[Adapted from the Explanatory Supplement]
The spectra in the Low-Resolution Spectrometer (LRS) Catalog
are divided into two bands (8-13 microns and 11-22 microns). The file
LRS_HEADER.DAT lists only the wavelengths corresponding to each channel
of the spectra (see Table 1 below). The file LRS_SPECTRA.DAT contains
the spectra and associated header information (see Table 2). The
spectra are in order of increasing right ascension of the corresponding
point source. As described below, the spectra are identified by the
name of the corresponding source in the Point Source
Catalog. Both files have 80-character ASCII records.
|Table 1. Catalog Header File
||Angle from center of slit (one number per sample)
||Wavelength in 8-13 micron corresponding to each sample
||Wavelength in 11-22 micron corresponding to each sample
- Distance from Slit: ANGLE(100)
- As described in detail in Chapter IX of the Explanatory Supplement,
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 baseline determination. The wavelength values
corresponding to these values of sample number (or ANGLE) are set to 0.
|Table 2. Spectrum Records
||Averaged non-color corrected flux densities (1 value per band)
||No. of observed spectra
||No. of observed spectrum halves
||Characterization of spectrum
||Quality of 8-13 micron and 11-22
||Percent of variability likelihood
||No. of associations (<25)
||Type of association
||4 spare bytes
|---- New Record ----
||Average of outer 20 samples of spectra. Short and long wave-length end for each spectrum half (from catalog)
||scaled by SCALE
||RMS noise per sample (one value per spectrum half)
||scaled by SCALE
||Signal-to-noise ratio (average signal in spectrum part divided by noise, one value per spectrum half
||Relative baseline asymmetry (difference of left and right baselines divided by average siganl; one value per band)
||Ratio of integrated LRS flux to 12 micron survey flux
||Scale factor for all flux densities
||W m-2 micron-1
||100 samples for each of the two bands (8-13 micron, 11-22 micron) scaled by SCALE
||scaled by SCALE
||Association field from main catalog
||Additional records as required
The entries starting at bytes 0 through 25 are identical to the
items with the same names in the
Point Source Catalog. This is also the case
for the entries starting at bytes 71, 73, 75, and 960. Their
explanations will not be duplicated here, but may be found in the
description of the format of the PSC.
- Number of Spectra: NSPECTRA, NACCEPT
- NSPECTRA is the number of spectra observed for the source;
NACCEPT is the number of 8-13 micron and the 11-22 micron 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 of the Explanatory Supplement. Table 3 lists the spectral
classes used to characterize the spectrum.
- Quality of the Spectrum Halves: SPQUAL
- Depending on the signal-to-noise ratio of the 8-13 micron and
11-22 micron halves of the spectra, the number of accepted spectrum
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
- 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-2 micron-1.
- Baseline of Spectrum Halves: BASELINE(4)
- These four values are the averages of samples of 1 through 20 (short
wavelength end) and 81 through 100 (long wavelength end) of both
the 8-13 micron and 11-22 micron 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 8-13
microns and 11-22 microns, respectively, are divided by the NOISE
- 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 spectral
flux densities with the 12 micron bandpass of the survey instrument)
in the spectrum and the 12 micron survey flux is given in this item.
Normally this value should be close to unity (section IX.C of the
Explanatory Supplement). As the 11-22 micron part of the LRS spectrum
hardly overlaps with the 25 micron survey band, a ratio of
LRS/survey for this band is not significant.
- The Spectrum: SPECTRUM(200)
- The integer values of the spectrum; they must be multiplied by
SCALE for conversion to W m-2 micron-1. The wavelengths
corresponding to the 100 samples given for the 8-13 micron and
the 11-22 micron halves of the spectrum are given in the catalog
header file LRS_HEADER.DAT. 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.