IRAS Explanatory Supplement
X. The Formats of the IRAS Catalogs and Atlases
E. Low-Resolution Spectra
Two files give the data for the low-resolution spectra (LRS) which, as described in Chapter IX, consist of two bands (8-13 µm and 11-25 µm). The first file on the tape consists of a single 80-character ASCII record giving the date and version of the LRS data. The next file lists only the wavelengths corresponding to each sample of the spectra (Table X.E.1a). The last file contains the spectra and associated header information (Table X.E.1b). The spectra are in order of increasing right ascension. Both files have 80-character ASCII logical records and 256 logical records per physical record. An atlas of the spectra will be published as an Astronomy and Astrophysics Supplement.
Distance from Slit. ANGLE(100)
As described in detail in
Chapter IX, each sample in the
spectrum corresponds to a certain in-scan distance of the source
from the centerline of the spectrometer entrance aperture. ANGLE
lists these angular distances.
Wavelength Calibration.: LAMBDA1, LAMBDA2
There is a non-linear relation between the displacement of
the source from the centerline of the spectrometer and the sampled
wavelength. The wavelengths corresponding to each sample (or
ANGLE) are given in LAMBDA1 and LAMBDA2 for the two wavelength
bands. The beginning and end of each spectrum contain measurements
that lie outside of the wavelength coverage of the instrument
but which can be used for electronic baseline determination. The
wavelength values corresponding to these values of sample number
(or ANGLE) are set to 0.
The items starting at bytes 0 through 25 are identical to
the items with the same names in the point source catalog (see
Section X.B.1).
This is also the case for the items starting at
bytes 71, 73, 75 and 960.
Number of Spectra: NSPECTRA, NACCEPT
NSPECTRA is the number of spectra of this source observed;
NACCEPT are the numbers of 8-12 µm and the 11-25 µm spectrum halves
ultimately averaged to make the entry in the catalog.
Characterization of the Spectrum.: LRSCHAR
A description of the method of characterization is given
in Section IX.D.
Table X.E.2 lists the spectral classes used
to characterize the spectrum.
Quality of the Spectrum Halves:. SPQUAL
Depending on the signal-to-noise ratio of the 8-12 µm and
11-25 µm halves of the spectra, the number of accepted spectra
halves and the difference in level of the baselines on either
side of the spectrum halves, a quality digit is assigned to each
half of the spectrum; 1 indicates good quality, 2 moderate quality,
and 3 barely acceptable.
Scale factor For All Spectrum Flux Densities: SCALE
Multiplying the integers BASELINE, NOISE, and SPECTRUM by
the factor SCALE converts the values into units of
W m-2µm-1.
Baseline of Spectrum Halves: BASELINE
These four values give the average value of samples 1 through
20 (short wavelength end) and 81 through 100 (long wavelength
end) of both the 8-12 µm and the 11-25 µm halves of the spectrum.
RMS Noise: NOISE
Using the twenty samples on the long wavelength end used
for BASELINE, the rms noise per spectrum half is determined.
Signal-to-Noise Ratio: SNR
The average value of the samples in the wavelength ranges
8-12 µm and 11-25 µm, respectively, are divided by the NOISE values
determined.
Baseline Asymmetry: ASYMM
This value indicates by what fraction of the average signal
the baselines on the short and long wavelength sides of the spectrum
halves differ. A large baseline asymmetry indicates that a confusing
source may have contaminated the spectrum. The baseline asymmetry
is usually large near the Galactic plane.
LRS/Survey Flux Ratio:. SRATIO
The ratio of the integrated flux (after convolution of the
spectrum flux densities with the 12 µm band pass of the survey
instrument) in the spectrum and the 12 µm survey flux is given
in this item. Normally this value should be close to unity
(Section
IX.C). As the 11-25 µm part of the LRS spectrum hardly overlaps
with the 25 µm survey band, a ratio of LRS/survey for this band
is not significant.
The Spectrum: SPECTRUM
The integer values of the spectrum must be multiplied by
SCALE for conversion to m-2µm-1. The wavelengths
corresponding
to the 100 samples given for the 8-12 µm and the 11-25 µm halves
of the spectrum are given in the catalog header file. Values for
non-significant wavelengths are set to zero. For baseline interpolation,
either the sample numbers or the ANGLE (distance in arcmin from
the spectrometer center line) can be used.
E.1. Catalog Header File
E.2. Spectra Records
Table X.E.1a
Byte Name Description Units Format
0 ANGLE Angle from center of slit (one number per sample) arc min 100F8.4
800 LAMBDA1 Wavelength in 8-12 µm corresponding to each sample µm 100F8.4
1600 LAMBDA2 Wavelength in 11-25 µmcorresponding to each sample µm 100F8.4
Table X.E.1b.
Byte Name Description Units Format
0 NAME Source name --- 11A1
11 HOURS RA 1950 hrs I2
13 MINUTE RA 1950 min I2
15 SECOND RA 1950 deci-sec I3
18 DSIGN Declination sign ± A1
19 DECDEG DEC 1950 arc deg I2
21 DECMIN DEC 1950 arc min I2
23 DECSEC DEC 1950 arc sec I2
25 FLUX Averaged non-color corrected flux densities (1 value per band) Jy 4E9.3
61 NSPECTRA No. of observed spectra --- I2
63 NACCEPT No. of accepted spectrum halves --- 2I2
67 LRSCHAR Characterization of spectrum --- 2I1
69 SPQUAL Quality of 8-12 µm and 11-25 µm parts of spectra --- 2I1
71 VAR Percent of variability likelihood (from catalog) ---
I2
73 NID No. of associations (<25) --- I2
75 IDTYPE Type of association --- I1
76 SPARE 4 spare bytes --- 4A1
(---------------new record--------------)
80 BASELINE Average of outer 20 samples of spectra. Short and long wavelength end for each spectrum half scaled by SCALE 4I4
96 NOISE RMS noise per sample (one value per spectrum half) scaled by SCALE 2I4
104 SNR Signal-to-noise ratio (average signal in spectrum part divided by noise, one value per spectrum half) --- 2E10.3
124 ASYMM Relative baseline asymmetry (difference of left and right baselines divided by average signal. one value per band) --- 2E10.3
144 SRATIO Ratio of integrated LRS flux to 12 µm survey flux --- F5.2
149 SCALE Scale factor for all flux densities W m-2µm-1 E11.5
(---------------new record---------------)
160 SPECTRUM 100 samples for each of the two bands (8-12 µm, 11-25 µm), scaled by SCALE scaled by
SCALE 200I4
(---------------new record--------------)
960 ID#1 Association field from main catalog
(see Section X.B.1) --- 40A1
1000 ID#2 additional records as required.
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