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

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

The Two Faces of the LWR Camera

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

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?

- Cathy Imhoff

Reading NEWSIPS Extracted Spectral Files

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

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

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

An IUE Final Archive Browser

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

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

news@iuedac.gsfc.nasa.gov

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

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