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

330 APPENDIX E. ISOCAM ASTROMETRY: ANGLES AND COORDINATES
By construction, The M+ axis is identical to the Y+ axis while the N+ axis is in opposition
to the Z+ axis.
• α is the roll of the camera, as in Figure E.1. In the raster PDS α is found in the fields
.ANGLE RASTER and .INFO.ROLL. In the the SCD and SSCD, it is simply the field
.ROLL.
• β is the so-called SSCD and raster PDS field .RASTER ROTATION. It is the position
angle of the M+ axis of the raster, i.e. the angle between the celestial North and the M+
axis, counted positively eastward.
In a Y-axis raster, RASTER ROTATION cannot be specified by the observer at the PGA
level, and a very simple relation links it with the roll angle:
β = α +90 ◦
or
RASTER ROTATION = ROLL + 90 ◦
These rasters can be reconstructed quite straightforwardly as the camera’s axes are parallel
to the raster axes.
E.1.3
Rasters referenced to the celestial North axis
These rasters are called in short ‘North axis’ rasters. An example of such a raster is shown in
Figure E.3. These rasters can generate quite some confusion.
First, to program them, the observer had to specify the raster’s position angle, but could
only supply one comprised between 0 ◦ and 180 ◦ (in PGA this parameter was called orientation
angle). Therefore there is a 180 ◦ uncertainty between programing and reality (see Section E.2 to
remove that uncertainty). In this section we are only concerned with ‘reality’, what has actually
been performed.
Second, before reconstructing the raster, images have to be rotated by a certain angle. This
angle is not written in the data but has to be derived from the two others that we know already,
α and β. Figure E.3 show these angles, their definition follows:
• α is the roll angle of the mosaic, also called ANGLE RASTER in CIA structures.
• β is the true RASTER ROTATION comprised between 0 ◦ and 360 ◦ .
• γ is the angle between the M+ and Y+ axis, measured in the direct trigonometrical sense
from the M+ axis to the Y+ axis.
Note that in that case, α and β cannot be deduced from one another.
Given that the rasters are reconstructed with the M+ axis as horizontal axis and the N+
axis as the vertical axis, prior to project an individual position in that image, the data have to
be rotated by γ. Looking at Figure E.3, one has:
γ = α − β +90 ◦
or
γ = ROLL - RASTER ROTATION + 90 ◦