IRAS Explanatory Supplement
X. The Formats of the IRAS Catalogs and Atlases
B. Point Sources
B.3 The Working Survey Data Base
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The most complete observational and data reduction history for point sources is contained within the Working Survey Data Base (WSDB). The WSDB is broken up into 20 files, one for each range of ecliptic longitudes called a Lune. Each file is preceded by a header file containing a single 80-character ASCII record giving the date and version of the WSDB (Table X.A.1). Except as noted below, the entries are similar to those listed above for the catalog tape. Table X.B.3a lists the entries, their variable type and length. A second file, called the Ancillary file (Table X.B.3b) contains additional flags and derived quantities obtained during final product generation. Most of the information in the ancillary file is the same as that in the catalog tape described in Section X.B.1. Character variables are in ASCII format. All arithmetic quantities are in integer format with the high order byte first.
It is important to note that the WSDB and Ancillary files are in binary format with variable length, blocked records (due to the variable number of hours-confirmed sightings or associations). Extra information has been added to the WSDB and Ancillary file tapes to make it possible to read in these variable length records. Each physical block on the tape begins with a Block Control Word of length I*4. Its high order two bytes give the length of the physical block in bytes; this length includes the 4-byte length of the Block Control Word itself. In addition, each logical record within the physical block is preceded by a Segment Control Word of length I*4. Its high order two bytes give the length of the logical record in bytes; again, this length includes the 4-byte length of the Segment Control Word itself. The inclusion of this information should make it possible for computers that can work with Variable Block formats to easily read the tape. In no case does the information for a single source span two physical blocks.
The remainder of this section describes WSDB and ancillary file entries not discussed above.
SDAS Lune Number: LUNEIn the data reduction the sky was divided into twenty "lunes" based on ecliptic coordinates. Lune 1 comprises that part of the sky with (X) > 60°; Lune 2 comprises that sky with (X) < -60°. Lunes 3-20 comprise that part of the sky with |(X)| 60° and ecliptic longitudes, (X) in 20° wide blocks. Lune 3 extends from 0° (X) < 20°, lune 4 from 20° < (X) < 40°. etc.
Ecliptic Bin Number: BINTo aid the data reduction the entire sky was divided into some 40,000 1° sq. bins. The bin structure is quite simple in ecliptic coordinates (Fig. X.Ap1.1) and an algorithm for generating bin numbers is given in Appendix X. 1.
Ecliptic Coordinates: ELAT, ELONGEcliptic coordinates are given in units of 10-8 radians in the equinox 1950.
Scan Angle: SCANAfter weeks-confirmation SCAN gives the average scan angle of the focal plane with respect to the south-going local ecliptic meridian.
Positional Uncertainty: SIGY, LZ, SIGZAs discussed in detail in Section V.D, the position refinement describes the source positional uncertainty in terms of 1(X) in-scan and cross-scan gaussian uncertainties (SIGY and SIGZ) and a uniform uncertainty (of half-width LZ) whose size depends on the exact paths of the source across the focal plane.
Number of LRS Extractions: LRSXEach time a source with a signal-to-noise ratio greater than 25 at either 12 or 25 µm transversed the focal plane, the data reduction software automatically triggered a request to extract a spectra from the low-resolution spectrometer data (Chapter IX). The threshold was purposefully set quite low so that sources with weak continuua but strong lines could be detected. Many sources with LRSX > 0 will fail to have meaningful spectra and will thus have NLRS = 0.
Known Source ID: KSIDThe positions and predicted brightnesses of some 32,000 point sources including SAO stars, IRC objects and asteroids were incorporated into the data reduction software to provide a check on the positional and photometric accuracy of the IRAS sources. Table X.B.5 lists the range of KSID values assigned to sources of various types.
The following values are given for each hours-confirmed sighting:
Flux and Flux Uncertainty: FLUX(4),SIGF(4)Flux and flux uncertainty measurements are given in units of 10-16 W m-2 for each hours-confirmed sighting. Note that an instrumental flux, not the flux density, is given. In order to convert the instrumental flux to flux density, one must divide the instrumental flux by 13.48, 5.16, 2.58, 1.00 × 1012 Hz at 12, 25, 60 and 100 µm respectively. The derivation of flux uncertainties and the averaging of the individual hour-confirmed fluxes to give the average value (see AVGFLUX, below) quoted in the catalog is discussed in Section V.H.5.
