IUE Ultraviolet Spectral Atlas of Standard Stars

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Project Information : Data Reductions

* MXLO Data * New Tools * Image Combine *

A new MXLO data reduction tool has been developed to carry out the IUE Standard Star Atlas Project under the IRAF (Image Reduction and Analysis Facility ) environment. It facilitates reducing and analyzing data, generating the graphics (gif and ps format) and wavelength_flux (w_f) table for MXLO spectra, retrieving information from various related catologues, and so on.

The NEWSIPS Low-Dispersion Merged Extracted Image (MXLO) were retrieved from the IUE Final Archive, supported by the Multi-Mission Archive at STScI (MAST).


Features of The NEWSIPS MXLO Data

FITS Table
The IUE NEWSIPS MXLO file is a 3-D FITS table, a binary table with fixed-length floating point vectors to contain the extracted fluxes and associated flags.
Data Points
The number of extracted points is always 640. Wavelength are linearly sampled to a uniform step size and measured in vacuum.
Double aperture low-dispersion spectra will contain two rows, with one row for each aperture.
Absolute calibration
It convers to the absolute fluxes in the range of 1150-1980 Å for short-wavelength spectra and 1850-3350 Å for long-wavelength spectra.
Data Quality Flags $\nu$
these flages indicate abnormal conditions in the data which can range from fairly minor to quite serious situations. The flags for the MXLO data should be examined carefully in order to ascertain whether or not a particular data point is good or bad. The data points with zero flag value indicate that the data have no know problem condition. In general, flag values of -8 or more negative are indicative of unreliable data. See Data Quality Flag Description for a detailed discussion.
Small Aperture Spectra
The absolute fluxes for small-aperture data are significantly less reliable than those of large-aperture data. Because centering errors in the small aperture can lead to large variations in the overall observed flux level for individual spectra, it is impossible to determine an absolute S/L ratio. Therefore, the average of S/L over all wavelengths is normalized to unity. As a result, the small-aperture fluxes are known in a relative sense but not in an absolute one. See NEWSIPS Aperture Response Corrections for detials.


A New IRAF Tool for the MXLO Data Reduction

We assumed that the IUE MXLO data have been well-calibrated by the IUE NEWSIPS pipline. And no additional calibration for MXLO data is made by this project.

The run sequence to genaerate the spctra and flux tables is

Convert Data Format
  • convert the IUE spectra from the MXLO format to the STSDAS Table formats using the IUETOOL, an IRAF subpackage provided by R. Shaw and H. Bushouse at STScI.
  • convert 3D FITS table to 2D STSDAS table, including the following table columns.
    * w : wavelength
    * fL : flux of large aperture image
    * fS : flux of small aperture image
    * qL : data quality flag for large aperture image
    * qS : data quality flag for small aperture image
    * fLSc : averge combined flux of the large aperture and the scaled small aperture
    * qLSc : data quality flag for the averge combined image
Make Spectra
  • wavelength ranges :
    * LWR : 1850 - 3200 Å
    * LWP : 1850 - 3200 Å
    * SWP : 1150 - 1980 Å
  • extract ralated information from image header and transfer it to graphics and tables.
  • make flux table
  • make graphics
  • combine images: utilized the STSDAS gcombine as a basic task to make image combine/merge. See Image Combine section for algorithms and parameter specifications.
    * Large Aperture Spectra
    * Double (Large and Small) Aperture Spectra


Image Combine: Algorithms and Parameters

The input images to be combined are specified by a list. A list of input mask images, specified by the `masks' parameter, can be used to mask bad pixels before rejection operations. The mask images should have the same number and size as the input images.
For input images, set
For each output image line,
  • Get input image lines
  • Write the combined image
Parameter Specifications

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last updated: Jan. 2003