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Explanatory Supplement to the

S. G. Kleinmann

University of Massachusetts

R. M. Cutri, E. T. Young, F. J. Low

University of Arizona

F. C. Gillett

National Optical Astronomy Observatories

Notes about this computer-readable documentation:

The following text is an excerpt from the original printed documentation, including Section I "Introduction" and Chapter V "Formats of the IRAS Serendipitous Survey Catalog", which was keyed-in at the Astronomical Data Center (ADC). The text makes references to other chapters in the printed documentation as well as to the IRAS Explanatory Supplement. These references are in the form: XX.XX.XX, for chapter, section and subsection, respectively. For example, IV.B.3a refers to chapter IV, section B, subsection 3a, and Suppl.V.H.9 refers to chapter V, section H, subsection 9 in the IRAS Explanatory Supplement. The reader is directed to the printed versions 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 V) was changed to reflect the version distributed by the ADC. At the end of this computer-readable text is a discussion of the IRAS Serendipitous Survey 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 
    Greenbelt, MD 20771 
    (301) 286-6953 
    Internet: zmleb@scfvm.gsfc.nasa.gov 
              or  zmleb%scfvm@dftsrv.gsfc.nasa.gov 


In 1983 the sky was surveyed by the Infrared Astronomical Satellite (IRAS) in four broad spectral bands centered at 12, 25, 60, and 100 microns. During the course of the 300 day mission, about two-thirds of the satellite time was used to carry out an unbiased sky survey which led to the production of the IRAS Point Source Catalog (IRAS/PSC) containing 245,889 sources. That catalog, along with many other aspects of the mission, are described in the IRAS Catalogs and Atlases Explanatory Supplement (1985) [Referenced below as the "Main IRAS Supplement"]. A portion of the remaining mission was used to conduct a series of Pointed Observations, usually directed at objects of interest, and using the various capabilities of the satellite. Many of these Pointed Observations were performed by repeatedly scanning the 0.5 deg. wide survey detector array over the selected object. These scans were combined to produce a sensitive map of area about one square degree around the selected source (or in some cases, a targeted "blank" field). By spatially coadding the repeated scans comprising each Pointed Observation a large data base was generated. Young et al. (1985) have described these data, the Pointed Observation program and the associated data processing at the Infrared Processing and Analysis Center (IPAC).

This document describes the IRAS Serendipitous Survey Catalog (IRAS/SSC) which has been constructed using the fortuitous observations at 12, 25, 60, and 100 microns of 43,866 point-like sources that happened to lie in 1813 of the individual fields included in the Pointed Observations program. Because the Pointed Observations resulted in longer integration times for each source in the field than was possible in the all-sky survey mode, the limiting sensitivity for the IRAS/SSC is typically better than that of the IRAS/PSC by a factor of about 4. Also, the photometric accuracy, especially for faint sources, is improved relative to that of the IRAS/PSC. The amount of sky sampled in the SSC is nearly 1400 square degrees but because of uneven sensitivity across the Pointed Observation fields, the effective sky coverage is 1108 square degrees. Excluding certain areas of great scientific interest to the team who planned the observations (specifically, the Galactic plane and the Magellanic Clouds), the Pointed Observations were widely distributed on the sky. Positional association of the SSC sources with those in the PSC reveal that 11,129 are in both IRAS catalogs; also, 5470 SSC sources have been cataloged at other wavelengths.

The positional accuracy of the SSC is not as high as that of the PSC, in part because of the restricted geometry of the observations themselves and also because of compromises made in processing the data. In those cases where more accurate positions are critically needed, it may be possible to obtain better results through re-processing the observations at one of the IRAS data centers.

The main objective of the Serendipitous Survey was to extend the detection threshold for point sources over a finite but significant portion of the sky so that those sources could be studied individually. No attempt has been made to construct an unbiased sample by removing the targeted sources or by correcting either for the non-random sky coverage or for the uneven depth of the survey corresponding to different characteristics of the observing modes. Because most of the SSC sources are faint and because it was inherently difficult to achieve a uniform level of completeness in this program, it was decided to emphasize high reliability and photometric accuracy. These important SSC objectives were achieved by retaining only those sources which were detected in each of two independent Pointed Observations and by averaging the two measurements. No fields were included where only a single Pointed Observation was available. This strategy also provided a means for rejecting some asteroids and any other moving objects.

