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

Appendix E
ISOCAM astrometry: angles and
coordinates
With the roll angle of ISO constrained by the sun position, CAM does not produce images
conveniently oriented to the standard astronomical convention. This appendix 1 should help you
to understand how your IMAGEs, EXPOSUREs and MOSAICs are oriented and and how to go
about changing their orientation. A description of the FITS convention and how it relates to
CAM angles can be found in Section E.3.
E.1 Definitions
Ambiguous definitions (sometimes intentional) of angles of importance in ISOCAM observations
are spreading confusion in minds and in routines. This appendix sums things up to the best of
our current knowledge for the LW part of CAM (the SW is ‘a priori’ and simply deducible from
the LW case).
Before the drawings, some points to note:
• We are using only the astronomical convention for projections. In this convention, when
the celestial North is pointing upward, the celestial East is pointing leftward. In this
representation δ increases upward and α increases leftward.
• ISOCAM rasters are reconstructed along axes such that the M scanning direction goes
from left to right and the N scanning direction from bottom to top. Therefore the
North axis is generally not pointing upward in these images. However a simple rotation is
enough to restore the astronomical convention (i.e. no mirroring of the image is needed).
• The + sign after a letter is a convention to specify an oriented axis.
• The Y and Z axes are defined by the spacecraft and are identical for all instruments
(though they project differently in their various focal planes). The X axis never appears
because it is pointing to the target. Note that the (XYZ) referential is a direct one in 3D
space, but that, due to the astronomical convention used, the (YZ) referential is indirect
in the drawings.
• Rotation of a positive angle rotates in the direct trigonometrical direction, or counterclockwise,
i.e. unlike the IDL rot function.
1 Taken from Sauvage M., 1996, Angles and ISOCAM-LW, v3.0
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