SKYMAP 2. Mapping

Projections

skymap knows about seven kinds of map projectionsm which are specified by the parameter MAPPROJ. The Aitoff projection is commonly used for maps of the entire sky. Mercator and Lambert Cylindrical projections are used to plot large areas where the center is the equator or galactic plane. The Gnomonic projection plots a plane tangent to a point on the sky and is useful for finder charts. The Orthographic projection shows one hemisphere of the sky at a time. The Polar projection is good for showing areas centered on the north or south pole. A Linear projection is provided because it is easy and it may be useful for artificially created images. Images may also be labelled with pixel coordinates instead of sky coordinates, but in that case, no source catalog or planet motion may be plotted over the image.

Coordinate Systems

Either a grid or a set of tick-marked axes can be plotted for any projection, except for the Aitoff projection which is always displayed with a grid. Equatorial (J2000 or B1950), galactic, ecliptic or orbital (arbitrary pole position) coordinates may be specified, using the parameter MAPCOOR. The axes are labelled at intervals which may be set from the parameter file or set automatically by the program. The region of the sky to be displayed is specified by the coordinates of its center and vertical and horizontal half-widths in the appropriate coordinate system. If the horizonal half-width is set to zero, a square region of the sky is plotted. The grid or axes may be omitted or read from a file previously created by skymap. Such a file can be created by plotting only the desired grid, with overplotted information if desired, and renaming the skymap.plot file. Because no new computation is needed, this file can be plotted much faster than a new grid can be computed and drawn, and it adds the capability of plotting a contour map of one image over a color display of another image.

Labels

The map may be labelled using terminal or computer-generated characters. Computer drawn characters use the Hershey font set and are, in decreasing detail, Complex, Italic, Duplex, and Simplex. The font type is specified by its first character. Standard font sizes are specified as 0-3 for the four Tektronix sizes from largest to smallest. Sizes larger than ten are assumed to be in display units and allow smaller or larger characters than the standard set to be used. Tektronix sizes 0-3 correspond to display sizes 52, 44, 36, and 28. The font size may be set separately for headings, tickmark labels, axis labels, time, plate scale, and magnitude scale table. Font type is the same for heading, tick, and axis labels.

Zooming

Portions of the displayed map may be enlarged using the Zoom command or the "Z" cursor command. The original display parameters are remembered and may be recovered using the Unzoom or "U" commands. If the default automatic grid spacing has been specified, an optimum spacing is program-selected when zooming. If in menu mode and plotting to a specific plate scale, a new plate scale is prompted for when zooming. Portions of images can thus be mapped in sky coordinates rather than in rows and columns.