Frequently, independent observations were made of nearly the same region of sky and were treated in this catalog as separate fields; this practice resulted in partially redundant data sets. These overlapping fields are noted in the SSC to facilitate their further study. Because each of the 1813 fields in the SSC is unique in the sense that it may differ from other fields, both according to the observing mode that was used and in the wide range of source densities encountered, the catalog is presented field-by-field along with data pertinent to each field. Enough information is retained to permit the user to reorganize the source lists as needed. Those fields which overlap other fields are cross referenced. The fields are ordered by the right ascensions of their centers and the source list within each field is also ordered by right ascension.

An important objective of this catalog is to present a list of reliable sources found in all the selected Pointed Observation fields despite effects of high source density and/or complex backgrounds. In those heavily crowded fields where source confusion degrades the sensitivity, reliability, completeness, positional accuracy or photometric accuracy, the guiding philosophy has been as follows:

This policy has the result that many of the SSC fields and sources must be analyzed with particular care by the user, but it also means that many faint sources are included in crowded regions of scientific interest.

In summary, the main features of the SSC relative to the PSC are as follows:

  1. Enhanced sensitivity for point-like sources in all four wavelength bands over a few percent of the sky.
  2. Excellent reliability in low surface density fields.
  3. Uneven sky coverage and completeness.
  4. Reduced positional accuracy.
  5. Improved photometric accuracy.
  6. Much greater depth in crowded fields at the expense of reliability and accuracy.
The user of the SSC should be aware of the following considerations. First, a set of high latitude, unique, less crowded fields, selected as described in Section IV, was used in much of the current analysis. This subset of the catalog is comprised of 450 fields containing 5100 sources. Second, for these high latitude fields, reliability of order 99% was achieved for sources brighter than the flux density limits of 100, 150, 120, and 400 mJy for the 12, 25, 60, and 100 micron bands, respectively; note that even within these fields the completeness limits vary. Third, the measured positional accuracy of SSC sources defines a search box with dimensions of 30" x 180" at 12 microns, increasing to 60" x 140" at 60 microns. The orientation of each search box is indicated by the grid orientation angle given in the appropriate field header. Fourth, for the high latitude subset of fields, the systematic photometric errors introduced by the absolute calibration are nearly identical with those in the PSC. The remaining systematic errors and uncertainties due to lack of reproducibility are less than +/- 10% on average for sources detected with high signal-to-noise, and the effects of noise are well quantified within each field. Fifth, for lower latitude fields, or fields where the underlying background has a complex structure, the reliability, completeness, positional, and photometric accuracies may be substantially degraded and each field must be analyzed with caution. Sixth, as in the PSC many sources are slightly extended and will have erroneous flux densities; other IRAS data bases such as the INTN mode grids (Young et al. 1985) should be used for the study of small extended sources.

In the preparation and presentation of the SSC, an effort was made, so far as possible, to conform to conventions established in the preparation and presentation of the PSC. In the following sections only that information not found in the Main IRAS Supplement (1985) or in the Users Guide to the IRAS Pointed Observation Products (Young et al. 1985) and needed for the use of this catalog will be given. This includes: information on the Pointed Observation data base from which the point sources were extracted and the extraction process (Section II), on the data processing used to develop the SSC from the list of extracted sources (Section III), on the reliability and sensitivity of the SSC, its positional and photometric accuracy and certain statistical properties (Section IV), on the format and content of the SSC in its machine-readable and printed forms (Section V), and warning to the user concerning confusion effects, asteroid and redundant sources (Section VI), and a table of overlapping fields (Appendix A).


A. Introduction

This chapter describes the formats of the IRAS Serendipitous Survey Catalog in its printed and machine-readable forms. A brief description is given of each entry in the catalog; 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. The catalog consists of three basic parts; a) field headers, b) source listings, and c) redundant fields (a printed list of the redundant fields is also available in Appendix A). The field header includes information on the global properties of each pair of reference/confirming grids. The source listing documents the properties of the individual confirmed sources found in each field. The redundant field listing indentifies those grid pairs with more than 5% overlapping coverage on the sky.

Because the Serendipitous Survey fields are non-uniformly distributed on the sky, the catalog has been arranged by fields, with the fields ordered by the right ascension of the field center. The field header is located at the beginning of the source listing for that field. Within a field the sources are arranged in order of right ascension. The redundant field listing is separate and follows the rest of the catalog. As much as possible, the conventions established for the IRAS/PSC have been adopted for the Serendipitous Survey.

B. The Machine-Readable Version

The machine-readable version of the SSC contains four files which are described in the following tables. Tables V.B, V.B.1a, V.C, V.C.1a, and V.D describe the formats of the field headers, source, association, and overlap field files respectively.

