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

14.6. IMAGE COMPARISON AND OVERLAYING 151
6. Now click (with the left mouse button) on this same star in the reference field. The
fields located below the ’Reference Field’ are now filled with appropriate values. In this
case the star’s FITS astrometry header is used to calculate the equator ial position. To
associate these values to the pixel positions identified in the target field, click on either the
’COMPUTE CENTROID+RA,DEC’ or ’COMPUTE RA,DEC only’ buttons. The former
calculates a simple brightness weighted mean position before comput ing the equatorial
co-ordinates. The latter does not compute any centroiding calculations. This option is
useful if you believe centroiding can negatively affect the calculations, e.g. in crowded
fields.
7. Repeat for as many stars as desired. The cross-correlation table can be printed to a file
or saved as an IDL save-set.
ii) To determine equatorial co-ordinate offsets between two images. Follow steps 1-7 to build
the cross-correlation table as listed above. Then click on the ’COMPUTE DISTORTION COR-
RECTION’ button. The jitter-correction fields located to the right of the table are filled with
the values for RA and DEC offsets. In these fields, C0 is the RA offset, and D0 is the declination
offset.
iii) To determine and apply ISOCAM jitter-correction and distortion coefficients. Follow steps 1-
7 above. Then follow the steps for determining the RA,DEC offset (example ii above). Now click
on the ’APPLY JITTER CORRECTION’ to change the astrometry structure in the ISO data
structure. This change is not permanent until you quit xco rr astro. Hitting the ’RESET’ button
will restore the original astrometry data structure. The old values are kept in an additional tag,
’OLD ASTR’.
The buttons, fields, etc. of xcorr astro The main window is shown in Figure 14.18. The
various buttons, fields and the hidden mouse commands (or short cuts) are described in this
section. There are four main sections of the widget.
1. Buttons on the top row.
’QUIT’ exit xcorr astro
’COLOR’ manipulate color tables for display
’HELP’ display a short help file
’REFRESH’ refresh all plots
2. The display panels. The images in the two display windows can be controlled via the buttons
and editable windows provided to the right and left of the display windows. Zooming
capabilities are provided for the object display. The FWHM option for the reference
window is used by the centroiding routine.
3. The selection panels. These are all the fields between the display panel and the table.
These fields provide information on the currently selected stars. When all values are filled
in the user can add the current star to the table by clicking on the appropriate button.
4. The cross-correlation table. This includes the table itself, the buttons at the bottom row
and the jitter-correction information to the right of the table. The tables can be edited.
The available edit operations are the buttons available below the table. To edit, first select
the appropriate row (by clicking on it) and followed by the edit operation.