9.7 SWET Output

The main output data product produced during SWET is the low-dispersion merged extracted image FITS file (MXLO). The MXLO contains the net (background-subtracted) integrated flux spectrum, the Chebyshev characterized background spectrum, the $\nu$ flag spectrum, the absolutely calibrated net flux spectrum, and the calibrated sigma spectrum stored in a FITS binary table extension. The net flux and background spectra are in units of FN, the flag spectrum is in the normal bit-encoded unitless values, and the calibrated flux and sigma spectra are in physical units of ergs sec^-1 cm^-2 Å^-1. The sigma spectrum, originally computed in FN units, undergoes identical calibration steps (including all corrections for sensitivity degradation, etc.) as the net flux spectrum. Chapter 11 details the process of absolute flux calibration and camera degradation correction.

The background flux spectrum is scaled to the same integrated slit length as the net flux spectrum and is evaluated over the entire wavelength space of the camera target region contained in the low-dispersion SI (1050Å to $\sim$2000Å for SWP and 1750Å to $\sim$3400Å for LWP and LWR). The background values for pixel locations past the long-wavelength end of the camera target are replications of the last valid background value. The net flux and sigma spectra are computed over the entire wavelength space of the low-dispersion SI, but locations that are outside the camera target area will have net fluxes equal to zero because these regions do not contain any valid image data. The edge of the camera target areas are encountered at wavelengths greater than (approximately) 3395, 3425, and 2000Å for the LWP, LWR, and SWP cameras, respectively, for large-aperture spectra. Due to the curvature of the target boundary, small-aperture spectra reach the target edge at different wavelengths: 3380, 3400, and 2015Å for the LWP, LWR, and SWP cameras. The absolute calibrations are valid over wavelength limits of 1150-1980Å for SWP and 1850-3350Å for LWP and LWR. Beyond these limits the calibrated net flux and sigma spectra are set to values of 0 and -1, respectively and have a $\nu$ flag value of -2. The valid wavelength limits of the calibrated spectra are consequently somewhat truncated as compared to the net flux spectrum.

The SWET module writes the following information to the HISTORY portion of the image label:

• noise model version number,
• aperture-dependent extraction/calibration information:
  • predicted and found line positions of spectrum center, line position of the peak flux, and average peak FN,
  • cross-dispersion profile information (blocksize, total number of blocks, and number of rejected blocks),
  • number profile nodes and sigma rejection,
  • line position of profile centroid,
  • line numbers of flux extraction window,
  • pixel rejection threshold,
  • total number of extracted pixels, number of pixels rejected as cosmic rays, and number of pixels rejected as bad,
  • absolute flux calibration version number,
  • absolute calibration mode (i.e, point or trailed),
  • absolute calibration epoch,
  • camera rise time,
  • effective exposure time,
  • THDA of image,
  • reference THDA,
  • temperature-dependent sensitivity correction coefficient,
  • temperature correction factor,
  • time-dependent sensitivity degradation correction version number,
  • time-dependent sensitivity degradation correction mode,
  • sensitivity degradation calibration epoch, and
  • observation date.