VLASS FITS Header Keywords

This document describes the header keywords added to VLASS FITS files as part of PIPE-2461 and PIPE-3040. These keywords provide additional metadata describing the data processing and improve the overall data characterization.

To distinguish the newly introduced keywords from existing FITS header keywords, all new keywords are prefixed with VLASS. Below is a list of the keywords, along with their possible values and associated comments.

  • VLASSITY: Specifies the VLASS image type. Allowed values are INTENSITY_PBCOR_TT0 | INTENSITY_PBCOR_TT1 | RMS_TT0 | RMS_TT1 | ALPHA | ALPHAERR.

  • VLASSPT: Specifies the VLASS product type, identifying the imaging mode of the data product (Quick Look, SE imaging, or cube imaging), with possible values QL | SE | CC.

  • VLASSTN: Identifies the VLASS tile name corresponding to the observed sky region. The value is decoded from the file name. If no tile name is found, it is set to an empty string ('').

  • VLASSEP: Specifies the VLASS epoch. The value is decoded from the file name. If no epoch is found, it is set to an empty string ('').

  • VLASSVR: Specifies the VLASS version number. The value is decoded from the file name. If no version is found, it is set to an empty string ('').

  • VLASSPC: Specifies the VLASS phase center corresponding to the pointing center of the observation.

  • VLASSPL: Specifies the VLASS Stokes/polarization parameter. Allowed values are I | Q | U | V | L | IQU | IQUV.

  • VLASSRJ: Indicates whether the plane is rejected for VLASS CC processing; value is a boolean (T | F).

  • VLASSSPW: Specifies the spectral window ID(s) used for image generation; value is <spw-id>.

  • VLASSRMS: Specifies the median RMS values computed from the Stokes I RMS images (RMS_TT0 and RMS_TT1). The RMS_TT0 value is used for *TT0, *RMS_TT0, ALPHA, and ALPHA_ERROR images, while the RMS_TT1 value is used for *TT1 and *RMS_TT1 images.

  • VLASSPK: Specifies the peak flux density computed from the Stokes I primary beam corrected images (INTENSITY_PBCOR_TT0 and INTENSITY_PBCOR_TT1). The TT0 peak value is used for *TT0, *RMS_TT0, ALPHA, and ALPHA_ERROR images, while the TT1 peak value is used for *TT1 and *RMS_TT1 images.

  • VLASSBWN: Specifies the nominal bandwidth of the VLASS observation. For SE and QL products, the bandwidth is 2 GHz, while for CC products it is 128 MHz.

  • VLASSBW: Specifies the effective bandwidth after flagging. It is computed as the number of non-flagged channels multiplied by the channel width.

Processing Notes

The hif_makeimages task adds the VLASSITY, VLASSPT, VLASSTN, VLASSEP, VLASSVR, VLASSPC, VLASSPL, VLASSRJ, VLASSSPW, VLASSBWN, and VLASSBW keywords to the CASA image header. However, the VLASSPK and VLASSRMS keywords are added in the hif_makecutoutimages task.

All keywords are carried over to the FITS header during the conversion process in the hifv_exportvlassdata task. In the case of cube imaging, the images are split by Stokes parameter, and for these products the VLASSPL, VLASSPK, and VLASSRMS keywords are updated during hifv_exportvlassdata.

Additionally, the comments associated with these keywords are added in the hifv_exportvlassdata task after creation of the FITS file, since CASA image headers do not support keyword comments.