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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
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
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
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
the raw image as the primary array.
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.
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.
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.