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

332 APPENDIX E. ISOCAM ASTROMETRY: ANGLES AND COORDINATES
E.2 Trouble shooting astrometry in CIA structures
You’ve read the previous section, you think you understand ISO’s angles and you have been told
that the current version of CIA complies with these definitions. Thus you confidently reconstruct
your raster and. . . it fails! If the astrometry is seriously wrong then most likely you are using
data that is not scientifically validated – data from OLP prior to version 4.0 is not scientifically
validated. Just get the latest (OLP 10) data and re-run your CIA processing.
E.2.1
Incorrect astrometry in beam-switch data
If after calibration the beam-switch MOSAIC (BS PDS field .RASTER) appears inverted and
incorrectly has the astrometry of the reference field then read Section 19.3.
E.2.2
Astrometry inaccuracies
If the coordinates appear to be systematically wrong by a small amount then the problem is
probably due to the repositioning accuracy of the lens and filter wheels. This can have a bigger
effect than ISO’s pointing accuracy. The error induced by the wheel jitter is typically less than
2 pixels, but can reach up to 3 pixels. For example, for a 6” PFOV measurement the coordinate
error can be up 3*6” + 4” = 22”.
E.2.3
Roll, image orientation and !ORDER
It is important to understand the effect of the IDL !ORDER system variable on the displaying
of images. The difference is simply that an image will be displayed with pixel (0,0) in the
upper-left corner when !ORDER=1, and in the lower-left corner when !ORDER=0. However,
this simple difference can cause confusion when it comes to changing the orientation of a CAM
image. Figure E.4 should help you understand the effects of !ORDER and how to correctly
change the orientation of a CAM image.
E.3 Using FITS in CIA – new problems
E.3.1
FITS convention and IDL’s astrolib
IDL comes with an astronomical package called the Astrolib which is in fact quite handy: most
projection routines are already coded. However these assume that we are manipulating images
that have a FITS header. As you know by now, with structures there is no need of a FITS header.
Therefore to use these astrometric routines we have to create a FITS header appropriate to the
images.
This is done with the idl routine fits header. The syntax of the call is the following, with
standard FITS keywords given with each parameter: