Image Statistics and Histograms

• Task histo is a fairly commonly used task, giving a number of image statistics and a simple histogram. Apart from the input image (keyword in), it can take a region of interest ( region), a range to determine the histogram over ( range), and the number of bins in the histogram ( nbins).

  

• If you do not like histo , or you would like a PGPLOT plot of the histogram, you could use the task imhist instead. This task will also optionally draw, on the histogram, a Gaussian with the same mean, rms and integral as the actual histogram. This task, and its friend imstat use the options keyword atypically to input numeric (rather than just logical) information -- see the help file and the example below.

  Here we plot the histogram with 20 bins with a connected line style. We also do not allow any values below zero to contribute to this histogram.

  

• Complementing imhist is imstat . This task gives you statistics about your image in the selected region. also It also has the useful feature of being able to plot a selected statistic as a function of some axis in the cube.
  1. For example, let us look at the rms value computed over each RA--DEC plane in the cube as a function of velocity. Note that it does not matter what order the cube is in. For example, it might be in vxy order. imstat will average over the xy planes regardless.

     

  2. imstat can also give you a statistic averaged over just one axis rather than two if you wish. For example, if you wanted to see the sum along the velocity axis plotted as a function of RA for each DEC position in your cube you could do

     

• You may wish to integrate your image in concentric ellipses, thus producing an azimuthally averaged profile. You can do this with ellint . Each plane of a cube is integrated separately. There is no graphical output, just some tables of numbers which you can optionally write to a log file and plot with your favouriteplotting program. In this example, we integrate 10 circular rings from 0 to 100 arcsec centred on a pixel offset from the reference pixel by -5 and 10.3 arcsec in RA and DEC. We also make a primary beam correction and write the results to a log file.