.TH SCAT 7
.SH NAME
scat \- sky catalogue
.SH SYNOPSIS
.B scat
.SH DESCRIPTION
.I Scat
looks up items in catalogues of objects
outside the solar system
and implements database-like manipulations
on sets of such objects.
.PP
Items are read, one per line, from the standard input
and looked up in the catalogs.
The result of the lookup becomes the set of objects available
to the database commands.
After each lookup or command, if more than two objects are
in the set,
.I scat
prints how many objects are in the set; otherwise it
prints the objects'
descriptions or cross-index listings (suitable for input to
.IR scat ).
An item is in one of the following formats:
.TP
.B ngc1234
Number 1234 in the Revised New General Catalogue of
Nonstellar Objects.
The output identifies the type 
.RB( eg =galaxy,
.BR pn =planetary
nebula, 
.BR gc =globular
cluster, 
.BR oc =open
cluster, 
.BR dn =diffuse
nebula or 
.BR nc =nebular
cluster),
possibly contained within the Large Magellanic Cloud 
.RB ( in\ lmc )
or Small
Magellanic Cloud 
.RB ( in\ smc ),
its position in 2000.0 coordinates
and galactic coordinates, and a brief description.
.TP
.B sao12345
Number 12345 in the Smithsonian Astrophysical Star Catalogue.
Output identifies the visual and photographic magnitudes,
2000.0 coordinates, proper motion, spectral type, multiplicity and variability
class, and HD number.
.TP
.B m4
Catalog number 4 in Messier's catalog.
The output is the NGC number.
.TP
.B planetarynebula
The set of NGC objects of the specified type.
The type may be a two-letter NGC code or a full name, as above, with no blank.
.TP 
\fL"α umi"\fP
Star names are provided in double quotes.
Known names are the Greek
letter designations, proper names such as Betelgeuse, and bright variable stars.
Greek letters may be spelled out, e.g.
.BR alpha .
Constellation names must be the three-letter abbreviations.
The output
is the SAO number.
For non-Greek names, SAO numbers and names are listed for all stars with
names for which the given name is a prefix.
.TP
.B 12h34m -16
Coordinates in the sky are translated to the nearest `patch',
approximately one square degree of sky.
The output is the coordinates identifying the patch,
the constellations touching the patch, and the NGC and SAO
objects in the patch.
The program prints sky positions in several formats corresponding
to different precisions; any output format is understood as input.
.TP
.B umi
All the patches in the named constellation.
.PP
The commands are:
.TP
.BI add " item"
Add the named item to the set.
.TP
.BI keep " class ..."
Flatten the set and cull it, keeping only the specified classes.
The classes may be specific NGC types,
all stars
.RB ( sao ),
all NGC objects
.RB ( ngc ),
all M objects
.RB ( m ),
or a specified brightness range.
Brightness ranges are specified by a leading
.B >
or
.B <
followed by a magnitude.
Remember that brighter objects have lesser magnitudes.
.TP
.BI drop " class ..."
Like
.I keep,
but keeps only the objects not in the specified classes.
.TP
.BI flat
Some items such as patches represents sets of items.
.I Flat
flattens the set so
.I scat
holds all the information available for the objects in the set.
.TP
.BI print
Print the contents of the set.  If the information seems meagre, try
flattening the set.
.TP
.BI expand " n"
Flatten the set,
expand the area of the sky covered by the set to be
.I n
degrees wider, and collect all the objects in that area.
If
.I n
is zero,
.I expand
collects all objects in the patches that cover the current set.
.TP
.BI plot " option"
Expand and plot the set on the screen.
The only option is
.B nogrid
to suppress the lines of declination and right ascension.
Symbols for NGC objects are as in Sky Atlas 2000.0.
.SH EXAMPLES
Plot the NGC objects and naked-eye stars in Orion.
.EX
	ori
	keep ngc <6
	plot nogrid
.EE
.PP
Draw a map of the Pleiades.
.EX
	"alcyone"
	expand 1
	plot
.EE
.SH FILES
.B /lib/sky/*.scat
.SH SEE ALSO
.IR astro (7)
.br
.B /lib/sky/constelnames
for the three-letter abbreviations of the constellation names.
.PP
The data was provided by the Astronomical Data Center at the NASA Goddard
Space Flight Center.