22 Automatically Scale Displays

A common problem with astronomical images is that a few pixels at the centre of a bright star or galaxy will dominate the dynamic range of the image. If the range of intensities displayed is simply set from the dynamic range the faint details close to the sky background will not be visible. Script scaledisp.csh addresses this problem and determines a range of intensities to be plotted which are closer to the background.

it uses histpeak in ESP to determine the sky background level and a measure of its variation. The measure used is the mean absolute deviation, α, defined as:

α = 1 ni=1nxi x¯ (6)

α is more robust than the more familiar standard deviation (because it is less sensitive to outlying values). It is also usually smaller. The image is plotted using KAPPA application display.

Script saodisp.csh is similar to scaledisp.csh, but uses the image display program SAOIMAGE (see SUN/166[21]).

The input for either script consists of the name of the image to be displayed and the minimum and maximum intensities to be displayed. The minimum intensity is specified as the number of α below the sky background and the maximum as the number of α above the sky background.


To display a scaled image with KAPPA’s display type:
  % scaledisp.csh ngc2336_r_2.sdf 1  5

File ngc2336_r_2.sdf is displayed. The minimum intensity is 1 ×α below the sky background level and the maximum intensity is 5 ×α above it.

To display the same image with SAOIMAGE and similar scaling type:
  % saodisp.csh ngc2336_r_2.sdf 1  5