February 1996                              Volume 4, No. 1

IUE DATA ANALYSIS CENTER NEWSLETTER

This is an electronic newsletter edited by the IUE Data Analysis Center (IUEDAC) intended to inform the numerous remote and local users of the IUEDAC software of recent software updates and project changes.

IUEDAC Software and Hardware Updates

Recent Software Changes (since 13-Dec-1995)

A routine called MAN is now available for graphics terminal users for getting help on intrinsic IDL commands. For example, typing MAN,'SMOOTH' will display information on the IDL SMOOTH command.
The IUE merged log has been updated. All Goddard entries should now be included as well as NEWSIPS status entries through 12/5/95. The new version also includes galactic coordinates which can be accessed via the "expert mode" database routines.
A new version of MGEX was implemented which corrects problems when individual spectra are extracted from multiple exposure NEWSIPS SILO files.

NOTICE: The IUEDAC staff continues to prepare the DAC analysis software for the arrival of the NEWSIPS high dispersion data. Efforts thus far have concentrated on the basic extraction and display utilities. Are there particular capabilities and/or software that you would like to see become part of the DAC software package? This is your chance to send us your input. Send correspondence to rthompson@iuedac.gsfc.nasa.gov or arquilla@iuedac.gsfc.nasa.gov.

IUE Image Processing News

Small NEWSIPS Error

Recently we discovered a small error in the processing history portion of the FITS headers for some images. The error affects only LWP low-dispersion data taken after Nov. 12, 1992 (i.e. data that may be affected by solar scattered light). There is a message in the processing history under RAW_SCREEN that states that, except for trailed and multiple exposure images, the spectral extraction is set to point source, in order to minimize any possible scattered light contamination. This is correct. In some cases, the RAW_SCREEN processing history also says that the extraction was set to "extended source"; this is incorrect. The affected images are being reprocessed to remove this possible source of confusion.

NEWSIPS Status

The IPC has now processed all of the available NASA SWP and LWP low dispersion data. Data which are still proprietary have not yet been processed through NEWSIPS, but this will be done as data become publicly accessible. Some images have not been processed because of missing database information or other problems. We are currently working on getting those images taken care of and processed. Note also that we are not processing VILSPA data; VILSPA is doing so.

If you need images that seem unavailable, please contact Cathy Imhoff . She can check on them and in many cases fix the problem so that the images can be processed.

NEWSIPS Data Availability

Recently NSSDC has placed a large volume of NEWSIPS data into the NDADS system (see below). If you have been trying to get some images which are listed as processed but not yet available from NDADS, you may wish to try again.

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IUE and Education

Are you interested in using the IUE satellite or its data to support science education? We are too! We have recently formed a small informal working group to exchange ideas and experiences. If you are interested in joining, please contact Cathy Imhoff.

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Final Archive News

High Dispersion Data

Much of the SWP and LWP low dispersion Final Archive data are now available to the community. The reaction to the NEWSIPS low dispersion processing techniques has been very positive. The high dispersion Final Archive processing system is in final testing for the SWP images. The processing system should be implemented in a few weeks. The SWP ripple correction and absolute flux calibration are being derived by our collaborators at VILSPA. Processing of SWP high dispersion images will begin when these calibrations are received and operational testing of the software is complete. There will be several new data products for the NEWSIPS high dispersion processing system. The SIHI image is a rotated and geometrically rectified image. Individual orders can be custom-extracted easily from this image, which includes wavelength calibration for the horizontal echelle orders. Another output product of the high dispersion NEWSIPS system is the COSMIC RAY image, which is created by masking out the spectral orders and flagging the deviant background pixels, allowing visual identification of grazing incidence cosmic ray events.

Re-Extracting Certain NEWSIPS Data

The NEWSIPS extraction technique for low dispersion was developed to handle all of the images in the archive in a fully automated way. However, some spectra are not properly extracted with the automated extraction and should be re-extracted using an interactive version of the optimal extraction method or an alternate extraction method. Below are 2 examples of situations where a re-extraction of the NEWSIPS data is required. Investigators are advised that the SILO image should be examined for every image used in their analysis to fully understand the data. Only examining the MXLO files can be misleading.