Signal-to-Noise Ratio: TSNR(4)If a source is detected (but not necessarily even seconds-confirmed) in a given band, then a value equal to ten times the maximum signal-to-noise ratio (SNR) observed on any detector in that band in the hours-confirmed sighting is retained. In relatively simple parts of the sky the noise estimator used to derive SNR gives reasonable values. In more complex regions near the Galactic plane (see Section V.C.2) the utility of SNR is very limited.
Correlation Coefficient: CORR
The maximum point source correlation coefficient (see Section V.C.4) obtained during a hours-confirmed sighting is retained for each band. The values for each band, expressed as percentages up to a maximum of 100, are encoded into a single integer according to the following algorithm:
Flux Status: FSTAT
In each band there is a hierarchy of measurement quality
depending on how many times a given source is observed within
a given hours-confirmed sighting. FSTAT plays a crucial role in
determining whether a source is included in the catalog at all,
whether a flux is of high, medium or low quality and how the flux
averaging was performed
(Section V.H.5). for the meaning of each
FSTAT value refer to Section V.D.8.
The values of FSTAT for the four bands are compressed into a single integer according to the following algorithm:
Each value of DETID contains an integer that must be decoded
according to the following algorithm to obtain the detectors observing
At various stages in the reconstruction of a point source
attempts are made to recognize (and remedy) the effects of confusion
between nearby sources. The confusion status word CSTAT plays
an important role in selecting sources to be treated by the
"clean-up" processor (Section V.H.2)
and in determining which sources to keep in regions of high source density
(see Section V.H.6). For
a detailed discussion of confusion processing see
Values of CSTAT and brief descriptions of their meaning are given
in Section V.D.8.
The values of CSTAT for each band are encoded into a single
integer according to the following algorithm:
(X.B.4) (X.B.5) (X.B.6)
Detector ID's: DETID
The identities of all detectors observing a given source
on the sightings (up to a maximum of 3 sightings) comprising a
given hours-confirmed observation are recorded in the array DETID(I,
The four bands run from I = 1 to 4 while the three sightings run
from J = 1 to 3.
Detector Name: DNAM, TNAM
Throughout the course of the data processing each hours-confirmed
sighting is known by a combination of the first detector measuring
the object (DNAM) and the time (in deci-UTCS since 1981, January
1, 0h UT) of that sighting (TNAM).
Confusion Status: CSTAT, PNEARH, PNEARW
Cirrus Flags:. CIRRUS, CIRR1, CIRR2, CIRR3
The three flags denoting the presence of extended 100 µm
emission ("cirrus") as discussed in
Section V.H.4, are
encoded in two bytes according to the algorithm:
Small Extended Source Flags: SES1, SES2
The two small extended source flags for each band discussed
in Section V.E.1 are encoded into two integers
according to the following algorithm:
Each value of DETID contains an integer that must be decoded according to the following algorithm to obtain the detectors observing the source:
At various stages in the reconstruction of a point source attempts are made to recognize (and remedy) the effects of confusion between nearby sources. The confusion status word CSTAT plays an important role in selecting sources to be treated by the "clean-up" processor (Section V.H.2) and in determining which sources to keep in regions of high source density (see Section V.H.6). For a detailed discussion of confusion processing see Section V.D.2. Values of CSTAT and brief descriptions of their meaning are given in Section V.D.8.
The values of CSTAT for each band are encoded into a single integer according to the following algorithm:
(X.B.4)The number of hours- and weeks-confirmed point sources within 6' × 4.5' (half-widths) of the quoted source, PNEARH and PNEARW, are encoded into a single byte:
(X.B.6)Values of CIRR2 = 0 and CIRR3 = 255 mean no data were available.
Clean up Processors: CLEAN, BRIGHT, ACCEPT, HSDPROC, MISC
In creating the WSDB from the weeks-confirmed data, several processors were applied in order to fix various known problems (see Section V.H). These processors set various flags that allow the user to understand what processing occurred. The clean-up processor allowed the weeks-confirmation of various WSDB sources that did not previously have an opportunity to weeks-confirm for purely technical reasons or which were incorrectly split asunder during band merging. The flags set by that processor are described in Table X.B.7a, and include flags indicating saturated fluxes. Optical crosstalk from certain bright objects such as Saturn and IRC+10216 produced a few spurious sources. These were marked for deletion. The byte BRIGHT (Table X.B.7b) notes these cases and also contains flags denoting final acceptance (ACCEPT) or rejection of the source in each band. The flags from the High Source Density processor (HSDPROC, see Section V.H.6) are given in 2 bytes per band (Table X.B.7c). Miscellaneous flags are set in MISC (Table X.B.7d) and include the presence of flux discrepancies in each band, whether the in-scan positional uncertainties needed to be increased (Section VII.C) and whether the source was accepted in the catalog.
