IRAS SMALL SCALE STRUCTURE CATALOG

                              Catalog Prepared by

                                  George Helou

                                      and

                                  D. W. Walker

                           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" and Chapter VII "Formats", which was
 keyed-in at the Astronomical Data Center.  Slight modifications may have been
 made to the text in order to describe the machine-readable version of the
 catalog.  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.

    At the end of this computer-readable text is a discussion of the IRAS Small
 Scale Structure 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

    The primary mission of the Infrared Astronomical Satellite (IRAS) was to
 conduct a sensitive and unbiased survey of the sky in four wavelength bands at
 12, 25, 60, and 100 microns.  Launched in January 1983, IRAS ceased operations
 in November 1983 after having successfully surveyed 96% of the sky.  The IRAS
 mission, the data processing, and the products are described in detail in the
 "Explanatory Supplement to the IRAS Catalogs and Atlases" (Vol.  I of this
 series), with which the reader is assumed to be familiar.  References to
 chapters and sections in the "Supplement" are given here prefixed with Suppl.,
 e.g., Suppl.I.A.

    Results from the IRAS survey are presented in various forms depending
 primarily on the angular scale of interest.  They include a catalog of infrared
 point sources (the Point Source Catalog), the present Small Scale Structure
 Catalog (hereafter the SSS Catalog or the Catalog), and an atlas of absolute
 surface brightness images of the infrared sky (the Sky Brightness Images).  The
 Point Source Catalog contains some 250,000 entries resulting from a search for
 spatially unresolved sources in the survey data.  At the other extreme, the Sky
 Brightness Images display the large scale appearance of the sky with a
 resolution of 4'.  The SSS Catalog attempts to fill the gap by listing sources
 resolved in any band up to a maximum size of 8', aiming in particular at an
 adequate description of the infrared emission from objects such as galaxies,
 planetary nebulae or compact HII regions.  Given the complexity of the infrared
 sky and the infinite variety of source shapes, the scope of a SSS Catalog
 remains by necessity vaguely defined, quite apart from the technical problems
 posed by the specifics of the telescope design and operation.  This
 introduction describes the Catalog and the manner in which it was constructed,
 discusses the uncertainties in the various quantities listed, and gives a
 statistical overview of the sources in the Catalog.

    The Small Scale Structure Catalog contains 16,740 sources which point out
 directions on the sky where IRAS resolved emission on the scale of 2' to 8', in
 at least one of the four bands, in a reliable and unconfused manner.  Although
 the objective was to find intuitively acceptable "small extended sources", the
 Catalog entries are in fact defined by excursions in sky brightness detectable
 above the local baseline on spatial scales smaller than 8', excluding point
 sources.  The Catalog lists for each entry and in each band separately:  a
 position accurate to about one arcminute (rms); a flux density accurate to 50%
 (rms) overall, and better at high signal to noise ratios; a rough indicative
 size if a point source is present at the same location; and a variety of
 warning and processing flags.  Completeness in the Catalog is not estimated
 since it varies with position on the sky as a function of source density and
 number of survey coverages.  Fewer than 3% of the Catalog entries are
 unreliable.  Almost all of these are weak point sources broadened by radiation
 hits or detector noise.

    In spite of every effort to select only resolved, well-defined, and isolated
 sources for inclusion in the Catalog, a few problems remain.  At low



 signal to noise ratios, the processor may have erred in deciding whether a
 source is resolved or not.  Many entries, mostly at 60 to 100 microns, refer
 only to emission features that are details in larger structures known as
 Galactic cirrus.  Naturally, some close double and multiple point sources
 appear in the Catalog.  There is also some evidence for flux densities at 12
 and 25 microns being overestimated by 0.5 to 1 Jy.  The greatest amount of
 caution perhaps is required in interpreting the ABSENCE of an entry in this
 Catalog, in one or in all bands.  Incompleteness is a complex function of band,
 brightness, and location so an absence does not necessarily imply the lack of
 detectable extended emission in any band.

    While Small Scale Structure cannot be defined or measured with the same
 finality as point sources, Catalog entries always represent correlated and
 confirmed events in the detector output, making them valuable pointers to the
 IRAS images and raw data.  The SSS processor has distilled from the survey a
 wide variety of objects and provided reasonable estimates of their parameters,
 but if a detailed shape description, or more accurate position, flux, or size
 are needed, it is necessary to go back to the fundamental reference, the raw
 detector ouptut.

