Specify distance between tic marks for astronomical coordinate overlays
User inputs number an approximate distance
between tic marks, and the axis length in degrees. TICPOS will return
a distance between tic marks such that the separation is a round
multiple in arc seconds, arc minutes, or degrees
TICPOS, deglen, pixlen, ticsize, incr, units
deglen - length of axis in DEGREES
pixlen - length of axis in plotting units (pixels)
ticsize - distance between tic marks (pixels). This value will be
adjusted by TICPOS such that the distance corresponds to
a round multiple in the astronomical coordinate.
ticsize - distance between tic marks (pixels), positive scalar
incr - incremental value for tic marks in round units given
by the UNITS parameter
units - string giving units of ticsize, either 'ARC SECONDS',
'ARC MINUTES', or 'DEGREES'
Suppose a 512 x 512 image array corresponds to 0.2 x 0.2 degrees on
the sky. A tic mark is desired in round angular units, approximately
every 75 pixels.
IDL> ticsize = 75
==> ticsize = 85.333, incr = 2. units = 'Arc Minutes'
i.e. a good tic mark spacing is every 2 arc minutes, corresponding
to 85.333 pixels.
written by W. Landsman November, 1988
Converted to IDL V5.0 W. Landsman September 1997
Don't use all capital letters W. Landsman May 2003
Fix case where incr crosses degree/minute or minute/degree boundary
A. Mortier/W.Landsman April 2005