Table V.B. Format of the Field Headers [1] (SSC M-R Version)

 Start      Name                Description               Units        Format 
   00     FNAME [2]    Field Name                                     13A1
   13     RGRID [2]    Reference Grid No.                               I5
   18     RDATE [2]    Obs. Date, Ref. Grid           Days              I3
						      JD 2445000 +
   21     CGRID [2]    Confirming Grid No.                              I5
   26     CDATE [2]    Obs. Date, Conf. Grid          Days              I3
						      JD 2445000 +
   29     MACRO [2]    Macro Type                                      1A1
   30     GLON [2]     Galactic Longitude             Degree            I3
   33     GLAT [2]     Galactic Latitude              Degree            I3
   36     PDRAS        Sign of R.A. Difference        +/-              1A1
		       Between Grid Centers
   37     PDRA         Amplitude of R.A. Difference   Arcsec            I3
		       Between Grid Centers
   40     PDDECS       Sign of Dec. Difference        +/-              1A1
		       Between Grid Centers
   41     PDDEC        Amplitude of Dec. Difference   Arcsec            I3
		       Between Grid Centers
   44     RANGLE       Reference Grid Scan            Degree            I4
		       Direction (E of N)
   48     CANGLE       Confirming Grid Scan           Degree            I4
		       Direction (E of N)
   52     EFFAREA [2]  100x Effective Area            Square Deg.       I3
		       of Grid Overlap
   55     RUNDF [2]    No. of Fields with             NN                I2
		       Overlap > 5%
   57     SPARES       23 spare bytes                                 23A1
   80     RNOISE       Median Noise of Ref.           mJy              4I5
		       Grid (1 value per band)
  100     CNOISE       Median Noise of Conf.          mJy              4I5
		       Grid (1 value per band)
  120     NSOURC       Number of Confirmed            NNN              4I3
		       Sources (1 value per band)
  132     NCONF [2]    Number of Confused             NNN              4I3
		       Confirmations (1 value per band)
  144     CIRRUS [2]   Number of 100 micron only      NN                I3
		       Confirmed Sources
  147     NMERGE       Number of Merged Sources       NNN               I3
  150     SPARES       10 spare bytes                                 10A1
[1] Fields are listed in order of increasing Right Ascension of the Reference Grid center. Field header records are located at the beginning of the source listing for each field.
[2] This quantity is listed in the printed version of the catalog.

The following is a brief description of the individual entries in the Field Headers of the machine-readable version of the SSC.

Field Name: FNAME

The IRAS/SSC field name is the position of the center of the reference grid, given in the form hhmmssSddmmss.

Grid Number: RGRID, CGRID

The identifying number (see Section III.A) for the reference (R) and confirming (C) grids for the field. The reference grid has the lower 60 micron median noise.

Observation Date: RDATE, CDATE

The observation dates for the reference (R) and confirming (C) grids in Julian Days - 2445000.

Macro Type: MACRO

The macro identifying code is given in Table II.A.

Galactic Coordinates: GLON, GLAT

Galactic coordinates rounded to the nearest degree.

Positional Differences: PDRAS, PDRA, PDDECS, PDDEC

The sign and magnitude of the position difference between reference and confirming grid centers, in the sense of (confirming - reference), in right ascension and declination.

Grid Orientation: RANGLE, CANGLE

The orientation of the in-scan direction of the reference and confirming grids on the sky, measured in degrees East of North.

Effective Area: EFFAREA

The effective area of the sky covered by both the reference and confirming grids.

Redundant Fields: RUNDF

The number of additional grid pairs in the Serendipitous Survey, i.e. with different OBSID's, which overlap this field by more than 5%.


The median noise of the reference and confirming grids.

Confirmed Sources: NSOURC

The number of confirmed sources in this field, in the 12, 25, 60, and 100 micron bands, respectively.

Confused Sources: NCONF

The number of confused confirmations in this field, in the 12, 25, 60, and 100 micron bands, respectively.

100 Micron Only Sources: CIRRUS

The number of 100 micron confirmed sources in the field that are not band merged. The density of such sources is taken to be a measure of the infrared "cirrus" in the field.

Band-Merged Sources: NMERGE

The number of band-merged sources in the field, i.e., the number of source records following the field header.

