MHO_DCBlock

Purpose

MHO_DCBlock removes the DC spectral point from each channel’s visibility data by zeroing the visibility at the spectral point nearest to the local oscillator frequency. The DC point corresponds to the spectral channel where the sky frequency equals the LO frequency, which typically contains spurious power or corrupted data. Which spectral point is the DC point depends on the channel’s sideband: for lower sideband (LSB) channels, the DC point is the highest-frequency spectral bin; for upper sideband (USB) channels, it is the lowest-frequency spectral bin (index 0).

Control File Trigger

  • Keyword: dc_block

  • Category: flagging

  • Priority: 3.5

Parameters for dc_block

Parameter

Type

Description

dc_block

boolean

When true, enables DC blocking; when false, the operator is not created.

Input Data

This operator acts on the visibility_type container.

Algorithm

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

Execution (``ExecuteInPlace``):

  1. Retrieve the channel axis and frequency axis from the visibility container.

  2. Iterate over every frequency channel:

    1. Retrieve the channel’s net_sideband label (U for upper sideband, L for lower sideband).

    2. Determine the DC spectral index:

      • USB (net_sideband = U): DC index is 0 (lowest spectral bin).

      • LSB (net_sideband = L): DC index is N_freq - 1 (highest spectral bin).

    3. Zero out all visibility values at spectral index dc_index for that channel across all polarization products and all accumulation periods. This is accomplished by multiplying a slice view covering all pol-products and all APs at (channel, dc_index) by zero.

Effect on Data

For each channel, exactly one spectral point (the DC point) is zeroed across all polarization products and all accumulation periods. The DC spectral index depends on the channel’s sideband: index 0 for USB, index N_freq - 1 for LSB. The operator does not modify data weights or bandwidth fractions, as the effect on signal-to-noise ratio from removing a single spectral point is typically negligible.