Page History

Background

As described in 

Action items

in Do not apply compression fix on 3-bit data the compression fix was applied to 3-bit data for several months, from September 2022 to August 2023. Upon discussion with telescope experts it was determined that the fix could not be reliably applied to 3-bit data due to the non-linearity inherent in the 3-bit system. The compression fix is thus not currently being applied in the pipeline for these data. The question then arose as to what should be done with the 3-bit calibrations for which the compression fix was applied. Affected datasets are listed here: spreadsheet for tracking 3-bit compression application

Tests on one particularly badly-affected observation were analyzed and written up by Lilia Tremou (note that she later communicated that her Tsys corrected data were corrected with the wrong formula, so should be ignored). These tests showed that, although the correction did approximately the right thing for the overall flux of the calibrator (and improved it relative to the uncorrected data), the spectral index was rendered incorrect. 

Decision

Any calibration errors produced by applying the 3-bit compression fix are likely to be small and within the absolute calibration accuracy of the VLA, and applying the fix may have actually improved the results. Thus we will not rerun the calibrations without the compression fix. Given the possibility of issues with, for example, in-band spectral indices though we will monitor the Helpdesk and rerun any calibrations if the user notices a discrepancy.