It is occasionally necessary to recenter an image object (reference star, psf star, or target) because the current position is not centered (misplaced). The available algorithms are specified in the configuration file ldrs.cfg. In this file, a special procedure group is defined, where each entry gives the name of an existing IDL procedure. Source 1 shows a sample of the configuration file:

The tag CALCCENTER is used in the framework and must not be changed. The label Calculate Image Center is used in the graphical user interface and can be changed to a different string. The tags for the available procedures (PEAK, MEAN, PHASE, ...) should not be modified or the existing parameter files will be jeopardized. The standard distribution of the LN DRS contains three methods:

• LDRS_CALC_CENTER_PEAK: Uses the pixel with the largest value inside a small area as the new center.
• LDRS_CALC_CENTER_MEAN: Uses the weighted center of all pixels in a small window that have values above a certain limit.
• LDRS_CALC_CENTER_PHASE: Uses the fourier phase of the smallest frequency in a small image part as a displacement vector.

The template in source 2 can be used as a starting point to implement a new recentering method.

In the configuration file, the following excerpt from source 3 must be appended immediately after the last method concerning recentering (see source 1).

The placeholders <MY_PRO_NAME>, <MY_TAG>, <MY_LABEL>, and <N> must be replaced by the correct values.

In the LN DRS distribution three methods to automatically recenter a graphical object (reference star, psf star, target) exist:

• Intensity Peak: Searches the highest pixel value inside a small image part which is used as the new position. The new position has integral coordinates. This method is sensitive to noise and can give incorrect coordinates for weak reference stars.
• Intensity Mean: Uses the weighted center of all pixels in a small window which have values above a certain limit. The new position has fractional coordinates. This method is not sensitive to a tilted background (for example, the object halo).
• Fourier Phase: Uses the fourier phase of the smallest frequency in a small image part as a displacement vector. This method is sensitive to a tilted background.