		      xsky 2.0 - a computerized star chart



First  and foremost, xsky draws  a resizeable  black box  on the  screen, and
plots a section of  the sky  from the Yale Catalog of Bright Stars.  The size
of each star  corresponds to its brightness, and the color corresponds to the
spectral class (a monochrome display gets any  color it wants, as  long as it
wants white).  The scale can be infinitely magnified or compressed, either by
zooming in or out or by directly  specifying  the  display scale, and you can
move around  the entire sky via scrollbars or by  entering  a center position
(right ascension and declination) for the display.  Clicking the  right mouse
button anywhere in the display recenters the display on the clicked-on point.

If you add in the Revised New General Catalog of Non-Stellar Objects, you get
a small figure drawn on  the screen in the position of each object.  Clicking
the left mouse button on any star or object brings up  a formatted display of
the  catalog information about that object, including  common names  for  the
stars and Messier  numbers for NGC objects.  RNGC objects and  YBS stars (and
the included quasar catalog)  can  be added to or removed from the display at
will with the "Catalogs" menu option.

Clicking the  center mouse  button on an object  brings  up  a  bit  of  text
identifying the object; stars  are  identified  by Bayer Greek  letter or  by
Flamsteed number,  if any,  or by  HD or BD number otherwise, and NGC objects
are identified by Messier number, if any, or by NGC number.  This text can be
positioned  anywhere near the object that the user desires.  The text may  be
removed from the  display by clicking the center  mouse button with the shift
key  depressed.  The "Hide  IDs" menu button removes the IDs from the  screen
temporarily, and the "Erase  IDs" menu option permanently clears the IDs from
the screen.

The "Info" menu option brings  up a panel which gives some  information about
the sky display and two text widgets.  You can edit one of these to specify a
magnitude limit for the stars; the other may be edited to change the scale of
the display.

The "Zoom  In"  and "Zoom Out" menu buttons change the scale  of the display;
zooming in magnifies the display  and zooming out  provides a more wide-angle
view.

The  "Grid"  menu  option  adds  coordinate  lines  of  right  ascension  and
declination to the display.

The "Find" menu option brings up  a text box  in which you can enter the name
of a star or object, and the program  will locate that object and  center the
display on  it.  This  allows finding  stars by  name, or objects  by catalog
designation (the RNGC catalog also allows finding objects by Messier number).

The final slick feature is that the "Chart" menu option will generate a Post-
Script file of the display,  with  the stars correctly  sized, including  any
labels you placed on any of the objects.



Initialization:

First, note that xsky takes a while to start - it must inhale  the entire YBS
catalog  (9110 stars) and  the entire RNGC  catalog (8193  objects),  do some
sorting, plot a bunch of stars, and then paint all that on  the screen.  Have
patience -  this  takes about 10 seconds on  a 20-SPECmark  machine with SCSI
disks (DECstation 5000/200).

This startup  time  is  lengthened if  additional catalogs are available.  To
speed things up, xsky looks  for a binary file which is a dump of the in-core
database for each catalog.  If present, xsky merely reads in the binary file;
if present but empty, xsky creates the binary data from  the catalog,  and if
not present, xsky reads  the  catalog and does  not save the binary database.
These binary filenames  are  ybs.bin,  rngc.bin,  and qso.bin, for the  three
supplied catalogs.

The environment variables YBS_PATH and RNGC_PATH are  used  to  determine the
paths to the YBS data (ybs.dat and starnames.dat) and the RNGC data (rngc.dat
and rngc_desc.tab).   If  either of these environment variables are not  set,
xsky will look for the associated files in the current directory, and give up
if they are not found.  YBS_BIN_PATH and RNGC_BIN_PATH are used to locate the
binary  file  mentioned  in  the  previous  paragraph.   Similar  environment
variables are defined for any other catalogs to be used by xsky.

xsky comes up with  a  500x500 pixel sky, at a scale of 20 pixels per degree,
with  a stellar magnitude limit of 8.0,  centered on Alnilam (the middle star
in Orion's belt).   These values are all  configurable in the resources file,
XSky, and have  been chosen so the familiar constellation of Orion appears in
the display on startup.



MAIN MENU:

In the upper left is a  button labelled  "Control".  Click  and hold the left
mouse  button to bring  down a menu.   Drag  the mouse button  down  and each
choice  is highlighted.  Release the  mouse button and the highlighted choice
is activated.  The choices are as follows:


Info: this brings up  a panel  showing  the current  size of the  display  in
degrees (note that the display is circular, as seen in a telescope - this may
change  in  the  future),  and three  text  boxes  giving the current  center
position,  star magnitude limit, and display  scale.  These  text   boxes are
editable; move  the mouse pointer into them,  change them  via  Emacs editing
commands, click on the "Apply" command button, and the display will update to
reflect your changes.  Clicking on the  "Cancel"  command button ignores your
changes to the text fields.


Catalogs:  brings up a  submenu allowing you to select inclusion or exclusion
of any of the catalogs.  The submenu selection is highlighted if that catalog
is currently included.  Click the left mouse button on a  selection to toggle
inclusion/exclusion of that catalog.


Zoom  In:   magnifies  the  display,  leaving  the  display  center  position
unchanged.


Zoom Out: pulls back to a more wide-angle view, again without  modifying  the
center position of the display.
 

Grid:  coordinate lines of right ascension  and declination are drawn on  the
display.


Find: brings up  a  subpanel with a text input  box to enter  the name of the
object to find.  Position the mouse pointer  into  it, enter the  name of the
object, and click the left mouse button on "Find" to move the display to that
object.  If the object cannot  be  found, a bell  sounds and the display does
not change.

Stars may be found by common name, Bayer letter (Greek  letter spelled out in
lower   case)   plus   constellation   designation   (standard   three-letter
abbreviation or full genitive-case name), Flamsteed number plus constellation
designation, or HR number (the YBS catalog number).  NGC objects may be found
by entering  their  Messier  number as "M nn" or their catalog number as "NGC
nnnn".  Objects  from  other catalogs are found by their designation  in that
catalog only.

Using this scheme, there are ambiguities between some Bayer Greek letters (mu
and  nu) and variable-star  designations of  the  same two-letter  pairs.  To
resolve  this, variable-star designations will  only be recognized  in  UPPER
CASE.  Thus,  "vv Puppis" will  win a  bell, but  "VV  Puppis" will  find the
variable star.


Chart: generates a file  named "starchart.ps" containing the current display,
including the names of any labelled objects (see below).


Quit: everyone's favorite by the time I get done bragging about this program.



OTHER ACTIONS IN THE MAIN WINDOW:

The text labelled "Display center" is editable; changing this text will move
the sky display to the specified location when the return key is depressed
while in the editing box.

The vertical and horizontal scrollbars will jump-scroll (not smooth-scroll)
the display to a new position depending on where the scrollbar was clicked.
The size of the scrollbar thumb is intended to give a rough indication of how
much of the total sky is visible.



MOUSE ACTIONS IN THE DISPLAY:

Clicking the left mouse button on a star or object will bring up a data panel
giving the catalog data for that  object.  Additionally, stars have a list of
common names, if any, and NGC objects include the Messier number, if any, and
galaxies  have the NGC description expanded into English  text (more or less;
the authors of  the  new NGC  descriptions quite bogusly and unashamedly used
ambiguous notations, and failed to document a few  of the  abbreviations they
used - xsky does the best it can).

Clicking the right mouse button on any point in the display will recenter the
display on that point, with no change to any other display parameters.

Pressing and holding the center mouse button on any object  brings up a small
rectangle outline which is sized to enclose the  name of the object.  The box
will follow the mouse  movement until you release the  button, at which point
the box will disappear and the actual  textual identification will  appear in
its place.  Identification of a star is chosen in  the following order: Bayer
letter,  Flamsteed  number, Henry Draper  (HD)  number, Bonner Durchmusterung
(BD) designation,  Henry Russell (HR)  number  (this last is the  YBS catalog
designation of  the star).   Identification of  an NGC object  is by  Messier
number   first, and NGC   number second.   Note   that   moving, resizing, or
rescaling the display  will wipe out  the identifications, as will adding new
catalogs to the display (to prevent the text from hiding an object in the new
catalog).

Clicking the center mouse button on a text identification,  while holding the
shift key down, will remove that text identification from the display.


USUAL X ACTIONS:

The  usual X actions, under control of your window manager, apply to the xsky
window.  It can be moved about the  screen, resized, iconified, and killed in
the  usual manner.  The usual assortment  of  X options,  such as  background
color and geometry, can be specified on the command line.