    The algorithms used to detect and measure the entries in this Catalog are
 described in Suppl.V.E, and summarized in Chapter II.  Chapter III examines the
 detection processing to assess the types of sources detected and the influence
 of noise on their reliability.  Chapter IV describes the strategy used to
 select the most reliable sources for the Catalog from the larger pool of
 confirmed detections.  Chapter V provides an overview of the statistial
 properties of the Catalog, a comparison with other IRAS data products, and
 estimates for the accuray of the measured quantities.  Chapter VI discusses
 reliability and completeness.  Chapter VII details the formats of the printed
 and machine-readable versions of the Catalog.



 VII. FORMATS

    The Small Scale Structure Catalog is available in three media, on tape, in
 print, and in machine-readable text.  On tape the Catalog is contained in one
 file ordered by right ascension and by declination in case of conflict.  The
 printed version has essentially identical contents to the tape copy, but is
 organized and formatted differently and shows only one association.  In both
 versions and for each source, data are shown only for those bands where
 detections have been found and have survived processing through "weeks
 confirmation".  No upper limits are given where no detections were found; none
 are given where detections were dropped because of confusion ("cluster
 analysis").  Absence of data from this Catalog at a given position in a given
 band does not imply absence of emission, but simply lack of information (see
 Chapter VI) for a discussion of completeness).  On the other hand, flags
 NEARPS, SES1 and HD, which depend only on position in the sky, are always
 estimated for all four bands, at the mean position of each source.

 A. Tape Version

    In general, the data for each source is organized in three 80-byte records
 followed by a variable number of association records; Table VII.A.1 shows
 detailed organization.  The first two records contain basic parameters and
 warning flags referring to the band-merged source, whereas the third record is
 made up of four 20-byte components containing position and quality flags for
 each of the bands.  When present, the 12 micron data is inserted into the first
 20 bytes of the record; the 25, 60, and 100 micron data go into the second,
 third, and last block of 20 bytes in the record, respectively.  Quarters
 corresponding to bands absent from the source are filled with blanks.  The
 fourth and subsequent records contain data on positional associations between
 the Catalog source and objects in a variety of astronomical catalogs listed in
 Suppl.V.H.9.  Each record holds data on two associations, in two 40-byte
 blocks.  All data entries for each source are described below in their order of
 appearance in Table VII.A.1

 B. Machine-Readable Version

    The machine-readable version of the catalog is divided into two files.  The
 first is a 240 byte record file containing the 16,740 sources.  The second is a
 58 byte record file containing the 5,178 associations.  The file formats are
 described below in Table VII.A.1 and Table VII.A.1a.

    Source Name:  NAME
    ------------------

    The letter "X" prefixes all names in this Catalog to distinguish them  from
 Point Source names.  The name is  then derived from position by combining  the
 hours and minutes from the right ascension with the sign, degrees, and decimal
 fraction of  degrees from  the declination.   In case  of name duplication the
 letters "A", "B", etc. are appended  to the name in order of  increasing right
 ascension, and of increasing declination in case of a tie.



    "Band Merging" Flag:  BMFLG
    ---------------------------

    The "band merging" flag gives the  number of bands in which the  source has
 an entry  in the  third record  (regardless of  flux quality  in these bands),
 together with indications  on the "band  merging" processing history  for that
 source.   BMFLG=C  or  D  indicates  3  or 4 mutually confirming (Suppl.V.E.6)
 components  in  the  source.   BMFLG=I,  J,  K  or  L  indicates  1, 2, 3 or 4
 components in  a source  having experienced  "band merging"  complications and
 having survived the modified "final selection" discussed in IV.B.3.a.

    Position:  RAHR, RAMIN, RASEC, DSIGN, DECDEG, DECMIN, DECSEC
    ------------------------------------------------------------

    Source position is the simple mean  of the positions of all the  individual
 band components in the source regardless of the individual flux quality flags.
 It  is  given  in  equatorial  coordinates  for  the 1950 equinox.  Positional
 accuracy is discussed in V.C.2.

    Number of Sightings:  NH(4)
    ---------------------------

    The  number  of  survey  coverages  contributing  a  sighting  to the weeks
 confirmed source is given for each band as a single character.  The order  for
 this and similar arrays in the Catalog  is from 12 microns (subscript = 1)  to
 100 microns (subscript = 4).

