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

E.3. USING FITS IN CIA – NEW PROBLEMS 335
my header = fits header (
Nx,
NAXIS1
Ny,
NAXIS2
RefX,
CRPIX1
RefY,
CRPIX2
RA,
CRVAL1
Dec,
CRVAL2
Angle,
CROTA2
ResolX,
CDELT1
ResolY,
CDELT2
equinox = my equinox
cd = cd)
Some explanations are required for those unfamiliar with FITS. To fully grasp them, please
consult also Figure E.5). NAXISn are the number of pixels on the 2 axes. Axis1 is the horizontal
rightward axis, and Axis2 is the vertical upward axis. In order to perform some astrometry, one
need to know the coordinates (α, δ) of a reference pixel in the image, the plate scale and an
orientation angle. CRPIXn are the pixel coordinates of that reference pixel (usually the central
one), while CRVALn are its (α, δ) coordinates in degrees. CROTA2 it the position angle of
the celestial North axis with respect to the Axis 2 counted positively eastward, i.e. it
is different from the definitions of angles used in CIA where the North axis was the reference.
Figure E.5 clearly shows that difference (it is the angle called θ). An incorrect value in CROTA2
is the first error possibility when you find that fits header ‘does not work’. CDELTn are the
plate scales on both axes. In the FITS header they will appear in degrees per pixel which are
the units you must give to fits header. Getting the units wrong is a second source of error
when the routine ‘does not work’.
The CDELT parameters must be given with the correct signs. These are:
LW
SW
CDELT1 > 0 CDELT1 < 0
CDELT2 < 0 CDELT2 < 0
The reason that CDELT2 is negative is that pixel (0, 0) is defined to be at the top of an
ISOCAM image. This is of course an arbitrary decision but to reverse it all the signs must be
changed.
Do not forget to specify the equinox keyword. IDL works by default in B1950.0 while ISO
only uses J2000.0. . .
Optionally you can pass in the CD matrix if you already know it. It is defined as:
cos(crota2)
sin(crota2)
−sin(crota2)
cos(crota2)
The easiest way to get the CROTA2/CDELT/CD parameters is to ask the roll to crota2
routine to give you them. Its calling sequence is:
crota2 = roll_to_crota2(roll [,channel, pfov, cdelt, cd])
pfov is the pixel-field-of-view. For example,
CIA> crota2 = roll_to_crota2 (scd_get(’roll’,scd), ’lw’, [3,3], cdelt, cd])