Byte Name Description Units Type
0* LUNE SDAS Lune number --- I*4
4 BIN Ecliptic Bin Number --- I*4
8 ELONG Ecliptic longitude 1950. 10-8 rad I*4
12 ELAT Ecliptic latitude 1950. 10-8 rad I*4
16 SCAN average scan angle with respect to ecliptic meridian milli-rad I*2
18 SIGY In-scan Gaussian position uncertainty µ-rad I*2
20 LZ Cross-scan Uniform position uncertainty µ-rad I*2
22 SIGZ Cross-scan Gaussian position uncertainty µ-rad I*2
24 LRSX Total number of LRS extraction requests --- I*2
26 KSID Known Source ID --- I*2
28 NHCON Number of Hours --- I*4
(<25) confirmed sightings
The following values repeat for each hours-confirmed sighting:
32 FLUX FLUX(I)=In-band power 10-16W m-2 4I*4
48 SIGF SIGF(I)=Uncertainty in FLUX(I) 10-16W m-2 4I*4
64 TSNR TSNR(I)=10x(max SNR) --- 4I*2
72 CORR Maximum correlation coefficient (1 value per band, Eq. X.B.1) --- I*4
76 FSTAT Flux status word (1 value per band, Eq.X.B.2) --- I*2
88 DETID Detector ID Array (4,3) for 4 bands
(Eq.X.B.3) --- 12I*2
102 LRSXNO Number of LRS extraction requests --- I*1
103 DNAM detector part of source name --- I*1
104 TNAM deci-UTCS part of source name deci-sec I*4
108 CSTAT Confusion status flags
(Eq.X.B.4) --- I*4
112--- 191 repeats bytes 36-115
192--- 271 for subsequent hours etc... confirmed sightings.
Byte Name Description Units Type
0 PNEAR Nearby hours and weeks-confirmed neighbors --- I*1
1 CLEAN Clean up Processor flags --- I*1
2 SES1 Number of nearby unconfirmed SES (4 bands encoded) --- I*2
4 SES2 Number of nearby weeks-confirmed SES (4 bands encoded) --- I*2
6 CIRR1 Number of 100 µm only WSDB sources,
spatially filtered 100 µm emission.
CIRR3 value of 100 µm half-degree beam total intensity MJy sr-1
8 AVGFLUX AVGFLUX(I) Averaged flux (in-band power) in band I. 10-16W m-2 4I*4
24 AVGUNC AVGUNC(I) Uncertainty in averaged (in-band) flux in band I. 10-16W m-2 4I*4
40 HSDPROC HSDPROC(I). Flags set by the high source density processor in band I --- 4I*2
48 RA Right ascension 1950 10-5" I*4
52 DEC Declination 10-5" I*4
56 NAME source name --- A*12
68 NLRS Number of meaningful LRS spectra --- I*2
70 LRSCHAR characterization of LRS spectra --- A*2
72 BRIGHT Bright Source Clean up flag for catalog sources accepted in the catalog --- I*1
73 VAR Percent likelihood of Variability --- I*2
74 FQUAL FLUX Quality flags (one per band)
BIT 0-1, 12 µm
BIT 2-3, 25 µm, etc.
--- I*1 75 MISC Miscellaneous Status bits, incl. DISC, ACCEPT,SIGY
See Table X.B.7d
76 LUNE SDAS Lune number --- I*4
80 BIN Ecliptic Bin Number --- I*4
84 ELONG Ecliptic longitude 10-8 rad I*4
88 ELAT Ecliptic Latitude 10-8 rad I*4
92 NID 2 Number of Associations --- I*2
94 IDTYPE Type of association --- I*2
96 CATNO Catalog Number --- I*2
98 SOURCE Source ID --- A*15
113 TYPE Source Type/Spectral Class --- A*5
118 RADIUS Position difference (") I*2
120 POS Position Angle ' E of N I*2
122 FIELD1 object field #1 --- I*2
124 FIELD2 object field #2 --- I*2
126 FIELD3 object field #3 --- I*2
128-160 continuation of association 3 in blocks of 32 bytes
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