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1985iasi.book.....Beichmann+
1988iasi.book.....Beichmann+

INFRARED ASTRONOMICAL SATELLITE (IRAS)

CATALOGS AND ATLASES

Explanatory Supplement

Prepared under the supervision of the Joint IRAS Science Working Group (JISWG)

Notes about this computer-readable documentation:

The following text is an excerpt from the original printed documentation, including Chapter I "Introduction", Parts A.1, A.2, and A.3, Chapter X "The Formats of the IRAS Catalogs and Atlases", Parts A, B, and B.1, and Chapter XII "Errata and Revisions as of 1987", Part A.

The text for Chapters X and XII were kindly supplied to the Astronomical data Center in the form of WordStar word processing files by Ms. Rosanne Fernandez, IPAC Librarian. The text for Chapter I was keyed-in at the astronomical Data Center (ADC). The text makes references to other chapters in she printed documentation. These references are in the form: XX.XX.XX, for chapter, section and subsection, respectively. For example, V.H.6 refers to chapter IV, section H, subsection 6. The reader is directed to the printed version of the IRAS documentation for these references.

Because of the restrictions of the ASCII character set, some notational differences appear between the printed documentation and this text. When appropriate, greek letters are written out as their english names, except the word "micron" appears instead of the term "Mu-meters" or "um". In general, subscripts appear in parentheses, for example, flux per unit frequency, "f subscript Nu", appears as f(Nu) and can be pronounced, "f sub Nu". Terms given in italics in the original text are enclosed in double quotation marks.

The description of the logical record format of the catalog (Chapter X) was changed to reflect the version distributed by the ADC. At the end of this computer-readable text is a discussion of the catalog as it appears in FITS-table format. This was authored at the ADC, and does not appear in the original documentation.

Please direct any reports of typographical errors to: 

   Lee Brotzman
   Code 930.3
   NASA/GSFC
   Greenbelt, MD 20771
   (301) 286-6953
   BITNET:   ZMLEB@SCFVM
   SPAN:     CHAMP::BROTZMAN
   Internet: zmleb@scfvm.gsfc.nasa.gov
             or  zmleb%scfvm@dftsrv.gsfc.nasa.gov

I. INTRODUCTION

A. General Overview

A.1 The IRAS Mission

The primary mission of the Infrared Astronomical Satellite (IRAS) was to conduct a sensitive and unbiased survey of the sky in four wavelength bands centered at 12, 25, 60, and 100 microns. The project was initiated in 1975 as a joint program of the United States, the Netherlands, and the United Kingdom. Launched in January 1983, IRAS ceased operations in November 1983 after having successfully surveyed more than 96% of the sky.

The results of several portions of the IRAS mission are given in a catalog of infrared point sources, in a catalog of extended sources smaller than 8', in a catalog of low-resolution spectra, and in an atlas of absolute surface brightness images of the entire infrared sky. These catalogs give the characteristics of some 250,000 point sources and 20,000 small extended sources down to a limiting flux density, away from confused regions of the sky, of about 0.5 Jy at 12, 25 and 60 microns and about 1.5 Jy at 100 microns for point sources, and about a factor of three brighter than this for small extended sources. The angular resolution of the instrument varied between about 0.5' at 12 microns to about 2' at 100 microns. The positional accuracy of sources detected by IRAS depends on their size, brightness and spectral energy distribution but is usually better than 20". Approximately 5000 8-22 micron spectra of survey sources brighter than 10 Jy at 12 and 25 microns are available.

A.2 The Explanatory Supplement

This Explanatory Supplement is intended to be a complete and self-contained description of the IRAS mission in relation to the products of the survey. In Chapter II, the IRAS satellite, telescope and focal plane instrumentation are reviewed. The elements of the mission profile -- the constraints, the design features, and the in-flight modifications to that design -- are described in Chapter III and are accompanied by a chronology of the events of the mission. In-flight tests of those aspects of the performance of the instrument directly associated with the survey are presented in Chapter IV. Chapters V and VI describe the processing performed on the data; the summaries that precede the detailed discussions should be sufficient to acquaint the user with the contents of the catalogs. Since the flux reconstruction and calibration of the instrument probably hold intrinsic interest for many readers, these are described separately in Chapter VI.

A preliminary analysis of some of the statistical properties of the catalogs is given in Chapter VII. Emphasis is placed on general statistics, such as positional and photometric accuracy and on easily derived number counts. A preliminary analysis of sky coverage and of the completeness and reliability of the catalog is given in Chapter VIII. The low-resolution spectrometer and the analysis of its measurements are described in Chapter IX. Chapter X explains the format and meaning of each of the entries in the catalogs. Each printed volume of the catalogs repeats the description of the formats of that catalog.

In order to produce the catalogs in a timely fashion, some processing errors and anomalies could not be fixed; those which were discovered before the release of the data in November 1984 are described in Chapter XI. A compilation of the names of people who worked on the IRAS project comprises Chapter XII. The last chapter provides a series of plots giving the details of the coverage of the sky by the IRAS survey.

