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A Brief Description of NEWSIPS

The NEWSIPS system was intended to provide a number of improvements to the IUE calibrations and data reduction techniques compared to IUESIPS. It was also required to operate in a standardized "pipeline-processing" mode to produce uniform output data products for users. Thus the standardized extraction techniques generally work best on well-exposed, point-source, continuum spectra similar to the calibration stars, and may not perform as well on other types of spectra. Output files were created at various intermediate processing steps so that users may apply their own customized reduction and extraction techniques (see description of files below). In many cases, customized extractions will provide better quality data.

A complete description of the NEWSIPS processing system is given in the NEWSIPS Image Processing Information Manual. In addition, several published papers and IUE Newsletter articles describe various aspects of the Final Archive algorithms, processing and calibrations.

Below the processing steps in NEWSIPS are summarized. The results of each step are recorded in the processing log portion of the FITS header for each image.

  • Raw screen. The raw image is screened for various problem conditions, such as missing telemetry and the presence of microphonic noise in the LWR camera. The spectrum is located in the raw image, if the signal is strong enough, and its location, width, and signal level are characterized. Pixels affected by specific problems are flagged.

  • Cross-correlation. Pattern-matching is performed using a cross-correlation technique between the raw science image and the ITF.

  • Photometric correction. Each pixel is corrected to a linear scale of Flux Numbers (FNs) and an overall flat-field correction is applied. The mapping of the ITF to the science image is provided by the results of the cross-correlation. Pixels affected by specific problems in the photometric correction step are flagged.

  • Geometric correction. The photometrically corrected image undergoes a single resampling to correct for displacements between the raw science image and the ITF, to rotate the spectral format, and to linearize the wavelength scale. Additional small corrections are included in the single resampling for low dispersion spectra to correct for spatial "wiggles" in the spectrum, align the large and small aperture data, correct for the "tilted" wavelengths in extended spectra, and apply a fixed starting wavelength and position in the image. For high dispersion spectra, the additional small corrections include echelle order "splay", shifts for order registration, and correction for cross-dispersion "wiggles".

  • Wavelength calibration. The wavelength calibration is parameterized as a function of time, camera head temperature, and (for high dispersion) order number, based on analysis of hundreds of platinum-neon calibration images. High dispersion spectra are also corrected for the motion of the Earth and spacecraft, providing a heliocentric wavelength scale.

  • Extraction. The extraction steps are substatially different between low and high dispersion. For low dispersion spectra, an optimal extraction technique is used to fit the spectrum and background with 3 to 15 nodes. If there is not enough flux, a default point-source or extended profile is used. The absolute calibration applied includes corrections for aperture, degradation with time, and temperature dependence. For high dispersion spectra, the background between the crowded echelle orders is characterized using swaths perpendicular to the orders and smoothed. Each order is located and extracted, using a slit length with depends on order. If the flux is insufficient, a default location is used for the order. A ripple correction is applied to the orders, and the absolute flux calibration is scaled by order from the low dispersion calibration scale.

  • FITS formatting. Various steps in the processing are output as data files in FITS format. The FITS header includes detailed documentation on the image, the object observed, and the processing. Each header includes FITS keywords describing the image, the spectra by aperture, the original VICAR header documenting the original observation, and the processing log.

    Output Files Description
    RILO, RIHI Contains the raw image as the primary array.
    LILO, LIHI Contains the photometrically corrected image as the primary array, with an image extension containing the associated quality flags determined at that point in the processing.
    SILO, SIHI Contains the photometrically and geometrically corrected image as the primary array. Wavelength information is given in a binary table extension, and quality flags in an image extension. Suitable for most re-extraction techniques.
    MXLO, MXHI Contains the extracted and calibrated spectra, wavelengths, data quality flags, and sigma or noise estimates in a binary table extension. A single MXLO file can contain spectra from both apertures.