    Flux Density:  FLUX(4)
    ----------------------

    An estimate  of the  spatially integrated  flux density  from the source is
 given  in  Jansky  for  each  band,  with  three  digit  precision.   This  is
 transformed from the fluxes  in the IRAS bands  assuming a spectrum with  Nu x
 f(Nu) = constant (see Suppl.VI.C).  To  obtain the actual flux density at  the
 nominal wavelength  for each  band, FLUX  must be  corrected according  to the
 prescription in  Suppl.VI.C, which  is reproduced  at the  end of this volume.
 The estimation algorithm is described in Suppl.V.E.3, 4 and 5; calibration  in
 II.C.  Accuracy is discussed in V.C.1.

    Optical Cross Talk Flag:  XTALK(4)
    ----------------------------------

    This quantity indicates  the extent and  kind of cross-talk  for which each
 component in the source was flagged (the printed version gives only a  summary
 described under "FCAT" below).  As  discussed in IV.B.2a above, the  processor
 accumulated the number NXT of detections that might have been caused simply by



 optical cross-talk,  along with  the number  NS of  detections.  If NXT=0 then
 XTALK=0 for  that band.   If NXT  > 0,  and either  (i) (NS-NXT)  > 2, or (ii)
 (NXT/NS) < 2/3 then XTALK=1 for moderate cross-talk.  If NXT > 0, and  neither
 (i) or (ii) is true then XTALK=2 for severe cross-talk.  If in addition a band
 component was flagged for cross-talk at the level of "final source selection",
 then XTALK is increased by 4. Note  that the component is given a low  quality
 rating if XTALK > 0.

    Near-by Point Sources:  NEARPS(4)
    ---------------------------------

    This flag gives a count of all weeks-confirmed point sources (regardless of
 whether or not  they are included  in the Point  Source Catalog) in  each band
 within a 9' radius of the  mean position of the SSS Catalog  source.  NEARPS=1
 indicates most often that the emission has been detected as a point source  as
 well as here.  Larger values are a warning of possible confusion affecting the
 source, or an indication that this  source combines emission from two or  more
 barely resolved point sources (see DBLPS below).  NEARPS is given as a  single
 character per band, and denoted by a letter when it exceeds 9 ("A" for 10, "B"
 for 11, etc.).

    Near-by Intermediate Small Extended Sources:  SES1(4)
    -----------------------------------------------------

    This flag  gives a  count in  each band  of all  hours-confirmed entries in
 FISES (Suppl.V.E.3) within a 9' radius of the mean position of the source.  In
 a  clean  sky,  SES1  would  be  equal  to NH; larger values implying a higher
 density of detections, either spurious  or due to complex structure,  point to
 areas where "cluster analysis"  was active.  Large values  of SES1 for a  band
 not  represented  in  the  source  may  indicate  reliable extended detections
 discarded in subsequent  processing.  SES1 appears  as a single  character per
 band in the same notation as NEARPS.

    Cirrus Indicator:  CIR
    ----------------------

    This cirrus flag gives the number of point source detections at 100 microns
 only, hours confirmed but not necessarily weeks confirmed, within a 30' radius
 of the SSS source.  Cirrus is rich in structure on all scales, and may  supply
 a  100  micron  or  a  60  micron  component  to  a  source  detected at other
 wavelengths, or may combine with the  100 micron or 60 micron emission  from a
 source and severely compromise its flux measurement.  Values of CIR above  six
 invite caution in interpreting both 60 and 100 micron detections.



    High Source Density Flag:  HD
    -----------------------------

    This single character flag denotes whether the source falls in a region  of
 high  source  density  in  the  sense  of  Point  Source  clean-up  processing
 (Suppl.V.H.6).  This flag  was not used  in SSS processing,  but warns against
 confusion by  pointing out  areas where  point source  density may  exceed the
 resolving  capability  of  the  instrument.   When  the flag is written out in
 binary notation the least significant bit refers to the 25 micron band, and so
 on.  Values greater  than 9 are  denoted by letters  as for NEARPS  (See Table
 Suppl.X.B.2).

    Double Point Source Flag:  DBLPS
    --------------------------------

    This  flag  indicates  the  possibility  that  this  Catalog  source is the
 combination  of  two  point  sources  (see  IV.B.2.e  for  definition).    The
 indication is given for each band, encoded as for HD, with the bit set to  one
 when  the  possibility  exists  for  the  corresponding  band.   This  flag is
 estimated only  for bands  represented in  the source,  regardless of the flux
 quality in these bands.