Each chapter of the Supplement was written by those members of the IRAS team whose names are appended to that chapter. The work described was obviously the result of efforts by many individuals and should not be ascribed to the authors alone.

A.3 Cautionary Notes

While it is unlikely that all aspects of the instrumental performance or the data processing will be of interest to all readers, even casual readers should familiarize themselves with the various caveats described in the chapters appropriate to the type of data in question. All users of the IRAS data should be cognizant of the following crucial facts:

a) The sky at 100 microns is dominated by filaments termed "infrared cirrus" which, although concentrated in the Galactic plane, can be found almost all the way up to the Galactic poles. The primary, deleterious effects of the cirrus are that it can generate well-confirmed point and small extended sources that are actually pieces of degree-sized structures rather than isolated, discrete objects and that it can corrupt 100 micron, and occasionally 60 micron, measurements of true point sources (Sections V.H.4, VIII.D.2).

b) The spectral bandwidths of the detectors were sufficiently wide that the quoted flux densities depend on the assumed energy distribution of the source. For the catalogs, the energy distribution was taken to be constant in the flux per logarithmic frequency interval. If the source has a different energy distribution than this, a "color correction", as large as 50% in extreme cases, must be applied to the quoted flux densities (Section VI.C.3).

c) The survey is clearly "confusion limited" within about 10 degrees of the Galactic plane and in several areas of the sky such as the Ophiuchus and Orion-Taurus regions. Considerable effort has been made to select only highly reliable sources in such areas, at the expense of completeness. The flags associated with sources with possible confusion-related problems should be examined very carefully (Sections V.D.8, V.H.6, VIII.C, VIII.D and X.B).

d) The algorithm used to estimate the detector noise suffered from a significant lag. This caused an under-estimate of the true noise when approaching regions of rapidly changing noise and an overestimate of the noise when leaving such areas. Regions with large and rapidly varying numbers of sources, such as the Galactic plane, also produced this effect. Since the source detection algorithm (Section V.C) thresholded on signal to noise ratio, the overestimated noise level resulted in a dearth of sources, or a "shadow", in the areas observed just after passage across the Galactic plane. At 60 and 100 microns, where the effect is worst, a "coverage hole" can extend as far as 2 degrees from the plane. The density of detected sources can differ, totally artificially, by as much as a factor of ten from one side of the plane to the other due to this shadowing (Section VIII.D).

e) While great pains were taken to confirm the reality of sources in the point and small extended source catalogs, no such attempt was made for the sky brightness images. Instead, separate images of the sky taken at times differing from weeks to months are given. It is the responsibility of the user to ensure that sources seen in the images are not due to transient sources such as asteroids.
Authors: G. Neugebauer and C. A. Beichman.

X. THE FORMATS OF THE IRAS CATALOGS AND ATLASES

A. Introduction

This chapter describes the formats of the point source and small extended source catalogs, the total intensity images and the low-resolution spectra in their printed and machine-readable forms. A brief description is given of each entry in the catalogs; tables describe each column of the catalog in more detail and give, for the machine readable versions, the logical type of each variable and its length in bytes.

IRAS images are presented in the FITS format (Wells, Greisen and Harten 1981). Sample headers are given for the sky plates, the Galactic plane map and the all-sky map. The format of the 1/2 degree beam zodiacal history file is described in this chapter. The format of the extragalactic specialty catalog is described at the beginning of that catalog. IRAS data products discussed in this chapter include: 

 Point Sources:
 Machine-readable version of the Catalog (Section X.B.1)
 Printed version of the Catalog (Section X.B.2)
 Machine-readable version of the Working Survey Data Base (Section X.B.3)
 Small Extended Sources:
 Machine-readable version of the Catalog (Section X.C.1)
 Printed version of Catalog (Section X.C.2)
 Extended Emission Images:
 16.5 degree Images (photographic representation - Section X.D.3a)
 16.5 degree Images (machine readable format - Section X.D.3b)
 Galactic Plane Images (all formats - Section X.D.4)
 Low-Resolution Maps (all formats - Section X.D.5)
 Zodiacal History File (machine readable - Section X.D.6)
 Low-Resolution IRAS Spectra:
 Catalog of spectra (machine readable - Section X.E)

B. Point Sources

The information about infrared point sources is presented in increasing detail, progressing from the printed volumes to the tape version of the catalog to the detailed description of the observational and processing history of each source in a file known as the Working Survey Data Base (WSDB) augmented by its ancillary file. The printed version (Section X.B.2) is intended for users at the telescope or at institutions without computerized information retrieval systems. The catalog tape (Section X.B. 1) is intended for astronomers desiring to make statistical studies and to search the catalog for large numbers of sources. The WSDB and ancillary file (Section X.B.3) are meant to give the sophisticated researcher all the available data on any given source such as its brightness on each hours-confirmed sighting, the detectors involved and the details of the data reduction such as confusion with neighboring sources.

