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FUSE is in low-Earth orbit, so astronomical spectra taken with FUSE
include various emissions that originate in the terrestrial atmosphere.
The strongest of these ``airglow'' emissions is the H I Lyman-beta line
at 1026 Angstroms, but many other airglow lines can be seen in FUSE
spectra, including other members of the H I Lyman series, various O I
multiplets, an N I multiplet near 1134 Angstroms, and two He I lines
(537 and 584 Angstroms) that appear in second order (i.e., at 1074 and
1168 Angstroms).
If you are searching for these same lines in an astronomical source, or if there is an accidental coincidence between the lines of interest and the airglow lines, then your scientific objectives may be affected by the presence of the terrestrial airglow. So that FUSE users and proposers can gauge the effects that airglow emissions may have on their programs, the FUSE Project is making various airglow spectra publicly available. Notes About the Spectra
Please note that the spectra presented below contain at least two
emissions that are not from the airglow: emission centered near 977
Angstroms is scattered light from the strong solar C III line, and
emission near 1034 Angstroms is scattered light from the strong solar O
VI multiplet. These weak solar lines occur only in the SiC channels.
The strengths of these two emissions depend strongly on the spacecraft
attitude; sometimes these emissions are undetectable.
An attempt was made to remove burst events (times when
anomalously high count rates are detected; the origin of these
occasional outbursts is unknown) from the spectra, but this effort was
not entirely successful. In particular, some of the daytime spectra
taken through the MDRS aperture seem to have anomalously high noise in
some regions, and this may be due to incomplete elimination of burst
events. These anomalous regions generally extend over a much larger
range in wavelength than true airglow emissions, which consist of
atomic and ionic lines.
The spectra presented here were taken at high beta angle (95.9 deg) and
at Earth-limb angles between 24 and 30 degrees. Spectra taken at
different geometries will presumably have somewhat different background
light levels and/or airglow emission line intensities from those
presented here.
Bottom line: If your analysis of the airglow spectra presented here
indicates that your scientific investigation may be compromised
by airglow emissions, you should send a message to
fuse_support@pha.jhu.edu
to discuss your concerns in more detail.
The Data
Eventually we hope to post plots and lists of the strongest airglow
emissions here, but for the time being we are simply pointing you to an
anonymous ftp site that has various FITS files of FUSE airglow
spectra. These spectra were taken during a background observation in
the Continuous Viewing Zone (CVZ) with no target in any of the
apertures and should be representative of the airglow emissions that
will be detected during FUSE astronomical observations. The data set
consists of 24 files:
The spectra for the LiF2 and SiC2 channels are identical to the LiF1 and SiC1 channels, with the exception of the change in effective area and slightly different wavelength coverage. Therefore, only spectra from side 1 are included here. (The wavelengths on side 2 that are not included on side 1 do not contain any airglow lines.) The files are in the standard 1-D extracted format of the CALFUSE pipeline. Each file contains a binary table in the first extension with 6 columns: wavelength, flux, 1 sigma error on the flux, quality flags, counts, and 1 sigma error on the counts. The relevant orbital parameters during the airglow observations are:
DAY exposure time approximately 25500 seconds for LiF1A The names of the files encode information about the data contained within them. For example, the filename
refers to LiF 1a spectra taken during daytime through the number 2 (MDRS) aperture. All the data discussed here are were taken in TIME-TAG mode and have been converted into absolute flux-calibrated spectra. The correspondence between the aperture number and its commonly used name is given in the following table:
Here is the list of all 24 filenames: I2050302DAY1alif2ttagfcal.fit I2050302DAY1alif3ttagfcal.fit I2050302DAY1alif4ttagfcal.fit I2050302DAY1asic2ttagfcal.fit I2050302DAY1asic3ttagfcal.fit I2050302DAY1asic4ttagfcal.fit I2050302DAY1blif2ttagfcal.fit I2050302DAY1blif3ttagfcal.fit I2050302DAY1blif4ttagfcal.fit I2050302DAY1bsic2ttagfcal.fit I2050302DAY1bsic3ttagfcal.fit I2050302DAY1bsic4ttagfcal.fit I2050302NIT1alif2ttagfcal.fit I2050302NIT1alif3ttagfcal.fit I2050302NIT1alif4ttagfcal.fit I2050302NIT1asic2ttagfcal.fit I2050302NIT1asic3ttagfcal.fit I2050302NIT1asic4ttagfcal.fit I2050302NIT1blif2ttagfcal.fit I2050302NIT1blif3ttagfcal.fit I2050302NIT1blif4ttagfcal.fit I2050302NIT1bsic2ttagfcal.fit I2050302NIT1bsic3ttagfcal.fit I2050302NIT1bsic4ttagfcal.fit You can download individual files now by clicking here. Or, you can download the entire set of files (4.5 MB) in gzipped tar format. Or, if you prefer, you can download the files by following these instructions from your terminal session:
ftp fuse.pha.jhu.edu Any questions regarding these airglow spectra should be sent to fuse_support@pha.jhu.edu Acknowledgement: These airglow spectra were compiled primarily through the efforts of Ed Murphy, Paul Feldman, and Dave Sahnow. A discussion of FUSE airglow spectra appeared in a poster presented at the January 2000 AAS meeting (Feldman, Murphy, Sahnow, Moos, et al., 2000). |
This web page prepared by Hal Weaver
Last updated: 31 May 2000
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