This document is intended to provide the International Ultraviolet Explorer (IUE) Guest Observer or archive user with the information necessary to understand the image processing operations which are performed on IUE data at the IUE Observatory at Goddard Space Flight Center (GSFC) and to interpret properly the various output products resulting from these reduction procedures. Its scope includes descriptions both of the significant steps in the overall reduction procedure and of the detailed nature of the end products (e.g., magnetic tape file formats, etc.)
An earlier version of this document (Version 1.0, CSC/TM-79/6301, published in January 1980 (Turnrose and Harvel, 1980) and reissued as Version 1.1, CSC/TM-81/6268, in November 1981 (Turnrose, Harvel, and Stone, 1981)) addressed similar topics relative to the old reduction software of the IUE Spectral Image Processing System (IUESIPS), which performed an explicit geometric correction of the raw image prior to photometric correction and spectral extraction. This version addresses the new IUESIPS reduction software which eliminates the explicit geometric correction step. For low dispersion spectra, the new reduction software was implemented on 3 November 1980 (see Turnrose, Bohlin, and Lindler, 1981; Bohlin, Lindler, and Turnrose, 1981). For high dispersion spectra, the new reduction software was implemented on 10 November 1981 (see Turnrose, Thompson, and Bohlin, 1982; Bohlin and Turnrose, 1982). Versions 1.0 and 1.1, along with the document IUE Image Processing Overview and Mathematical Description_, CSC/TM-77/6250 (Perry and Turnrose, 1977), remain the primary sources of information for data processed at GSFC prior to the implementation dates for the new software. Chronological descriptions of all changes to the system made since the beginning of the Guest Observer phase of IUE operation on 3 April 1978 are catalogued elsewhere (see Section 10). Version 2.0 consequently complements Versions 1.0 and 1.1 in extending the system documentation to the new software, but does not supersede those earlier versions.
The primary scientific goal of IUESIPS is to provide the IUE Guest Observer (GO) with data reduced from the observations in an accurate and standard manner so as to be as free of instrumental effects as possible. Toward this end, image processing operations requiring either detailed knowledge of the performance of the IUE scientific instrument or large-scale computing facilities are performed by the resident image processing staff on a Xerox Data Systems Sigma-9 computer. These processing operations include implicit compensation for the geometric distortion of the camera system by reference to fiducial reseau marks which are superimposed on the images; photometric calibration of the images by means of pixel-by-pixel intensity transfer functions (ITFs) which correct for the SEC vidicon shading and non-linear response; wavelength calibration of the images with analytical dispersion relations determined from platinum-neon-lamp spectral images; extraction of spectral flux as a function of wavelength from the photometrically corrected images; and reduction of the extracted spectral flux to an absolute flux system by means of absolute calibrations obtained from observations of photometric standard stars (low dispersion only). Figure 1-1 summarizes the image processing functions performed on IUE images. A detailed description of the procedures used for these operations is given in the sections which follow.
The various end products of the image processing operations performed on each
image are assembled into a Guest Observer Data Package which is delivered to
the appropriate GO. This data package is designed to be amenable to
immediate astronomical analysis and includes photographic hardcopy images
(Photowrite images), CalComp plots of extracted spectra (optional), magnetic
tape files containing both raw and processed data, and listings of the tape
file contents and header. Each of these items is discussed in detail in
Section
8.
Figure 1-1.
IUESIPS Functional Overview
In order to satisfy the needs of the basic IUE data reduction operation,
that is provide a homogeneous data package for all images in a timely fashion,
the range of image processing functions which are performed on IUE data is
purposely limited and standardized. Within this framework, several basic
processing options are available to the GO. For this reason, it is
imperative
that the section on each Observatory Record Sheet ("script") labeled
"PROCESSING SPECIFICATIONS" be filled out by the GO. These specifications
are the means by which the basic processing options are selected, and no
defaults are permitted. Images for which specifications are lacking will
not be processed. The various options are explained in the relevant
sections of this document.