    Coincident Point Source:  PTSRC
    -------------------------------

    A  small  extended  source  will  often  trigger the point source detection
 processor, either because  it is still  small enough to  produce an acceptable
 fit to the  template (bright point  sources with low  correlation coefficients
 are examples of this), or because it actually contains one or more  point-like
 components.   When  an  entry  from  the  Point  Source  Catalog lies within a
 distance D of  the SSS Catalog  source position it  is indentified as  a point
 source counterpart to the SSS Catalog source, and its IRAS name is entered  in
 this column.  The  distance D used  is half the  largest value of  UNC (V.C.3)
 among the bands in  the source.  When two  or more counterpart candidates  are
 found, the nearest one is chosen, and an asterisk precedes PTSRC as a warning.
 Clearly, the  source descriptions  here and  in the  Point Source  Catalog are
 complementary, and neither can be ignored.

    Inferred Source Size:  PSIZ(4)
    ------------------------------

    An estimate of the source size is given in deci-arcminutes for those  bands
 where the point Source counterpart is detected.  The estimate is obtained from

                                                           1/2
              PSIZ(i) = FWHM(i) x (SSSFLUX(i)/PTSRCFLUX(i))



 where FWHM(i) is the full width at  half maximum of a point source in  band i,
 namely 0.82', 0.84', 1.44',  and 3.14', respectively, at  12, 25, 60, and  100
 microns; SSSFLUX and PTSRCFLUX are the fluxes in the two catalogs.  See  V.B.1
 and V.C.3 for details and a discussion of accuracy.

    Positional Associations:  NID, IDTYPE
    -------------------------------------

    Positional  associations  between  objects  in  the  Small  Scale Structure
 Catalog and objects in astronomical catalogs were obtained following the  same
 strategy used for  Point Sources (Suppl.V.H.9).   The only difference  is that
 the search radius around sources in this Catalog is never less than 120".  NID
 is the total number of matches found.   IDTYPE ranges from 1 to 4 to  indicate
 an association found  in an extraglactic  catalog (1), a  stellar catalog (2),
 other catalogs (3),  or matches in  multiple types of  catalogs (4).  Data  on
 each associated  object appear  in a  40 byte  block whose  format is detailed
 further  below.   Each  record  starting  with  the fourth data record for the
 source contains two of these blocks.  The format for the association is almost
 identical to the Point Source Catalog (see Suppl.X.B.1).

    Flux Quality Flag:  FQLT
    ------------------------

    This is  the quality  class assigned  to the  band component  by the scheme
 described  in  Chapter  IV  and  Table  IV.A.1.   High,  intermediate  and low
 qualities are denoted, respectively, by A, B, and F. Starting with this  flag,
 data for each band appear separately in the corresponding quarter of the third
 data record for the source.

    Final Catalog Selection Flags:  FCAT
    ------------------------------------

    The results of the three  main stages of "final selection"  (Figure IV.B.1)
 are presented  in this  flag which  appears in  the tape  version as  a single
 character ranging from 0 to V,  and is decomposed in the printed  version into
 three parts (VII.B).  When FCAT is  written out in binary notation, the  least
 significant bit indicates the result of  the flux test (IV.B.2.d), and is  set
 if failed; the next bit refers to the detection count test (IV.B.2.c), and  is
 set if failed.  The next two bits refer to the repeatability test  (IV.B.2.b),
 and signal one of  four possible outcomes: 00  for intermediate values of  N/M
 except N/M = 2/2; 01 for low N/M; 10 for high N/M; and 11 for N/M = 2/2 (Table
 IV.B.3).  The most significant bit in FCAT is set if the detection is  flagged
 for  cross-talk  of  any  kind  (IV.B.2.a).   Table VII.A.2 shows all possible
 values of  FCAT, their  representation both  on tape  and in  print, and their
 meaning.



    Right Ascension Offset:  DRA
    ----------------------------

    This gives in seconds the amount to add to the mean right ascension of  the
 source (shown in the first record) to obtain the right ascension measured  for
 the emission in this band.

    Declination Offset:  DDEC
    -------------------------

    This gives in arcseconds the amount  to add to the mean declination  of the
 source (shown in the first record) to obtain the declination measured for  the
 emission in this band.

