This is a toggle switch which alters whether the object is centered in the aperture before doing the measurement. Centroiding is controlled by the parameters SEARCH, POSITIVE, MAXSHIFT, MAXITER and TOLER. These cannot be changed from within the program, and if alternative values are required they should be given on the command-line when starting PHOTOM. For instance:
would make PHOTOM centroid on objects whose signal was negative (not a very likely choice).% photom positive=f
The choice of mode is indicated by the messages `Centroiding in stellar aperture', or `No centroiding'. When starting PHOTOM one of these modes will be chosen as the default. Centroiding cannot be turned off when using optimal extraction.
Unless the field under investigation is very crowded, or there are other special conditions, it is probably best to leave the centroiding option on all the time. This is more important if the measurements are being made in non-interactive mode (command F); unless you're certain that the positions are accurate for all images.
PHOTOM --- A Photometry Package