New International Ultraviolet Explorer Final Archive Data Now Available!

Volume 10, Numbers 3 & 4, December 1994
by Michael Van Steenberg

The short-wavelength prime camera of the International Ultraviolet Explorer provided this raw image of a high-dispersion echellegram. The spectrum is from an O-star.

Since its 1978 launch, the still-active International Ultraviolet Explorer (IUE) spacecraft has obtained about 100,000 distinct observations of astronomical sources. The IUE project is currently reprocessing all short- and long-wavelength prime (SWP and LWP) low-dispersion archival images through the new IUE Final Archive Processing System, known as NEWSIPS. The data are being delivered to NSSDC, where the files are being placed into the NASA Data Archive and Delivery Service (NDADS) nearline system. The original IUE dataset represents the most-requested astronomical dataset in the history of NSSDC (more than 19,000 requests through NDADS in just the last three years, representing over 55% of all NDADS requests). The new reprocessed data are now becoming available to the NASA scientific community.

The IUE project is currently processing the NASA SWP and LWP low-dispersion data obtained before 1990. The European Space Agency (ESA) is likewise reprocessing its IUE data, but the files have not yet been delivered to NSSDC. At this time, about 10,000 of the more than 16,000 SWP low-dispersion images obtained at Goddard Space Flight Center have been processed and archived at NSSDC. Over the coming months, we will continue to receive and archive the rest of the SWP and LWP low-dispersion data. The long wavelength redundant (LWR) low-dispersion datasets, as well as the high-dispersion datasets, will follow over the next few years. Meanwhile, the IUE mission continues and the archive continues to grow. Even with modern machines and techniques, it will take a few years to reprocess all the archival data.

There are many differences between the original IUESIPS (current)-processed data and NEWSIPS-processed data. Principal among them are: a cross- correlation technique is used to register the science data with the intensity transfer function (ITF), yielding a significantly more accurate registration and helping to remove pixel-to-pixel noise; a single-step geometric resampling, including all geometric corrections, reduces the "artificial" smoothing caused by repeated resampling/interpolations; a signal-weighted extraction technique is used, with a noise model that more accurately samples the spectral data and generally improves the signal to noise; and new calibrations are used, including wavelength calibrations and absolute calibrations.

Also in contrast to the original IUE data format (IUESIPS GO) the NEWSIPS-generated output files are in the astronomical community's standard format, Flexible Image Transport system (FITS). Because the newly accepted FITS image and binary table extensions are used, some FITS readers may not handle these yet. However, the FITS readers available through the IUE project do correctly handle these files. The FITS headers include extensive, verified information documenting the observation and the data processing. The IUE project is continuing to process all new data being taken with IUE through the original IUESIPS processing system. This is expected to continue until the final archive processing system can handle the full range of data types, including high-dispersion data, which will probably be nine to 12 months from now. Requesters are cautioned about intercomparisons of original IUESIPS and NEWSIPS data, given all the differences between the two systems.

There are a number of useful documents available on request. Please contact us or the IUE project for copies of the following: The IUESIPS Image Processing Information Manual The NEWSIPS Image Processing System Information Manual: Low Dispersion Data Version 1.0, NASA IUE Newsletter (No. 54 is the most recent issue) The IUE Observing Guide (newsletter No. 47)

Nichols-Bohlin gives a more detailed description of the final archive philosophy (1993a, NASA IUE Newsletter, No. 51, p. 27) and describes the FITS file formats (1993b, NASA IUE Newsletter, No. 51, p. 31). Imhoff gives an example of a FITS header (1993, NASA IUE Newsletter, No. 51, p. 43).

You may request the NEWSIPS data using the NDADS/ARMS E-mail request procedure. For instance, to request the fully extracted and calibrated data file for images SWP 30548 and LWP 3289, send an e-mail message to: nssdca::archives or archives@nssdca.gsfc.nasa.gov with a subject line of: "REQUEST IUE MXLO". The body of the message should contain the list of IUE images desired (i.e., SWP30548 and LWP03289). For example:

	MAIL> SEND
	TO:  NDADSA::ARCHIVES
	SUBJ:  REQUEST IUE MXLO

	SWP30548
	LWP03289
The files SWP30548.MXLO and LWP03289.MXLO will then be written to our anonymous FTP/DEFAULT DECnet IUE data distribution area ([data_dist.iue]): on nssdca.gsfc.nasa.gov or ndadsa::anon_dir:[data_dist.iue].

You also may access IUE archival data through the World Wide Web. The NDADS/ARMS and IUEDAC homepages (at URLs http://nssdca.gsfc.nasa.gov/archives_homepage.html and http://iuesn1.gsfc.nasa.gov/iue/iue_homepage.html, respectively). Both allow you to connect to a form for requesting IUE data. You then may electronically transfer the data via Default DECnet or FTP to a node of your choice. This process is similar to the NDADS/ARMS e-mail procedure. These pages are also accessible from the NASA Goddard Space Flight Center Home page ( http://www.gsfc.nasa.gov/GSFC_homepage.html).

Questions concerning the calibrations and processing algorithms may be sent to Joy Nichols (nichols@iuegtc.gsfc.nasa.gov, iuegtc::nichols). Questions concerning the processing status and availability of an image may be sent to Cathy Imhoff (imhoff@iuegtc.gsfc.nasa.gov; iuegtc::imhoff). Questions concerning the access to the archived data on NDADS may be sent to Michael E. Van Steenberg (mev@mystry.gsfc.nasa.gov; mystry::mev). Much of the information provided here comes from the IUE project, in particular Cathy Imhoff, Joy Nichols, and Andy Michalitsianos.

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Author:Miranda Beall
Curators: Erin Gardner and Miranda Beall
Responsible Official: Dr. Joseph H. King, Code 633
Last Revised: 21 Nov 1996 [EDG]