    Positional Uncertainty from Intensity Distribution:  UNC
    --------------------------------------------------------

    The  second  moments  about  major  and  minor  axes of the roughly modeled
 intensity distribution are  estimated as explained  in Suppl.V.E.3, and  their
 geometric mean is given here as a diameter in deci-arcminutes.  Because of the
 coarse grid used in  the definition, UNC is  best interpreted as a  positional
 uncertainty, namely the 95% confidence diameter for the position in this  band
 (see V.C.3 for a discussion).

    Number of Detections:  NS
    -------------------------

    This  total  number  of  seconds-confirmed  detections  in  the  source  is
 accumulated through "cluster analysis" and "weeks confirmation", and  includes
 detections that failed to seconds-confirm because of a failed detector.

    Association File:  CATNO,SOURCE,TYPE,RADIUS,POS,FIELD1,FIELD2,FIELD3
    --------------------------------------------------------------------

    For each match in an astronomical catalog, CATNO is the number  identifying
 that catalog in Tables Suppl.V.H.1 and Suppl.X.B.4.  SOURCE is the name of the
 object in that  catalog, and TYPE  its character or  spectral or morphological
 type  if  available.   RADIUS  is  the  distance  in  arcseconds from the IRAS
 position to  the associated  object.  POS  is the  position angle  measured in
 degrees East of North of the direction from the IRAS source to the  associated
 object.  FIELD1-3  carry values  depending on  the catalog  in question (Table
 Suppl.X.B.4).  Typically FIELD1 and FIELD2 contain magnitudes in decimag,  and
 FIELD3 a size in arcseconds.



        TABLE VII.A.1  Format of the Small Scale Structure Sources File

 Start
 Byte      Name             Description                  Units       Format

 1       NAME          IRAS source name                  ---         10A1

 11      BMFLG         Number of bands in source         ---         A1 [1]
                       and "band merging" warning flag

 12      RAHR          Right ascension (1950)            Hours       I2

 14      RAMIN         Right ascension (1950)            Minutes     I2

 16      RASEC         Right ascension (1950)            Seconds     F4.1

 20      DSIGN         Declination (1950)                ---         A1

 21      DECDEG        Declination (1950)                Degrees     I2

 23      DECMIN        Declination (1950)                Arcminutes  I2

 25      DECSEC        Declination (1950)                Arcseconds  I2

 27      NH(4)         Number of hours-confirmed         ---         4A1
                       sightings

 31      FLUX(4)       Averaged, spatially               Jy          4E8.2 [1]
                       integrated flux density
                       (no color correction)

 63      XTALK(4)      Cross-talk flag                   ---         4A1 [1]

 67      NEARPS(4)     Number of near-by weeks-          ---         4A1
                       confirmed point sources

 71      SES1(4)       Number of near-by FISES entries   ---         4A1

 75      CIR           Number of hours-confirmed,        ---         I2
                       100 micron only point sources

 77      BLANK         Four spare bytes                  ---         4A1

 81      HD            High source density flag          ---         A1
                       (encoded, one bit per band)

 82      DBLPS         Possibility this is a double      ---         A1
                       point source (1 bit per band)



  TABLE VII.A.1  Format of the Small Scale Structure Catalog Sources File

 Start
 Byte      Name             Description                  Units       Format

 83      PTSRC         Name of IRAS Point Source         ---         12A1
                       counterpart and conflict flag

 95      PSIZ(4)       Size estimate from comparison     Deciarc-    4I3
                       of FLUX with PTSRC flux           minutes

 107     NID           Number of associations            ---         I2

 109     IDTYPE        Type of objects associated        ---         I4

 113     BLANK         48 spare bytes                    ---         48A1

 161-180               (blank if no 12 micron component)

 161     FQLT          12 micron flux quality class      ---         A1 [1]

 162     FCAT          12 micron "final selection" flags ---         A1 [1]

 163     DRA           Right ascension offset from       Seconds     F6.1
                       mean position to 12 micron

 169     DDEC          Declination offset from mean      Arcseconds  I4
                       position to 12 micron

 173     UNC           95% confidence diameter for       Deciarc-    I3
                       position at 12 micron             minutes

 176     NS            Number of individual detections   ---         I3
                       in 12 micron component

 179     BLANK         Two spare bytes                   ---         2A1

 181-200               25 micron equivalent of bytes 161-180
                       (blank if no 25 micron component)

 201-220               60 micron equivalent of bytes 161-180
                       (blank if no 60 micron component)

 221-240               100 micron equivalent of bytes 161-180
                       (blank if no 100 micron component)