Another catalog available only in machine readable form lists the WSDB entries for all sources that failed one or more of the confirmation and confusion criteria and were not, thus, included in the main catalog. This file of REJECTED sources includes spurious objects, including: processing failures, space debris, asteroids and comets, and celestial sources that, due to incompleteness at faint levels or to variability, failed to achieve the minimum criterion of two hours-confirmed sightings. In regions of high source density the file includes sources rejected by the more severe criteria for reliability applied there (Section V.H.6). Caveat emptor.

B.1 The Machine Readable Version of the Point Source Catalog

The machine readable version of the catalog is divided into two files. The first is a 160 byte file containing the 252,889 sources arranged in order of increasing right ascension. The second is a 58 byte file containing 142,228 positional associations.

Tables X.B.1 and X.B.1a describe each entry in the catalog files. Those columns that are also included in the printed version are marked. In the table the column "Format" refers to the length and type of the (FORTRAN) character field used to read or write each entry. Figure X.B.1 describes the format of the printed version of the catalog.

In general, for quantities that have a value in each wavelength band, subscripts or array indices range from 1 to 4 and refer, respectively, to 12, 25, 60 and 100 microns. A number of the flags discussed below have values in each of the four wavelength bands. For compactness these are encoded into a single base-16 (Hex) digit (values 0-F) in the following manner (Table X.B.2). The four bits of that hex digit correspond to the four wavelength bands with bit 0 (Least Significant Bit) for 12 microns, bit 1 for 25 microns, bit 2 for 60 microns and bit 3 for 100 microns. The presence of a flag in a band is denoted by setting its bit to 1. Thus a source with a particular flag, e.g. CONFUSE, set at 12 and 25 microns would have CONFUSE = 0011 = 3 (Hex) while one confused in 25, 60 and 100 microns would have CONFUSE = 1110 = E (Hex). A flag encoded in this manner will be referred to as "hex-encoded by band".

The remainder of this section discusses individual entries in the catalog. At the end of the section a few remarks relevant only to the printed version of the catalog are given.

Table X.B.1 Format of Point Source Catalog Data File 

Start
Byte    Name            Description               Units         Format

00    NAME [1]    Source Name                     ---           11A1
11    HOURS       Right Ascension 1950.           Hours           I2
13    MINUTE      Right Ascension 1950.           Minutes         I2
15    SECOND [1]  Right Ascension 1950.           deci-Seconds    I3
18    DSIGN [1]   Declination Sign                +/-             A1
19    DECDEG      Declination 1950.               Arc Deg         I2
21    DECMIN      Declination 1950.               Arc Min         I2
23    DECSEC [1]  Declination 1950.               Arc Sec         I2
25    MAJOR [1]   Uncertainty ellipse             Arc Sec         I3
		  major axis
28    MINOR [1]   Uncertainty ellipse             Arc Sec         I3
		  minor axis
31    POSANG [1]  Uncertainty ellipse             Degree          I3
		  position angle                  (East of
						   North)
34    NHCON [1]   Number of times observed         ---            I2
      (<25)
36    FLUX [1]    Averaged non-color corrected    Jansky         4E9.3
		  flux densities (1 value per     (1.0E-26 W/m**2/Hz)
		  band)
72    FQUAL [1]   flux density quality.           ---            4I1
		  (1 value per band)
76    NLRS        Number of significant LRS       ---             I2
		  spectra
78    LRSCHAR [1] Characterization of averaged    ---            2A1
		  LRS spectrum.
80    RELUNC [1]  percent relative flux density   ---            4I3
		  uncertainties (1 value
		  per band).
92    TSNR        ten times the minimum signal-   ---            4I5
		  to-noise ratio in each band
112   CC [1]      point source correlation        ---            4A1
		  coefficient (1 value
		  per band).
116   VAR [1]     percent Likelihood of                           I2
		  Variability
118   DISC        Discrepant Fluxes flag          ---             A1
		  (one per band, hex-encoded)
119   CONFUSE [1] Confusion flag (1 flag          ---             A1
		  per band, hex-encoded)
120   PNEARH [1]  Number of nearby hours-         ---             I1
		  confirmed point sources
121   PNEARW [1]  Number of nearby weeks-         ---             I1
		  confirmed point sources
122   SES1 [1]    Number of seconds-confirmed nearby   ---       4I1
		  small extended sources.
		  (1 value per band)
126   SES2 [1]    Number of nearby weeks-              ---       4I1
		  confirmed small extended
		  sources.
		  (1 value per band)
130   HSDFLAG [1] Source is located in high            ---        A1
		  source density bin.
		  (1 value per band, hex-encoded)
131   CIRR1 [1]   Number of nearby 100 micron          ---        I1
		  only WSDB sources
132   CIRR2 [1]   Spatially filtered 100 micron        ---        I1
		  sky brightness ratio to flux
		  density of point source
		  (see text)
133   CIRR3       Total 100 micron sky                 MJy/sr     I3
		  surface brightness
136   NID [1]     Number of positional                 ---        I2
      (<25)       associations.
138   IDTYPE      Type of Object                       ---        I1
140   MHCON       Possible number of HCON              ---        I2
142   FCOR        Flux correction factor (times 1000)  ---       4I4
		      (1 value per band)
158   SPARE       3 Spare blanks                       ---       3A1

Table X.B.1a Format of Point Source Associations File 


00        NAME        IRAS source name                     ---       A11
11        RECNO       Record Number of source in data file ---        I6
18        CATNO [1]   Catalog number                       ---        I2
20        SOURCE [1]  Source ID                            ---      15A1
35        TYPE [1]    Source Type/Spectral Class           ---       5A1
40        RADIUS1     Radius Vector from IRAS              Arc Sec    I3
		      Source to Association
43        POS         Position Angle from IRAS             Degree E   I3
		      Source to Association                of N.
46        FIELD1 [2]  object field #1                      catalog    I4
		      (magnitude/other)                    dependent
50        FIELD2 [2]  object field #2                      catalog    I4
		      (magnitude/other)                    dependent
54        FIELD3      object field #3                      catalog    I4
		      (size/other)                         dependent
[1] Quantities listed in printed version of catalog.
[2] FIELD1 is listed in printed version of catalog, except for catalogs 2 and 19, where FIELD2 is listed.

Table X.B.2 Meaning of Hex Encoded Flags 

     Flag Set in A Particular Band (microns)    Resultant Value
  100        60        25         12            of Encoded Flag
(Bit 3)    (Bit 2)   (Bit 1)    (Bit 0)         xxxx = HEX = Decimal

   0          0         0          0              0000 = 0 =  0
   0          0         0          1              0001 = 1 =  1
   0          0         1          0              0010 = 2 =  2
   0          0         1          1              0011 = 3 =  3
   0          1         0          0              0100 = 4 =  4
   0          1         0          1              0101 = 5 =  5
   0          1         1          0              0110 = 6 =  6
   0          1         1          1              0111 = 7 =  7
   1          0         0          0              1000 = 8 =  8
   1          0         0          1              1001 = 9 =  9
   1          0         1          0              1010 = A = 10
   1          0         1          1              1011 = B = 11
   1          1         0          0              1100 = C = 12
   1          1         0          1              1101 = D = 13
   1          1         1          0              1110 = E = 14
   1          1         1          1              1111 = F = 15

Source Name: NAME

The IRAS source name is derived from its position by combining the hours, minutes and tenths of minutes of right ascension and the sign, degrees and minutes of the declination. In obtaining the minutes of right ascension and declination for the name, the positions were truncated. The letters 'A','B','C', etc. are appended to names of sources so close together that they would otherwise have had identical names. Names were uniquely assigned to both catalog and reject file sources, with catalog sources receiving letters first.

Position: (HOURS,MINUTE,SECOND,DSIGN,DECDEG,DECMIN,DECSEC)

Positions are given for the equinox 1950.0, and epoch 1983.5. Hours (HOURS) and minutes (MINUTE) of right ascension are given as integers while seconds (SECOND) are rounded to integer deciseconds. The declination is given as a character sign (DSIGN) followed by integer values of degrees (DECDEG), minutes (DECMIN) and seconds (DECSEC). Values of 60 seconds are given when the truncated minutes (given here) disagree with the minutes in the rounded position.

Position Uncertainty: MAJOR, MINOR, POSANG

As discussed in Section VII.B.2, the uncertainty in the position for a source depends on its brightness in the various wavelength bands, its path across the focal plane and the number of sightings. The final uncertainty after position refinement is expressed as a 95% confidence uncertainty ellipse (see Section V.D.9) whose semi-major (MAJOR) and semi-minor (MINOR) axes are given in seconds of arc. The orientation (POSANG) of the ellipse on the sky is expressed in terms of the angle between the major axis of the ellipse and the local equatorial meridian. It is expressed in degrees east of north.

Number of Sightings: NHCON

The number of hour-confirmed sightings is given. This number of flux entries will be found in the WSDB.

Flux Density: FLUX(4)

Each of the four wavelengths has a non-color-corrected flux density in units of Janskys, (1 Jy = 1.0E-26 W/m**2/Hz). The quoted value is an average of all the hours-confirmed sightings as obtained by the prescription described in Section V.H.5. The quality of each flux density is designated by FQUAL (see below).

The flux densities have been calculated assuming an intrinsic source energy distribution such that the flux density f(Nu) is proportional to 1/Nu. Corrections to other spectral shapes can be made by consulting Section VI.C.

The flux densities for sources so bright that they saturated the analog-to-digital converter on every sighting are lower limits based on the brightest value recorded. The uncertainties are given as ten times the quoted flux density and a flag is set indicating saturation has occurred (Table X.B.7a).

Signal-to-Noise Ratio: TSNR(4)

The signal-to-noise ratio given for an individual hours-confirmed sighting is the highest value of the detections comprising that sighting (Section V.C.2). The values quoted in the catalog are ten (10) times the minimum of the signal-to-noise ratios for the various sightings (HCONs) of the source. A value is given for each wavelength band with a high or moderate quality measurement and for those upper limits coming from a non- seconds-confirmed detection. Values of TSNR greater than 30,000 are given as 30,000.

