. The reference station receives the phasor directly; the remote station receives the conjugated phasor. The delay value is also stored as metadata on the channel axis for later inspection.