In the same way that it was useful to display images at intermediate stages in the data reduction, it is often useful to be able to calculate and list gross statistics (mean value, standard deviation etc.) for images as they are processed. There are a couple of applications which provide this facility. Proceed as follows.
The following information should be displayed:
ngc2336_r_2.log will be created containing a copy of the output.
statshas a ‘-sigma clipping’ option which allows outlying values, such as star images, to be rejected when computing the statistics. If a clipping level is given,
statswill compute statistics using all the available pixels, reject all those pixels whose values lie outside standard deviations of the mean and then re-evaluate the statistics. An array of up to five clipping levels may be given, which are applied sequentially. For example, to reject values outside two standard deviations type:
The statistics will again be displayed, but this time the clipping will have been applied before they were computed.
statswill be adequate. However, application
histpeakin the ESP package (see SUN/180) gives further details. To load ESP type:
histpeak and respond to the prompts:
Output similar to the following should be produced:
and a plot similar to Figure 11 should appear. For the purpose of determining the sky
background level it is probably best to use either
stats with the clipping option.
Also note that the
histpeak pre-amble contains the dimensions and bounds of the