Source Variability: VAR

VAR is the percent probability (0-99) that a source is variable based on an analysis of the 12 and 25 micron flux densities and their uncertainties (see Section V.H.5). The value "-1 " indicates that the source was not examined for variability.

Discrepant Fluxes: DISC(4)

The DISC flag indicates whether any one of the fluxes in a given band disagrees with others in that band, hex-encoded by band (Section V.H.5).

Flux Density Quality: FQUAL(4)

As described in Section V.H.5, a flux density measurement can be either high quality (FQUAL=3), moderate quality (FQUAL=2) or an upper limit (FQUAL=1).

Low-Resolution Spectra: NLRS,LRSCHAR

The Low-Resolution Spectrometer obtained 8-22 micron spectra of bright 12 and 25 micron sources (Chapter IX). NLRS gives the number of statistically meaningful spectra available for the source. LRSCHAR gives a short characterization of the nature of the spectrum (Table IX.D.1). All of the LRS spectra are available in tape and printed forms (Astronomy and Astrophysics Supplement, 1985) as described below (Section X.E) and in Chapter IX.

Flux Density Uncertainties: RELUNC (4)

Each flux density measurement other than an upper limit has an associated uncertainty expressed as a 1 Sigma value in units of 100 x [Delta f(Nu)/f(Nu)] Uncertainties are discussed in Sections V.H.5. and VII.D.2.

Point Source Correlation: CC(4)

As described in Section V.C.4, the point source correlation coefficient can have values between 87-100%. These are encoded as alphabetic characters with A=100, B=99...N=87, one value per band. The value quoted is for the highest correlation coefficient seen for that source on any sighting.

Confusion: CONFUSE,PNEARN,PNEARW,HSDFLAG

As described in Section V.D.2, a great deal of care went into trying to untangle instances of confusion between neighboring sources. In parts of the sky where the source density is low, confusion processing was sometimes able to separate sources that are quite close together. The CONFUSE flag is set if two or more sightings of the source in a given band had confusion status bits set indicating confusion in the seconds-confirmation or band-merging processes. This flag is hex-encoded by band.

Other indicators of possible confusion are given by PNEARN and PNEARW which are, respectively, the numbers of hours-confirmed and weeks-confirmed point sources located within a 4.5' cross-scan and 6' in-scan (half-widths) window centered on the source. Values larger than 9 are given as 9.

Regions of high source density received special processing to improve the reliability of the quoted sources (see Section V.H.6). The regions are band-dependent. If a particular band of a given source went through high source density processing, then the appropriate bit in HSDFLAG (Table X.B.7) is set. HSDFLAG is hex encoded by band.

Small Extended Sources: SES1(4),SES2(4)

SES1 is the number of seconds-confirmed, small extended source detections in a given band found within a window centered on the source. The size of the window is 6' in-scan x 4.5' cross-scan (half-widths). As described in Sections V.H.3-4 and VII.H.1, values of SES1 greater than 1 should caution the reader that significant extended structure may exist in the region and that the source in question may be a point-source like piece of a complex field.

SES2 is the number of weeks-confirmed small extended sources in a given band, located within a 6' in-scan x 4.5' cross-scan window (half-width) centered on the source. SES2 greater than 0 means that the point source flux measurement should be treated with caution as the source in question may, in fact, be extended. The flux quoted in the catalog of small extended sources may provide a better value for the source.

Cirrus Indicators: CIRR1,CIRR2,CIRR3

Over a large range of Galactic latitudes the infrared sky at 100 microns is characterized by emission from interstellar dust on a wide range of angular scales. The so-called "infrared cirrus" can seriously hamper efforts to extract point source detections from the data. To aid the user in interpreting the quoted 100 micron measurements three quantities have been established (Section V.H.4 and VII.H).

CIRR1 gives the number of 100 micron-only WSDB sources located within a 1/2 degree x 1/2 degree box in ecliptic coordinates centered on the source. The sources included in this count are the weeks-confirmed sources prior to high source density region processing (if applicable) plus those sources hours-confirmed but not weeks-confirmed. Values of CIRR1 greater than 3 may indicate contamination by cirrus with structure on the point source size scale.

CIRR2 gives a cirrus indication on a larger scale than CIRR1 and compares a "cirrus flux" with the source flux at 100 microns (see (Eq. V.H.2)). Values larger than 4-5 indicate the presence of considerable structure in the 100 micron emission on a 1/2 degree scale. A value of 0 indicates that no 1/2 degree data were available for the source in question.

CIRR3 is the total surface brightness of the sky surrounding the source in a 1/2 degree beam at 100 microns, clipped to exclude values greater than 254 MJy/sr. Values of CIRR3 greater than 30 MJy/sr indicate emission from dust with an appreciable column density. A value of CIRR3 = 255 means that no data were available.

