The graph is labeled with astronomical coordinates taken from the FITS
headers or from the astrometry information stored in the raster data
structure. The type of coordinate display is controlled by the keyword
RADEC. In the default setting (RADEC=0) each axis is marked with the
corresponding distances from the center of the image. ISOCONT
automatically sets the appropriate units. The reference coordinates
of the image center (in the epoch J2000 by default) are included in a
legend displayed just below the image. The north-east directions are
indicated by arrows. (This can be suppressed if the keyword /NONORTH
is used, as explained in section 3.3).
If the RADEC=1 option is used, then an orthogonal transformation (in most cases a simple rotation) is first performed to the input image. Then the image is displayed with its north and east directions towards the top and the left of the display window respectively.(This option can also be selected by typing: /RADEC). The display window is a square equal in size to the input reference image. The axes are Right Ascension and Declination, with RA increasing from right to left and Dec from bottom to top of the display window. (See Fig. 2)
Since the display window is always square, the previous transformation
can result in displaying areas which are not covered by the original
image. By default, a value of -1 is put in all undefined pixels. If
the dynamic range of the input image is small (i.e. there is a small
difference between the minimum and maximum values of the image) then
the color table is not adapted properly in the transformed image. In
this case it is better to put in undefined pixels a value which is
close to the minimum value of the input image. This can be done using
the MISSING keyword.
(See Fig. 2)
All coordinate labels can be precessed to the equinox specified by the
EPOCH keyword. ISO images stored in CIA raster data structures are in
J2000 coordinates. Using EPOCH=1950 can be useful to quickly compare
an ISO image with another image extracted from an older publication
(usually in B1950 coordinates).
(See Fig. 3)
The number of tics along each axis is controlled by the NTICS keyword.
The default number of tics is 5. A coordinate grid can also be
overplotted by using the /GRID keyword.
(See Fig. 4)
It is possible that an ISO raster map (or a FITS file) may have a
rectangular rather than a square shape. This is the case in a
6 x 2 raster map. When the RADEC=0 option is used then the image
is fully displayed in a rectangular frame. If RADEC=1 the image may
have to be rotated and only a part of it is displayed in the
rectangular frame. This problem can be avoided using the /RECT
option. With this option the rectangular image is first copied in a
square one with size equal to the diagonal of the previous image and
after that it's rotated.
(See Fig. 5)