1) Multiple spectra

This situation occurs when two or more spectra are recorded through the large aperture, or when the aperture is filled with an extended source with a continuum point source also in the aperture (e.g., PN with central star). The IUESIPS extraction technique was a simple boxcar extraction. If multiple spectra were present in the large aperture, the flux from all spectra was summed for the IUESIPS extraction. The extraction technique used in NEWSIPS is based on the optimal extraction technique (Kinney et al. 1991). The NEWSIPS code assumes the cross-dispersion profile varies slowly with wavelength and that only one spectrum is present in the large aperture. In the case of multiple spectra, the NEWSIPS extraction will center on the strongest spectrum. If the additional spectrum or full aperture illumination from an extended source includes strong emission lines, the NEWSIPS extraction will possibly flag the emission lines as deviant pixels based on the statistics of the noise model and the deterimined profile. The flux from the pixels in the emission lines flagged as cosmic ray hits (deviant pixels) is not included in the extraction of the spectral flux. Therefore, the total flux in the emission lines can be less with NEWSIPS than with IUESIPS if a strong continuum source is also present in the aperture.

For this situation, the investigator should use an alternate extraction technique, depending on the goals of the science. A boxcar extraction would yield the total flux in the aperture in the case of an extended source and a point source. If multiple point source spectra are present, they can be extracted individually with an interactive version of optimal extraction or a gaussian extraction.

The fact that the NEWSIPS extraction requires that the cross-dispersion profile vary slowly with wavelength and that it cannot model more than one spectrum in the large aperture at a time in its automated mode, as well as the algorithm used for flagging deviant pixels for exclusion from the flux calculation, are presented in the NEWSIPS Image Processing Information Manual, Newsletter #53.

2) Two-node spline fit in NEWSIPS extraction

The NEWSIPS extraction method uses splines to fit the spectrum in successive lines parallel to the dispersion. The fits are used to create the array of cross-dispersion profiles used to weight the extracted data. The number of nodes used in the spline fits is determined from the signal-to-noise ratio, with 15 being the maximum number of nodes for high signal-to-noise data, and 2 being the minimum for low signal-to-noise data. We have found that when 2 nodes have been used in the spline fit, the spectrum may not be well modeled by the fit and the resulting flux can be less accurate than flux determined using an assumed profile, such as a gaussian profile, the default optimal profile, or a boxcar profile. Investigators are advised to use caution when using NEWSIPS data that has been extracted with a 2-nose spline fit. This information is recorded in the HISTORY portion of the FITS label. It would be a good idea to re-extract such spectra with an alternate extraction technique to verify the NEWSIPS results.

Questions concerning the NEWSIPS image processing techniques, the NEWSIPS output products, and IUE calibrations can be directed to Joy Nichols (nichols@nebula.gsfc.nasa.gov).

Availability of NEWSIPS Data Via NDADS

Most low dispersion SWP and LWP data processed by GSFC with NEWSIPS through early January, 1996, are now available from NDADS. The status flags accessed by the project status report marker are being updated and should be up-to-date early in February. Some VILSPA SWP low dispersion data are now available through NDADS and more will become available over the next few weeks.

Some users attempting to retrieve NEWSIPS data from NDADS may have been told that the data was proprietary. Due to a programming error, some data was incorrectly flagged as proprietary. All of the erroneous flags have now been reset.

The archive has received 19th episode data from VILSPA, but none of it is yet available through NDADS. This data should become available sometime in February. High dispersion data have been processed with IUESIPS and the low dispersion data have been processed with NEWSIPS.

Please send questions concerning retrieval or availability of IUE data from NDADS to Karen Levay. kll

Staff Changes

Dr. Michael Carini has left the IUEDAC staff to assume an academic position. Ms. Lyla Taylor has left the DAC to pursue a graduate degree. The IUE project wishes both of them well and both will be missed.

Mr. James Caplinger joins the DAC staff. Many IUE users probably know Jim as one of IUE's excellent group of Telescope Operators.

Dr. Richard Arquilla is the current IUEDAC Task Leader.

Editor: Dr. Richard Arquilla
Contributors: Mr. Randy Thompson; Mr. Jim Caplinger; Dr. Cathy Imhoff; Ms. Karen Levay; Dr. Joy Nichols

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Mr. James Caplinger, Computer Sciences Corporation

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Dr. Don West, IUE Operations Scientist