Positional Associations: NID,IDTYPE,CATNO,SOURCE,TYPE,RADIUS,POS,FIELD1-3
Much of the utility of the IRAS catalog comes from the association of infrared objects with sources known to exist from other astronomical catalogs. As described in Section V.H.9, a large number of catalogs have been searched for positional matches. The total number of matches found is given by NID. Each match results in a forty-character description (2 per record).

IDTYPE ranges from 1 to 4 and states whether an association was found in an extragalactic catalog (1), a stellar catalog (2), other catalogs (3), or matches in multiple types of catalogs (4). CATNO is the number of the catalog in which the match was found (Tables V.H.1, X.B.4).

SOURCE is the name of the object in that catalog and TYPE its character or spectral type, if available. A vector is drawn from the IRAS position to the associated object. RADIUS is the length of that vector in arc seconds. POS is the angle between the vector and the local equatorial meridian expressed in degrees east of north. Three fields (FIELD1-3) have values depending on the catalog in question (Table X.B.4). Typically FlELD1,2 are magnitudes (in decimag) and FIELD3 a size.

Table X.B.4 Meaning of the Source Association Fields 
Catalog             Field and Meaning*

1 GCVS             Type             Blank

                   Field1 =         Code gives meaning for Fields 2-3
                   if Field1 = 1    Field2 and Field3 are B mag [decimag] at
                                       max,min
                             = 2    Field2 and Field3 are V mag [decimag] at
                                       max,min
                             = 3    Field2 and Field3 are photographic mag
                                       [decimag] at max,min
                             = 4    Field2 and Field3 are estimated V mag
                                       [decimag] at max,min
                             = 5    Field2 is 999 and Field3 is 0
------------------------------------------------------------------------------
2 Dearborn         Type             Blank
  Obs.
                   Field1           Code for Field2 (1,2)
                   Field2           if Field1 is 1, Field2 is red magnitude
                                       [decimag]*
                                    if Field1 is 2, Field2 is 999
                   Field3           0
------------------------------------------------------------------------------
3 Revised          Type             Blank
  AFGL
                   Field1           Magnitude at 4.2 microns [decimag]
                   Field2           Magnitude at 11 microns [decimag]
                   Field3           Magnitude at 27 microns [decimag]
------------------------------------------------------------------------------
4 2-Micron         Type             Blank
  Sky Survey
                   Field1           K magnitude [decimag]
                   Field2           I magnitude [decimag]
                   Field3           0
------------------------------------------------------------------------------
5 Globules         Type             Blank
  (Wesselius)
                   Field1           999
                   Field2           Minimum diameter [arcsec]
                   Field3           Maximum diameter [arcsec]
------------------------------------------------------------------------------
6 R C 2            Type             Blank

                   Field1           Harvard V magnitude [decimag]
                   Field2           B(T) [decimag]
                   Field3           D(0) [arcsec]
------------------------------------------------------------------------------
7 Stars with       Type             Blank
  em. lines
                   Field1           V magnitude [decimag]
                   Field2           999
                   Field3           0
------------------------------------------------------------------------------
8 Equatorial       Type                  Blank
  IR Cat.
                   Field1           Flux density [1.0E-16 W/cm**2/micron]
                                       at 2.7 microns
                   Field2           999
                   Field3           0
------------------------------------------------------------------------------
9 UGC              Type             Blank
                   Field1           Zwicky magnitude [decimag]
                   Field2           Minimum diameter [arcsec] in B
                   Field3           Maximum diameter [arcsec] in B
------------------------------------------------------------------------------
10 MCG             Type             Blank

                   Field1           999
                   Field2           Minimum diameter [arcsec] in B
                   Field3           Maximum diameter [arcsec] in B
------------------------------------------------------------------------------
11 Strasbourg      Type             Blank
Planetary Nebulae
                   Field1           V magnitude of Nebula [decimag]
                   Field2           B magnitude of Center Star [decimag]
                   Field3           Minimum diameter of Nebula [arcsec]
------------------------------------------------------------------------------
12 Zwicky          Type             Blank

                   Field1           Zwicky magnitude [decimag]
                   Field2           999
                   Field3           0
------------------------------------------------------------------------------
13 SAO             Type             Spectral Type

                   Field1           V magnitude [decimag]
                   Field2           p(g) magnitude [decimag]
                   Field3           0
------------------------------------------------------------------------------
14 ESO/            Type             First 3 characters of object type
Uppsala
                   Field1           B magnitude [decimag]
                   Field2           Maximum diameter [arcsec]
                   Field3           Minimum diameter [arcsec]
------------------------------------------------------------------------------
15 Bright          Type             Spectral Type
Stars
                   Field1           V magnitude [decimag]
                   Field2           B-V [centimag]
                   Field3           U-B [centimag]
------------------------------------------------------------------------------
16 Suspected       Type             Spectral Information
Var.
                   Field1           V magnitude at maximum [decimag]
                   Field2           999
                   Field3           0
------------------------------------------------------------------------------
17 Carbon          Type             Spectral Type (May be truncated)
Stars
                   Field1           p(g) magnitude [decimag]
                   Field2           V magnitude [decimag]
                   Field3           I magnitude [decimag]
------------------------------------------------------------------------------
18 Gliese          Type             Spectral Type (May be truncated)

