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IUE Project Electronic Newsletter

NASA-Goddard Space Flight Center (GSFC)

Vol. 5, No. 3 - March 1997


Processing of LWR Low Dispersion Images Begins!

We have started to process Goddard LWR low dispersion images through NEWSIPS.

You are welcome to send us a list of your favorite images for priority processing. There is a NEWSIPS user request Web page at http://hypatia.gsfc.nasa.gov/IUE/search/IUE_usrreq.html. Your request goes into a semi-automated system, which will send you email reports on the status of the processing. The data may be retrieved from the NDADS system once it has been archived, usually just a few days after it has been processed.

- Cathy Imhoff


IUEDAC Software Changes

03-Feb
New versions of ifitsrd, iue3drow, & nstwrt were added to the experimental library to allow fields from a FITS binary table extension to be stored in an IDL structure or an array of structures. This can simplify the calling sequence when requesting multiple rows and columns from a FITS binary table extension.
11-Feb
A new version of subroutine isndads was implemented which allows archive requests for high dispersion NEWSIPS files using the program search.
24-Feb
A new version of readmx was added to the experimental library which uses an improved method for deriving splice points when merging multiple orders. The previous version simply spliced orders at the mean wavelength within the overlap region, whereas the new version uses the wavelength at which the difference in the applied ripple correction within the overlap region is a minimum. The result is a significant reduction in the apparent noise at the ends of each order.
24-Feb
The following programs were implemented:
ripple
corrects an error in the IUESIPS ripple-correction algorithm as reported in Newsletter 33, p. 71.
iuerip
new routines for deriving K & ALPHA parameters for IUESIPS and SWP NEWSIPS high dispersion spectral orders.
new_splice
new routine used by readmx for calculating splice points between merged spectral orders.
readsi
contains updated prolog documentation.
- Randy Thompson

About High Dispersion NEWSIPS Processing

Many of you are using the NEWSIPS processed SWP high dispersion data which are available from the NDADS system. We are working hard on the documentation for the processing and hope to have it available in a few months. In the meantime, some of the major items that you should be aware of are listed below.

One of the major differences between the NEWSIPS and IUESIPS high dispersion processing is in how the background is handled. In IUESIPS, a localized background based on the pixels between the orders was used. This led to many well-known problems, since in the area of crowded orders the background is filled in by light from the adjacent orders. The background estimate was therefore too high, and absorption lines were often seen to go well below zero flux. In NEWSIPS, swaths have been chosen across the image to sample the background, and the area affected by interorder light is corrected for that effect. A "global" determination of the background is then made. This produces a more accurate background, and you will see that the absorption line depths are more realistic.

There are limitations to this new technique, of course. If there are local perturbations of the background, such as small cosmic ray hits, they are interpolated over and the background estimate is too low around that small area. If there are large perturbations, such as glancing cosmic ray hits (the ones that look like comets), they may distort the global background fit by producing mild ringing. In some portion of the image the background estimate may then be too high or too low. Flares in late-epoch LWR images, and the lack of any nearby pixels with reliable background fluxes in the short-wavelength corner of the SWP camera, are specialized problems for which there is no good background estimator - by any algorithm.

In addition, images which have only emission lines with no continuum are better handled with a localized background determination. The NEWSIPS software determines whether the image has a continuum spectrum or not, then chooses a "global" or "local" background determination appropriately. This is recorded in the processing history.

You can easily see that NEWSIPS has a more sophisticated background estimation algorithm, but it isn't perfect. This is to be expected in a production system designed to process all the widely divergent images in the IUE archive. A customized background and extraction may be expected to do a better job for individual images, depending on their characteristics. In addition, an improved version of readmx (described above) uses an updated method for splicing high dispersion orders, thus reducing the apparent noise. A future issue of the Newsletter will give details on how customized background extractions might be done easily.

One of the new data products from NEWSIPS is a high dispersion line-by-line (sihi) file, similar in concept to the line-by-line files for low dispersion (IUESIPS elbl and NEWSIPS silo files). These files can be used as a basis for those users interested in generating customized extractions.

A side note about absorption line depths: First, the noisiness of the data in the depth of an absorption line is much higher than on the wings and nearby continuum, so you may well see flux values both above and below zero in the depth of a dark line. Second, the actual minimum in a dark absorption line is generally not exactly zero, due to grating scattered light. Those of us who measure line depths and equivalent widths in high resolution data normally have to consider that there is a few percent effect from the line being filled in by grating scattered light. In addition, IUE's detectors have a significant amount of halation, so even a perfectly extracted dark absorption line may have a minimum that is a little above zero flux.

If you have any questions, please feel free to contact us.

- Cathy Imhoff


NEWSIPS File Differences

Users should be cautioned that because of differences in the extraction algorithms used for low and high dispersion NEWSIPS data, the low dispersion (mxlo) files contain absolutely-calibrated "sigma" vectors, while the high dispersion (mxhi) files contain uncalibrated "noise" vectors. Because no spectral profile information is available as in low dispersion, the high dispersion noise spectra are based solely on the camera noise models.

As a reminder, all NEWSIPS data sets use vacuum wavelengths, whereas IUESIPS files use air wavelengths longward of 2000 Ångstroms. This can cause an apparent wavelength differences of 0.65 to 0.9 Ångstroms, when comparing IUESIPS and NEWSIPS long wavelength spectral data.

Complete documentation on the high dispersion processing system is currently being completed by the IUE project and will be available in the near future.

- Randy Thompson


The IUE Electronic Newsletter, edited by James Caplinger, is primarily intended to inform the numerous remote and local users of the IUEDAC software of recent software updates, as well as let them know about news relating to the IUE project in general. If you want your name to be added to, deleted from, or changed on our distribution list, please send us e-mail to

news@iuedac.gsfc.nasa.gov

or send a postcard to:

IUE Data Analysis Center (IUEDAC)
Code 684.9, NASA-GSFC
Greenbelt, MD 20771


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