X.E. Low-Resolution Spectra

IRAS Explanatory Supplement
X. The Formats of the IRAS Catalogs and Atlases
E. Low-Resolution Spectra

Chapter Contents | Authors | References
Table of Contents | Index | Previous Section | Next Section

  1. Catalog Header File
  2. Spectra Records

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.

E.1. Catalog Header File

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.

E.2. Spectra Records

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.


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.

Header Information for Catalog of Spectra
Table X.E.1a
0ANGLEAngle from center of slit (one number per sample)arc min100F8.4
800LAMBDA1 Wavelength in 8-12 µm corresponding to each sample µm 100F8.4
1600LAMBDA2 Wavelength in 11-25 µmcorresponding to each sample µm 100F8.4

Format of Spectra in Catalog
Table X.E.1b.
Byte NameDescriptionUnitsFormat
0NAMESource name--- 11A1
11HOURSRA 1950hrs I2
13MINUTERA 1950minI2
15SECOND RA 1950deci-secI3
18DSIGN Declination sign± A1
19DECDEG DEC 1950arc degI2
21DECMIN DEC 1950arc minI2
23DECSEC DEC 1950arc secI2
25FLUXAveraged non-color corrected flux densities (1 value per band)Jy 4E9.3
61NSPECTRANo. of observed spectra--- I2
63NACCEPT No. of accepted spectrum halves --- 2I2
67 LRSCHAR Characterization of spectrum--- 2I1
69SPQUALQuality of 8-12 µm and 11-25 µm parts of spectra--- 2I1
71VARPercent of variability likelihood (from catalog)--- I2
73NIDNo. of associations (<25) ---I2
75IDTYPEType of association ---I1
76SPARE4 spare bytes---4A1
(---------------new record--------------)
80BASELINEAverage of outer 20 samples of spectra. Short and long wavelength end for each spectrum halfscaled by SCALE4I4
96NOISERMS noise per sample (one value per spectrum half)scaled by SCALE2I4
104SNR Signal-to-noise ratio (average signal in spectrum part divided by noise, one value per spectrum half) ---2E10.3
124ASYMMRelative baseline asymmetry (difference of left and right baselines divided by average signal. one value per band)---2E10.3
144SRATIORatio of integrated LRS flux to 12 µm survey flux--- F5.2
149SCALEScale factor for all flux densities W m-2µm-1 E11.5
(---------------new record---------------)
160SPECTRUM100 samples for each of the two bands (8-12 µm, 11-25 µm), scaled by SCALEscaled by SCALE200I4
(---------------new record--------------)
960ID#1 Association field from main catalog (see Section X.B.1)--- 40A1
1000ID#2 additional records as required.

Chapter Contents | Authors | References
Table of Contents | Index | Previous Section | Next Section