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

150 CHAPTER 14. IMAGE ANALYSIS AND DISPLAY
14.6.4 xcorr astro
xcorr astro is a graphical tool which displays two images side-by-side and allows the user to:
1. Cross-correlate two source catalogs.
2. Determining shifts between two images by identifying stars common to both images.
3. Determine PSF fits and centroids for sources on the images.
The ultimate goal of xcorr astro is to correct for the astrometric shift induced the wheeljitter
offsets. The astrometry information within the ISO structure is modified to reflect these
corrections.
How to use xcorr astro?
Calling syntax
CIA> xcorr_astro, cia_pds, ’reference.fits’
Issuing the command without any arguments, or by setting the ’/help’ flag in the command line
call produces a short help on xcorr astro syntax.
Required and optional data:
1. ISO data structure with or without astrometry information (the data/target field).
2. A reference image with pre-determined astrometry information (the reference field).
3. Catalog of point sources [optional]
Step-by-Step guides
i) To build a cross-correlation source table between two image. Please consult usage syntax and
required data sections (above) before starting. Once xcorr astro gets going, follow these steps
1. Set the display settings which suit your needs. The min/max (floor/ceiling) buttons, etc.
can be used for this.
2. Visually examine images to find stars common to both images.
3. Select one of these common stars in the target field by clicking on it with the left mouse
button. The ’current selection’ fields located immediately below the target field are populated
by approximate values for the star’s position. The IX and IY fields are integer pixel
references (in IDL convention). The X and Y values are fitted centers (see next step; we
use the CIA positioning convention for these).
4. Improve the star’s centroid by either using the ’FIT PSF’ or the ’CENTROID’ option.
’FIT PSF’ will use an appropriate PSF library to find the best-fit PSF match to your
input star. ’CENTROID’ performs a simple brightness weighted mean.
5. Once satisfied with the centroiding, select ’ADD TO TBL’ to place this star in the working
table for cross-correlation. Note that only pixel positions are currently identified for this
star in the cross-correlation table.