spacer link to MAST page spacer logo image spacer
 
link to STScI page


IUE Project Electronic Newsletter

NASA-Goddard Space Flight Center (GSFC)

Vol. 5, No. 4 - April 1997


IUE Image Processing News

Processing of LWR Low-Dispersion Images Flows On

As of April 8, we have processed 32% of the NASA LWR low-dispersion images through the NEWSIPS processing pipeline.

You are welcome to send us a list of your favorite images for priority processing. Feel free to use the 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 The data may be retrieved from the NDADS system once it has been archived, usually just a few days after it has been processed.

The Two Faces of the LWR Camera

For LWR low-dispersion processing, the NEWSIPS software makes use of 2 ITFs. This is different from the other cameras. Several studies over the last many years have shown that the fixed-pattern noise of the LWR has a "bimodal" behavior. The geometric format tends to follow one of two modes. To accommodate this, 2 ITFs were built. The first uses the original ITF data taken in 1983, and is constructed just as the other ITFs were done. The second ITF uses an alternate set of null images, chosen to exemplify the "second mode" that the LWR exhibits. The other eleven ITF levels were created by resampling the 1983 ITF images to the geometric format of this alternate null level.

In the NEWSIPS software, the image to be processed is cross-correlated with both ITFs. The ITF with the larger mean cross-correlation coefficient, i.e. with the better match for the geometric format of the image, is chosen and the image is processed with that ITF. The difference in the cross-correlation between the two ITFs is often large; the mean cross-correlation coefficient may be around 0.8 (an excellent match) for one ITF and 0.4 (a poor match) for the other ITF. The modified ITF (listed in the FITS header as LWR83R96A) is most often chosen for early and late epoch data, while the original ITF (LWR83R94A) is most often chosen for images in the middle. However during any given timespan, both ITFs are chosen by various images, emphasizing the true bimodal behavior of the camera.

The use of 2 ITFs should insure the best signal-to-noise ratio in the fully processed data. Since the two ITFs have somewhat different geometric formats, each ITF has its own, independently derived wavelength calibration. Because the same images were used for both ITFs, the photometric behavior should be the same. However, to insure that there are no subtle differences, the other calibrations (sensitivity degradation, absolute calibration) were derived independently for each ITF.

What about LWR high-dispersion processing? The high-dispersion images do not seem to show the same bimodal behavior as the low-dispersion images. This may be because the presence of the high-dispersion orders obscures this behavior. In testing, it was very difficult to find any images that clearly preferred the modified ITF. Thus it has been decided to use only one ITF for LWR high-dispersion processing, the original ITF (LWR83R94A).

Coming Attractions: Updated SWP Sensitivity Degradation for 1993-1996

The SWP sensitivity degradation correction currently implemented in NEWSIPS was derived several years ago and must extrapolate for data taken in the last couple years of the IUE mission. Recent analysis by Matt Garhart has shown that the SWP sensitivity degraded at an increased rate, so that the current calibration underestimates the SWP fluxes by up to 5% in late 1996. Note that this affects both low- and high-dispersion data.

Matt has derived an update to the SWP sensitivity degradation correction that will soon be implemented into NEWSIPS processing. The update implements a linear correction with a larger slope for data from 1993 onward, defined to be continuous with the correction up to 1993 so that there is no "jump" in the fluxes for images taken before and after the new correction is applied. The details of this analysis will be published in the NASA IUE Newsletter and made available on the Web.

We are planning to reprocess all SWP low- and high-dispersion data from 1993 onward with the new sensitivity degradation correction, beginning sometime in the next few weeks. The new version of the data will be identifiable from the processing history NEWSIPS version (2.5.3 for SWP low dispersion, 3.1.1 for SWP high dispersion).

What about the LWP? Testing shows that there is no need to change the sensitivity degradation correction. The extrapolation for later data works fine.

Questions?

If you have any questions about IUE processing or the Final Archive, please feel free to contact me at imhoff@iuegtc.gsfc.nasa.gov, or at (301) 794-1470

- Cathy Imhoff


Reading NEWSIPS Extracted Spectral Files

Many IUE users seem to be under the impression that the NEWSIPS MXLO and MXHI files are written in a non-standard FITS format that can only be read using the IUEDAC software. This is not true. The FITS format used for these files was approved by the IAU in 1994 and there are currently many publicly-available FITS readers that can read them.

