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
Hernandez, IPAC Librarian.  The text for Chapter I was keyed-in at the
Astronomical Data Center (ADC).  The text makes references to other chapters in
the 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 245,889 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

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

 Start
 Byte      Name                    Description                Units     Format

 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

 Start
 Byte      Name                    Description                Units     Format

 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



      Table X.B.4 Meaning of the Source Association Fields (Continued)

 Catalog                     Field and Meaning *

 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]



         Table X.B.4 Meaning of the Source Association Fields (Continued)

 Catalog            Field and Meaning *

 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]



        Table X.B.4 Meaning of the Source Association Fields (Continued)

 Catalog                Field and Meaning *

 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