Table V.C. Format of Source Listings [1] (SSC M-R Version)

 Start      Name                Description               Units        Format 
   00     FNAME          SSC field name                                 13A1 
   13     FLDRECNO       Record number, SSC field file                    I6 
   20     NAME [2]       Source Name                                    11A1 
   31     HOUR           Right Ascension 1950           Hours             I2 
   33     MINUTE         Right ascension 1950           Minutes           I2 
   35     SECOND         Right ascension 1950           Deci-seconds      I3 
   38     DSIGN          Declination Sign               +/-              1A1 
   39     DECDEG         Declination 1950               Degree            I2 
   41     DECMIN         Declination 1950               Arcmin            I2 
   43     DECSEC         Declination 1950               Arcsec            I2 
   45     SPARE          1 spare byte                                    1A1 
   46     ANGLE          Position Angle of              Degrees           I3 
                         Source Error Box               (E of N) 
   49     SPARE          1 spare byte                                    1A1 
   50     FLUX [2]       Averaged Non-color             Jansky           4E9.3 
                         Corrected Flux Densities       (1.0E-26 W/m**2/Hz) 
                         (1 value per band) 
   86     FQUAL [2]      Flux Density Quality                            4I1 
                         (1 value per band) 
   90     RGRID          Reference Grid Number                            I5 
   95     SPARE          9 spare bytes                                   5A1 
  100     RELUNC [2]     Percent Relative Flux                           4I3 
                         Density Uncertainties 
                         (1 value per band) 
  112     TLSNR          10x Local Signal-to-Noise                       4I4 
                         Ratio (1 value per band) 
  128     CC [2]         Point Source Correlation                        4A1 
                         Coefficient (1 value per band) 
  132     TRFLUX         10x F(c)/F(r)                                   4I2 
  140     POSDRS12       Right Ascension Delta Sign     +/-              1A1 
  141     POSDR12        Right Ascension Delta          Arcsec            I3 
  144     POSDDS12       Declination Delta Sign         +/-              1A1 
  145     POSDD12        Declination Delta              Arcsec            I3 
  148                    Repeat for 25 micron band      +/-              1A1 
 -155                                                   Arcsec            I3 
  156                    Repeat for 60 micron band      +/-              1A1 
 -163                                                   Arcsec            I3 
  164                    Repeat for 100 micron band     +/-              1A1 
 -171                                                   Arcsec            I3 
  172     PNEARC [2]     Number of Sources in                            4I1 
                         Confusion Window 
                         (1 value per band) 
  176     NID [2]        Number of Positional                             I2 
  178     IDTYPE [2]     Type of Object                                   I1 
  179     SPARE          1 spare byte                                    1A1 

Table V.C.1a Format of Associations (SSC M-R Version)

 Start      Name                Description               Units        Format 
   00     NAME           SSC source name                                11A1 
   11     RECNO          Record number, source listings file              I6 
   18     CATNO          Catalog Number [4]                               I2 
   20     SOURCE         Source ID                                      15A1 
   35     IDTYPE         Source Type/Spectral Class [5]                  5A1 
   40     RADIUS         Radius Vector from             Arcsec            I3 
                         SSC Position to Association 
   43     POS            Position Angle from            Degree            I3 
                         SSC Position to Association    (E of N) 
   46     FIELD1         Object Field #1                Catalog           I4 
                                                        Dependent [6] 
   50     FIELD2         Object Field #2                Catalog           I4 
                                                        Dependent [7] 
   54     FIELD3         Object Field #3                Catalog           I4 
                                                        Dependent [8] 
[1] Sources are listing in order of increasing Right Ascension within each field.
[2] This quantity is listed in the printed version of the SSC.
[3] For associations with the IRAS/PSC, this value is 41.
[4] For associations with the IRAS/PSC, this value is 3.
[5] For associations with the IRAS/PSC, this field is left blank.
[6] For associations with the IRAS/PSC, this value is a flag indicating the bands in which the source was detected with medium or high quality; it is encoded as indicated in the PSC Supplement Table X.B.2.
[7] For associations with the IRAS/PSC, this value is the PSC 2.0 Flux Density in the shortest (first) wavelength band in which it was detected. Flux Densities higher than 10 Jy are encoded 9999.
[8] For associations with the IRAS/PSC, this value is the PSC 2.0 Flux Density in the second wavelength band in which it was detected. Flux Densities higher than 10 Jy are encoded 9999.
The following is a brief description of the individual entries in the Source Listings of the tape version of the SSC.

Source Name: NAME, APPNAME

The IRAS/SSC source name is constructed as for the IRAS/PSC sources and 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 tuncated. If the sources within a field have duplicate positional names, they are distinguished by an appended letter (APPNAME), starting with the letter A.


Positions are, as in the IRAS/PSC, given for the equinox 1950.0 and epoch 1983.5. Hours (HOUR) 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).

Position Angle: ANGLE

The position angle of the major axis of the SSC source error box expressed in degrees East of North.

Flux Density: FLUX(4)

Each of the four wavelengths has a NON-COLOR-CORRECTED flux density in units of Janksys, (1 Jy = 1.0E-26 W/m**2/Hz). The quoted value is the noise weighted average as defined in Section III.C.2.

