S -- sky

This is used to choose between the different methods of estimating the background level in the sky aperture. There are four possible choices selected by the integers 1 to 4 which have the following meanings :

1. Simple mean.
2. Mean with 2 sigma rejection.
3. Mode.
4. A constant.

The simple mean uses all the values in the sky aperture. The mean with 2 sigma rejection excludes all those points which are more than 2 standard deviations from the mean. Because one or more wayward outliers can affect the size of the standard deviation, the mean and standard deviation are recalculated after each stage of clipping up to a maximum of three times. The mode is superficially calculated from the empirical relation $mode = 3 * median - 2 * mean$, but because this can be fooled by excessive skewness in the histogram there are rejection and averaging schemes in the algorithm to ensure stability. The final option is to supply a constant for the sky which is used for all subsequent measurements. This value is used until either a new value is chosen or one of the other methods of estimation is selected. The sky variance is also requested so that if the errors are calculated from the sky variance (command P) then a realistic error can be assigned. Both the sky value and variance should be given in data units.

When using a concentric background aperture it is recommended that the mode or mean with $2\sigma$ rejection is used as these offer protection against contamination from other objects in the sky aperture.

For optimal extraction it is currently recommended that you use the modal sky estimate.