usemask

This parameter is a logical flag which indicates whether a mask is to be used when estimating the background. The purpose of the mask is to block out contaminating objects from the background aperture. In this way bright stars can be excluded from the estimation of the sky, which would otherwise introduce contamination. Note that the sky estimators that perform clipping of the pixel histogram, the mode and the mean with $2\sigma$ rejection, also exclude contaminating pixels, but using the mask along with the mean estimator allows this to be done in a controlled way.

If the USEMASK flag is TRUE then a file containing a list of positions is requested (MASKFILE). The format of the file is the same as for inputting a list of positions to measure (command F), namely an index number followed by an x and y position. The given coordinates define the centres of circles and any pixel with its centre within a circle will be excluded from the sky estimation. The radius of the masking circle is defined by another parameter (MASKRAD).

The mask only affects pixels in the background aperture, it does not exclude any pixels from the measurement aperture. This means that identical lists can be used to create the mask and to provide a source for measurement. The output from an automatic object finding package could be used in this way.

It is important not to confuse this mask with the point spread function masks discussed as part of the optimal extraction algorithm.

Other ways of masking out pixels are to use the KAPPA facilities ARDGEN and ARDMASK, or the ARD and PATCH toolboxes of the GAIA (SUN/214) image display tool.