Flux Quality: FQUAL(4)

Each flux density measurement is designated high quality, moderate quality, or upper limit (FQUAL = 3, 2, or 1, respectively) based on the prescription discussed in Section III.C.

Flux Density Uncertainty: RELUNC(4), TLSNR(4)

Each flux density measurement other than an upper limit has an associated uncertainty expressed in two ways; RELUNC is the uncertainty expressed as a 1 sigma value in units of 100x/ (see Section III.C.2), TLSNR is ten times the local signal to noise ratio (see Section II.C) as determined from the reference or confirming grid, whichever is least.

Point Source Correlation Coefficient: CC(4)

As discussed in Section III.B, SSC sources can have point source correlation coefficients between 70-100%. These are encoded as alphabetic characters with A=100, B=99... Z=75-70, one value per band. The quoted correlation coefficients come from the reference or confirming grids, whichever is higher, for high quality sources.

Flux Density Ratio: TRFLUX(4)

As discussed in Section III.B, SSC sources can have flux density ratios 0.5 < F(c)/F(r) < 2.0. TRFLUX is the flux density ratio for high quality sources expressed as 10 x F(c)/F(r).

Position Differences: POSDRS12, POSDR12, POSDDS12, POSDD12 ...

The quoted positions of SSC sources are determined from a weighted average of the positions of the confirmed sources in each band with a high quality flux density (see Section III.C.1). POSDRS12, POSDR12, POSDDS12 and POSDD12 give the sign and amplitude (in arcsec), of the difference in right ascension and declination respectively, between the final band-merged position and the 12 micron confirmed source position. The following 12 entries repeat the above format for 25, 60, and 100 micron confirmed source components.

Confusion: PNEARC(4)

In regions of high source density, the Pointed Observation source extraction process, as well as the Serendipitous Survey Confirmation and Band Merging processing, can result in degraded positions and incorrectly band merged sources. PNEARC is 1-(number of confirmed sources in the confusion and band merge window). (See Section III.D.) Any value greater than zero is indicative of potential confusion in the processing and the resulting source information should be examined carefully, e.g. by inspection of the grids in question.


The positional associations formats and definitions are done as per the IRAS/PSC (Chapter X, Supplement), with the exceptions noted in Section III.F.

Table V.D. Format of Overlapping Fields File (SSC M-R Version)

Start Name Description Units Format Byte 00 GRID0 Prime Ref. Grid I5 05 SPARE 1 spare byte 1A1 06 GRID1 Overlapping Field #1, Ref. Grid I5 11 SPARE 1 spare byte 1A1 12 AOVLP1 Overlapping Sky Coverage Square Arcmin I4 16-26 Repeat of 5-15 for Overlapping Field #2 27-37 Repeat of 5-15 for Overlapping Field #3 . . 60-70 Repeat of 5-15 for Overlapping Field #5 71-79 SPARE 9 spare bytes 9A1 The quantities in the Overlapping Fields List are as follows:

Prime Reference Grid: GRID0

The grid against which other reference grids are checked for overlapping sky coverage.

First Overlapping Grid: GRID1

The number of the first reference grid with more than 5% overlap with GRID0.

Overlap Area: AOVLP1

Area of overlapping sky coverage between GRID0 and GRID1 in square arcminutes.

The format is repeated for each additional grid overlapping with GRID0. If more than 5 grids overlap with GRID0, they are listed in succeeding records, as necessary; the first six bytes of each of these additional records begins with a string of 6 spaces.

FITS-Table-Formatted Version of the IRAS SSC.
The IRAS Serendipitous Survey Catalog, as described above, does not translate 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 format of the field headers differs from the source data records following, and the number of association fields varies for each object. 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 SSC Catalog into FITS format, the two 80-byte records for the field headers were contatenated to form a single 160-byte record with a uniform format (in other words, NAXIS1=160 for the field header table). The records for each SSC source were split off into two separate files -- one for the source data and one for the associations. The first two records which always appear for each source were concatenated into bytes 21 through 180 of the source data table records. Each source record was prefixed with (a) the field header name (bytes 1-13), and (b) the sequential record number in the field header table where that field header could be found (bytes 14-19). Then, the associations were removed from the source data and written in a uniform format to bytes 19-58 of the associations table. Each association record was prefixed with (a) the SSC source name (bytes 1-11), and (b) the record number in the source data table where that SSC source could be found (bytes 12-17).

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 source data and associations records with the field headers, either by using the field header/source name as a key, or the sequential record numbers.

Please note that the separation of source data and 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