                   Field1           V magnitude [decimag]
                   Field2           B-V magnitude [millimag]
                   Field3           U-B magnitude [millimag]
------------------------------------------------------------------------------
19 S Stars         Type             Blank

                   Field1           p(g) magnitude [millimag]
                   Field2           V magnitude [decimag]
                   Field3           I magnitude [decimag]
------------------------------------------------------------------------------
20 Parkes HII      Type             Blank
Survey
                   Field1           999
                   Field2           Minimum diameter [arcsec]
                   Field3           Maximum diameter [arcsec]
------------------------------------------------------------------------------
21 Bonn HII        Type             Blank
Survey
                   Field1           Flux density at 4.875 GHz (Jy)
                   Field2           Diameter [arcsec]
                   Field3           0
------------------------------------------------------------------------------
22 Blitz           Type             Blank

                   Field1           Diameter [arcsec]
                   Field2           V(co) [Km/s]
                   Field3           Peak T(A) [degrees K]
------------------------------------------------------------------------------
23 OSU             Type             Blank

                   Field1           999
                   Field2           999
                   Field3           Diameter [arcsec]
------------------------------------------------------------------------------
24 IRC             Type             C if 2.2 micron sources are possibly
                                       confused, w/good pos. blank otherwise

                   Field1           Right ascension difference (IRC-IRAS)
                                       [deciseconds of time]
                   Field2           Declination difference (IRC-IRAS)
                                       [seconds of arc]
                   Field3           0
------------------------------------------------------------------------------
25 DDO             Type             Blank

                   Field1           999
                   Field2           999
                   Field3           0
------------------------------------------------------------------------------
26 Arp             Type             Blank

                   Field1           999
                   Field2           999
                   Field            0
------------------------------------------------------------------------------
27 Markarian       Type             Blank

                   Field1           999
                   Field2           999
                   Field3           0
------------------------------------------------------------------------------
28 Strong          Type             Object type (GAL or QSO)
5 GHz
                   Field1           V magnitude [decimag]
                   Field2           5 GHz flux density [deciJy]
                   Field3           0
------------------------------------------------------------------------------
29 Veron-          Type             Object classification
Veron
                   Field1           V magnitude [decimag]
                   Field2           Redshift x 1000
                   Field3           0
------------------------------------------------------------------------------
30 Zwicky          Type             Blank
8 Lists
                   Field1           999
                   Field2           999
                   Field3           0
------------------------------------------------------------------------------
31 VV              Type             Blank

                   Field1           Special flag, (see below) 999 otherwise
                   Field2           999
                   Field3           0
------------------------------------------------------------------------------
* In the printed version FIELD1 is listed if present except for catalogs 2 and 19, where FIELD2 is given.

VV Catalog Flags (Catalog 31)

FIELD1 Explanation 

10       VV 10 has the same coordinates  as VV 29 in the VV  Atlas.  The
	 UGC was used to confirm  that the coordinate is correct  for VV
	 29 and erroneous for VV 10.   The UGC position for VV 10  = UGC
	 10814 was adopted.

11       The  VV  position  is  substantially  different  (>400")   from
	 positions for the  object in other  catalogs.  The VV  position
	 has  been  assumed  to  be  in  error because two or more other
	 catalogs agree on a  different position.  The UGC  position has
	 been adopted.

12       Same as for 11, but the OSU position has been adopted.

13       The position in the VV  Atlas, and the position listed  for the
	 VV object in  the OSU are  in disagreement.  The  true position
	 has been established to be close to that of the OSU by the  use
	 of overlay  transparencies on  the POSS.  The OSU  position has
	 been adopted.

14       Same as for 13, but the  OSU position is not very good  either,
	 so a new position has been measured (accurate to about 1').

XII. ERRATA AND REVISIONS AS OF 1987

The second release of some portions of the IRAS data reflects a number of changes, corrections and improvements to the data. This chapter, written in late 1986-early 1987, describes changes to each of the major datasets and records a number of changes to the formats of the data which will be of importance to people using the various products. The history of the releases of the IRAS data is described in Table XII.A.1.

A. Version 2.0 of the Point Source Catalog

The most important difference between the first and second versions of the IRAS Point Source Catalog (hereafter denoted PSC-1 and PSC-2) is the application of a statistical correction to reduce the overestimation of the flux densities of sources near the detection threshold (see XI.J. for a discussion of the effect); changes of as much as a factor of two were made for some weak sources.

A number of other, less significant changes were also made. Flux densities for almost all sources changed by a few percent due to refined calculations of certain calibration factors; a few sources changed by as much as 10%. Corrections to errors in some calibration and confused source processing algorithms resulted in the loss of 6 sources from and the addition of 56 new objects for a revised total of 245,889 individual sources in PSC-2. Table XII.A.5 lists 12 sources which were omitted from both PSC-1 and PSC-2 due to a software error, but which are of sufficient quality to be noted for completeness. A number of new catalogs were added to the list used for associations. Finally, a number of minor errors in calculations of some of the quantities associated with point sources, e.g. the number of neighboring sources, were corrected.

