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

336 APPENDIX E. ISOCAM ASTROMETRY: ANGLES AND COORDINATES
E.3.2
From CIA structures to FITS images
The main problem is to know which angle to supply to the fits header routine. It obviously
depends on the type of data.
E.3.2.1
Individual SCDs and the like
This works also for an SAD that would be the average of an SCD, for a reduced beam-switch or
CVF. The only angle that is valid in that case is the ROLL, or α. The obvious way to transfer
the data is to avoid unnecessary rotations during the transfer, thus the Y+ axis will be Axis1
and the Z- axis will be Axis2 (see Figures E.1 and E.5).
The angle that needs to be given to fits header, CROTA2, is then the position angle of
the North axis with respect to the Z- axis. Thus:
θ = 360 ◦ − α
or
CROTA2 = 360 ◦ - ROLL
E.3.2.2
Reconstructed raster images
In that case, the meaningful angle is .RASTER ROTATION, β. The individual images can no
longer be distinguished therefore their ROLL is irrelevant to the raster (it is only coincidentally
meaningful for Y-axis rasters). Here also the simplest way to store the image is to have the M+
axis be Axis1 and the N+ axis be Axis2. Thus, from Figures E.3 and E.5 one has:
θ =90 ◦ − β
or
CROTA2 = 90 ◦ -RASTERROTATION