.TH SCAT 7
.SH NAME
scat \- sky catalogue and Digitized Sky Survey
.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.
It also provides an interface to
.IR astro (7)
to plot the locations of solar system objects.
Finally, it displays images from the
Space Telescope Science Institute's
Digitized Sky Survey, keyed to the catalogues.
.PP
Items are read, one per line, from the standard input
and looked up in the catalogs.
Input is case-insensitive.
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 New General Catalogue of
Nonstellar Objects, NGC2000.0.
The output identifies the type 
.RB( Gx =galaxy,
.BR Pl =planetary
nebula, 
.BR OC =open
cluster, 
.BR Gb =globular
cluster, 
.BR Nb =bright
nebula,
.BR C+N =cluster
associated with nebulosity,
.BR Ast =asterism,
.BR Kt =knot
or nebulous region in a galaxy,
.BR *** =triple
star,
.BR D* =double
star,
.BR ? =uncertain,
.BR - =nonexistent,
.BR PD =plate
defect, and
(blank)=unverified or unknown),
its position in 2000.0 coordinates,
its size in minutes of arc, a brief description, and popular names.
.TP
.B ic1234
Like NGC references, but from the Index Catalog.
.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 abell1701
Catalog number 1701 in the Abell and Zwicky
catalog of clusters of galaxies.
Output identifies the magnitude of the tenth brightest member of the cluster,
radius of the cluster in degrees, its distance in megaparsecs,
2000.0 coordinates, galactic latitude and longitude,
magnitude range of the cluster (the `distance group'),
number of members (the `richness group'), population
per square degree, and popular names.
.TP
.B planetarynebula
The set of NGC objects of the specified type.
The type may be a compact NGC code or a full name, as above, with no blank.
.TP 
\fL"α umi"\fP
Names are provided in double quotes.
Known names are the Greek
letter designations, proper names such as Betelgeuse, bright variable stars,
and some proper names of stars, NGC objects, and Abell clusters.
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, catalog numbers and names are listed for all objects 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 Abell, 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.
.TP
.B mars
The planets are identified by their names.
The names
.B shadow
and
.B comet
refer to the earth's penumbra at lunar distance and the comet installed in the current
.IR astro (7).
The output is the planet's name, right ascension and declination, azimuth and altitude, and phase
for the moon and sun, as shown by
.BR astro .
The positions are current at the start of
.I scat 's
execution; see the
.B astro
command in the next section for more information.
.PP
The commands are:
.TF print
.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 ),
all Abell clusters
.RB ( abell ),
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 ..."
Complement to
.BR keep .
.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 meager, 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 astro " option"
Run
.IR astro (7)
with the specified
.I options
(to which will be appended
.BR -p ),
to discover the positions of the planets.
.BR Astro 's
.B -d
and
.B -l
options can be used to set the time and place; by default, it's right now at the coordinates in
.BR /lib/sky/here .
Running
.B astro
does not change the positions of planets already in the display set,
so
.B astro
may be run multiple times, executing e.g.
.B "add mars"
each time, to plot a series of planetary positions.
.TP
.BI plot " option"
Expand and plot the set in a new window on the screen.
Symbols for NGC objects are as in Sky Atlas 2000.0, except that open clusters
are shown as stippled disks rather than circles.
Abell clusters are plotted as a triangle of ellipses.
The planets are drawn as disks of representative color with the first letter of the name
in the disk (lower case for inferior planets; upper case for superior);
the sun, moon, and earth's shadow are unlabeled disks.
Objects larger than a few pixels are plotted to scale; however,
.I scat
does not have the information necessary to show the correct orientation for galaxies.
.IP
The option
.B nogrid
suppresses the lines of declination and right ascension.
By default,
.I scat
labels NGC objects, Abell clusters, and bright stars; option
.B nolabel
suppresses these while
.B alllabel
labels stars with their SAO number as well.
The default size is 512×512; options
.B dx
.I n
and
.BR dy
.I n
set the
.I x
and
.I y
extent.
The option
.B zenithup
orients the map so it appears as it would in the sky at the time and
location used by the
.B astro
command
.RI ( q.v. ).
.IP
The output is designed to look best on an LCD display.
CRTs have trouble with the thin, grey lines and dim stars.
The option
.B nogrey
uses white instead of grey for these details, improving visibility
at the cost of legibility when plotting on CRTs.
.TP
.B "plate \f1[[\f2ra dec\f1] \f2rasize\f1 [\f2decsize\f1]]"
Display the section of the Digitized Sky Survey (plate scale
approximately 1.7 arcseconds per pixel) centered on the
given right ascension and declination or, if no position is specified, the
current set of objects.  The maximum area that will be displayed
is one degree on a side.  The horizontal and vertical sizes may
be specified in the usual notation for angles.
If the second size is omitted, a square region is displayed.
If no size is specified, the size is sufficient to display the centers
of all the
objects in the current set.  If a single object is in the set, the
500×500 pixel block from the survey containing the center
of the object is displayed.
The survey is stored in the CD-ROM juke box; run
.B 9fs
.B juke
before running
.IR scat .
.TP
.BI gamma " value"
Set the gamma for converting plates to images.  Default is \-1.0.
Negative values display white stars, positive black.
The images look best on displays with depth 8 or greater.
.I Scat
does not change the hardware color map, which
should be set externally to a grey scale; try the command
.B getmap gamma
(see
.IR getmap (9.1))
on an 8-bit color-mapped display.
.PD
.SH EXAMPLES
Plot the Messier objects and naked-eye stars in Orion.
.EX
	ori
	keep m <6
	plot nogrid
.EE
.PP
Draw a finder chart for Uranus:
.EX
	uranus
	expand 5
	plot
.EE
.PP
Show a partial lunar eclipse:
.EX
	astro -d
	2000 07 16 12 45
	moon
	add shadow
	expand 2
	plot
.EE
.PP
Draw a map of the Pleiades.
.EX
	"alcyone"
	expand 1
	plot
.EE
.PP
Show a pretty galaxy.
.EX
	ngc1300
	plate 10'
.EE
.SH FILES
.B /lib/sky/*.scat
.SH SOURCE
.B /sys/src/cmd/scat
.SH SEE ALSO
.IR astro (7)
.br
.B /lib/sky/constelnames\ \ 
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, except for NGC2000.0, which is Copyright © 1988, Sky
Publishing Corporation, used (but not distributed) by permission.  The Digitized Sky Survey, 102
CD-ROMs, is not distributed with the system.