The format of the Catalog tape has been revised to incorporate these new quantities. Table XII.A.3 lists the changes to the original format (Table X.B.1, page X-4). Table XII.A.3 Revised Portion of Catalog Tape Format (see Table X.B.1) Start Byte Name Description Units Format 139 MHCON Possible number of HCONs --- 12 141 FCOR Flux Correction Factors Applied (x 1000) (1 value per band) --- 4I4 149 SPARE 3 Spare bytes --- 3A1 Table XII.A.4 New or Deleted Point Sources New Sources 01241-7332 05265-6840 08220-4404 18373-0918 04531-6708 05299-6830 08573-4718 19514+4306 04532-6710 05303-6951 09040-7402 21097+4953 04534-6657 05305-6952 09101-5100 21099+4954 05044-7012 05341-6632 09179-7034 21314+5805 05046-7014 05371-6944 09215-7028 21329+5126 05122-6829 05374-6946 09437-6034 21345+5706 05150-6629 05375-6650 10075-5747 21445+5653 05150-6631 05392-6847 10075-5824 21446+5655 05207-6636 05393-6930 10182-5742 21478+5649 05236-6702 05448-6720 12376-6122 22126+6905 05239-6939 05486-7001 13191-6245 22299+6440 05242-6940 08003-5012 18092+4419 22510+7138 05264-6730 08005-5013 18285-0830 22512+7140 Deleted Sources 04526-6951 13046-6222 20250+4316 21503+5105 04538-6952 15574-0052

Table XII.A.5

Bright Sources Missing from the Point Source Catalog with |b| > 10 Degrees 
-- Name --                Gal          -Flux Density in Janskys-
  (1950)      R.A. Dec.  Coord           (Not Color-Corrected)
HHMMT DDMM     (s) (")    l b    NH   12um    25um     60um    100um

00125-0723    33.5  10   98-68   2    .25L     .44L    1.46     4.62
11434+2042    26.3  50  234+74   2    .25L     .46:    3.19:    6.45
12337+2616    47.4  51  230+86   2    .52:     .53     4.50    23.31
21492+3716    13.4   1   88-12   2    .36:     .25L     .48     2.95
22261+8025     9.1  59  117+20   2   1.01      .28L     .40L    1.63L
22308+4105    50.3  26   97-14   2   2.05      .54      .40L    6.60L
22324+4024    28.4  35   97-15   2   4.43    18.49    29.10    64.39
22325+4054    32.0  25   97-15   2    .25L     .26L    1.08     7.01
22326+4031    37.8   9   97-15   2    .41L     .25L    1.10     7.33L
22376+2426    36.8   3   89-29   2    .29L     .25L    1.05     2.68
23019+3405    57.7  32   99-23   2    .25L     .25L    1.45     2.50
23132+2449    14.4  12   97-33   2    .25L     .25L     .48     1.35

-- Name --              N  C   -Association-
  (1950)       C  P  S  I  A   -Name & Type MAG
HHMMT DDMM     1  W  2  D  T

00125-0723     0        1  10  M-01-01-060  999
11434+2042     1        2  10  M+04-28-050  999
12337+2616     0  1  7  6   9  U07772       103
21492+3716    15     4  1   2  D0 20951       1
22261+8025     7        1  13  3746   K0     67
22308+4105     3        2  13  52130  K0     77
22324+4024    12        2  27  MKN 914      999
22325+4054     8  1  4  1  32  X2232+408      4
22326+4031     9        1  23  DG 187       999
22376+2426     0
23019+3405     0
23132+2449     4        3   9  U12460       155

FITS-Table-Formatted Version of the IRAS/PSC Catalog

The IRAS Point Source Catalog, as described above, is not in a format that translates easily into FITS Tables format (see Harten, R.H., Grosbol, P., Greisen, E.W. and Wells, D.C. 1988, A. & A. Suppl. 73, 365). The number of association fields varies for each object, and this runs counter to the stipulation that all records in a FITS table be of fixed length and have a single uniform format.

In order to process the IRAS/PSC into FITS format, the first two 80-byte records which always appear for each source were contatenated to form a single 160-byte record with a uniform format (in other words, NAXIS1=160 for the main data table). Then, the associations were removed from the main body of the catalog and written to a second file in a uniform format. Each association record was prefixed with (a) the IRAS/PSC source name, and (b) the record number in the main data file where that PSC source could be found. If the FITS table formatted version of the catalog were loaded into some database managagement system, a join operation should be applied to "re-associate" the associations with the records of the main data file, either by using the source name as a key, or the main data file record numbers.

Please note that the separation of the association fields from the main data file is only for the purposes of placing the catalog in FITS tables format. The original, multi-record format is preserved in copies of the catalog that are not FITS formatted.
-- Lee E. Brotzman, Astronomical Data Center