One of the best supported, and most extensive, FITS readers is the FITSIO and FTOOLS software written in C and FORTRAN by Bill Pence from HEASARC. For information on downloading these programs, see the HEASARC web page at

http://heasarc.gsfc.nasa.gov

Listed below are examples of how to extract and plot flux vs. wavelength from an IUE MXLO file using several different IDL FITS readers. All of the listed programs contain additional documentation in the procedure prologs. The first three readers are available in the ASTRON library, maintained and distributed by Wayne Landsman, which is available from

http://idlastro.gsfc.nasa.gov/homepage.html

The last two programs are from the IUEDAC software library. Only the last program (readmx) was written specifically for reading IUE extracted spectral files.
1) readfits and tbget
written by J. Woffard & Wayne Landsman from the UIT project:

tab = readfits('lwp05426.mxlo',h,/exten)
w0 = tbget(h,tab,'wavelength',0)
dw = tbget(h,tab,'deltaw',0)
w = w0 + findgen(640)*dw
f = tbget(h,tab,'flux',0)
plot,w,f
2) fxbopen and fxbread
written by Bill Thompson of the SOHO project:

fxbopen,un,'lwp05426.mxlo',1,h
fxbread,un,w0,'wavelength',1
fxbread,un,dw,'deltaw',1
w = w0 + findgen(640)*dw
fxbread,un,f,'flux',1
plot,w,f
3) mrdfits
written by Tom McGlynn from HEASARC:

res=mrdfits('lwp05426.mxlo',1,h,range=1)
w = res.wavelength + findgen(640)*res.deltaw
plot,w,res.flux
4) ifitsrd
from the IUEDAC library:

ifitsrd,'lwp05426.mxlo',1,h,e,res,/struc
; 
; Reading binary table row:       1
;Column   1:   field = APERTURE data type = A  npoints = 5
;Column   2:   field = NPOINTS  data type = I  npoints = 1
;Column   3:   field = WAVELENGTH data type = E  npoints = 1
;Column   4:   field = DELTAW   data type = E  npoints = 1
;Column   5:   field = NET      data type = E  npoints = 640
;Column   6:   field = BACKGROUND data type = E  npoints = 640
;Column   7:   field = SIGMA    data type = E  npoints = 640
;Column   8:   field = QUALITY  data type = I  npoints = 640
;Column   9:   field = FLUX     data type = E  npoints = 640
w = res.wavelength + findgen(640)*res.deltaw
plot,w,res.flux
5) readmx
from the IUEDAC software:

readmx,'lwp05426.mxlo',h,w,f,nu,sigma
;LARGE aperture data to be extracted.
; 
;Trimming uncalibrated data regions (nu flag = -2).
;77 points removed.    563 data points remain.
plot,w,f
- Randy Thompson

An IUE Final Archive Browser

The Astrophysics Data Facility (ADF) of the SSDOO is furnishing a browser for the Final Archive IUE data as they are ingested into NDADS. The browser provides a preview of the spectra, selected header information, and an abbreviated version of the extracted spectra as a GZIPPED ASCII file. These files will enable a researcher to decide whether it is worthwhile to request the full, unextracted data from NDADS, and to perform immediate, initial analysis.

The browser files are accessible through the ADF search engine, WISARD, which can be found at the URL

http://hypatia.gsfc.nasa.gov/wisard/wisard.html

To access a browser file, simply click on the B hyperlink in the table returned by a WISARD search.

So far, more than 50,000 preview files have been produced. These are low dispersion SWP and LWP spectra. The ADF is currently processing Final Archive high dispersion SWP images, and will provide preview images and GZIPPED ASCII versions of extracted, combined order spectra for these.

- Derck Massa


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


<- Back to the IUEDAC Information Page
<- Back to the IUE Project Page

Webmaster

Mr. James R. Caplinger, Computer Sciences Corporation

Responsible NASA Organization/Official

Dr. Don West, IUE Operations Scientist