  TABLE VII.A.1a Format of the Small Scale Structure Catalog Associations File

 Start
 Byte      Name             Description                  Units       Format

 1       NAME          IRAS Source Name                  ---         A10

 12      RECNO         Rec. Num. of source in data file  ---         I6

 19      CATNO         Catalog identifier                ---         I2

 21      SOURCE        Object ID in that catalog         ---         15A1

 36      TYPE          Object type in that catalog       ---         5A1

 41      RADIUS        Distance from IRAS source         Arcseconds  I3
                       to associated object

 44      POS           Position angle from IRAS          Degrees     I3 [1]
                       source to object                  E of N

 47      FIELD1        Object data field Nr 1            [2]         I4 [1]
                       (magnitude or other)

 51      FIELD2        Object data field Nr 2            [2]         I4 [1]
                       (magnitude or other)

 55      FIELD3        Object data field Nr 3            [2]         I4 [1]
                       (magnitude or other)

 [1] This quantity appears with a different format or representation, or is
     omitted in the printed version of the catalog.

 [2] The definition and units of the quantities in FIELD1-3 depend on the
     individual catalog in which the association is found.  See Table
     Suppl.X.B.4.



          TABLE VII.A.2  Interpretation of Final Source Selection Flag

 Optical    N/M    Detection      Flux          Tape      Printed     Binary
  Cross-    Test     Count      Threshold      Version     FCAT      Notation
   Talk     [1]
                                             FQLT   FCAT    XEI        FCAT
 -----------------------------------------------------------------------------

    NO     MED*      PASS         PASS        B      0       00       00000
    NO     MED*      PASS         FAIL        F      1       01       00001
    NO     MED*      FAIL         PASS        F      2       02       00010
    NO     MED*      FAIL         FAIL        F      3       03       00011
    NO     LOW       PASS         PASS        F      4       10       00100
    NO     LOW       PASS         FAIL        F      5       11       00101
    NO     LOW       FAIL         PASS        F      6       12       00110
    NO     LOW       FAIL         FAIL        F      7       13       00111
    NO     HIGH      PASS         PASS        A      8       20       01000
    NO     HIGH      PASS         FAIL        B      9       21       01001
    NO     HIGH      FAIL         PASS        B      A       22       01010
    NO     HIGH      FAIL         FAIL        B      B       23       01011
    NO     2/2       PASS         PASS        B      C       30       01100
    NO     2/2       PASS         FAIL        F      D       31       01101
    NO     2/2       FAIL         PASS        F      E       32       01110
    NO     2/2       FAIL         FAIL        F      F       33       01111
    YES    MED*      PASS         PASS        F      G      X00       10000
    YES    MED*      PASS         FAIL        F      H      X01       10001
    YES    MED*      FAIL         PASS        F      I      X02       10010
    YES    MED*      FAIL         FAIL        F      J      X03       10011
    YES    LOW       PASS         PASS        F      K      X10       10100
    YES    LOW       PASS         FAIL        F      L      X11       10101
    YES    LOW       FAIL         PASS        F      M      X12       10110
    YES    LOW       FAIL         FAIL        F      N      X13       10111
    YES    HIGH      PASS         PASS        F      O      X20       11000
    YES    HIGH      PASS         FAIL        F      P      X21       11001
    YES    HIGH      FAIL         PASS        F      Q      X22       11010
    YES    HIGH      FAIL         FAIL        F      R      X23       11011
    YES    2/2       PASS         PASS        F      S      X30       11100
    YES    2/2       PASS         FAIL        F      T      X31       11101
    YES    2/2       FAIL         PASS        F      U      X32       11110
    YES    2/2       FAIL         FAIL        F      V      X33       11111

 [1]  In  the  repeatability  test,  "MED*"  indicates  that N/M is in the
      intermediate range (Table IV.B.2)  excluding N/M = 2/2;  this latter
      case in indicated by "2/2".



 FITS-Table-Formatted Version of the SSS Catalog

    The IRAS Small  Scale Structure 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 SSS  Catalog into  FITS format,  the first  three
 80-byte records which always appear for each source were contatenated to  form
 a single 240-byte record with a uniform format (in other words, NAXIS1=240 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 SSS Catalog source name, and  (b)
 the record number in the main data file where that SSS 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 as described above is preserved  in
 copies of the catalog that are not FITS formatted.

 -- Lee E. Brotzman, Astronomical Data Center