ISOCAM Interactive Analysis User's Manual Version 5.0 - ISO - ESA

328 APPENDIX E. ISOCAM ASTROMETRY: ANGLES AND COORDINATES
N
Y+
α
E
Column 24
Z+
Figure E.1: Schematic of an ISOCAM-LW image. Y+ and Z+ are the satellite axes. Column
24 is indicated to help you visualize the array. α, called the roll angle, is the position angle
of the Z+ axis, i.e. the angle between the celestial North and the Z+ axis, counted positively
eastward.
• We assume that you have already used CIA and are quite familiar with its structure
organization and basic routines.
E.1.1
Definition of the roll angle for CAM-LW
The roll angle for ISOCAM’s LW detector is defined in Figure E.1. Note that while this diagram
is correct, the orientation of your display can change the direction in which the axes point. In
IDL, the orientation of a displayed image depends on the system variable !ORDER. Refer to
Section E.2.3 for different axis orientations and the effects of !ORDER.
E.1.2
Rasters along the satellite axes
These rasters are called for simplicity’s sake ‘Y-axis’ rasters and an example is given in Figure
E.2. The sky is scanned along the satellite Y and Z axis. The raster axes are called M+
and N+ and the referential (M,N) is direct. In that referential ISO always starts its raster at
the lower left corner.
The number of points in the M direction of the raster are held in the raster PDS field
.RASTERCOL, which takes its value from the FITS keyword ATTRNPTS. The raster pds field
.M STEPCOL (or the FITS keyword ATTRDPTS) gives the distance between adjacent columns
in arcsecs. Likewise, the number of points in the N direction of the raster are held in the in the
raster PDS field .RASTERLINE, which takes its value from the FITS keyword ATTRNLNS. The
raster pds field .N STEPLINE (or the FITS keyword ATTRDLNS) gives the distance between
adjacent lines in arcsecs.