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).

  HISTO
  in=vela.imap	Input image
  region	Select region of interest
  range=-0.1,1	Range for histogram. Default is image
  	min and max.
  nbins=#	Number of bins in the histogram
  	Unset for default of 20

• 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.

  IMHIST
  in=vela.imap	Input image
  region=#	Select region of interest
  options=nbin,20,style,connect	Specify desired options
  cutoff=0.0	Discard negative pixels
  device=/xs	Plot on X window

• Complementing imhist is imstat. This task gives you statistics about your image in the selected region. It computes the statistic either over a row or a plane of data.
  1. For example, let us look at the rms value of a cube computed over each RA-DEC plane. 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.

     IMSTAT
     in=zeeman.vcub	Input image cube
     region=image(50,450)	Select region of interest
     plot=rms	Plot rms
     options	Unset
     axes=ra,dec	Compute statistic over RA-DEC plane
     cutoff	Unset
     device=/xs	Plot on X window
     log=numbers.log	Write numbers to log file

  2. By setting the axes keyword to a single axis, imstat will compute the statistic over a single row. Be warned that this can produce a lot of output. For example, if you wanted to see the sum along the velocity axis plotted as a function of RA and DEC position in your cube you could do

     IMSTAT
     in=zeeman.vcub	Input image cube
     plot=sum	Compute the sum.
     axes=velocity	Compute statistic over the velocity axis.
     device=/xs

• 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 favourite plotting 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.

  ELLINT
  in=ngc1313.mom0	Input image
  region	Full region
  center=-5,10.3	Offset from reference pixel
  pa	Unset for face-on
  incline	circular rings
  radius=0,100,10	Radii of rings
  options=pbcorr	Correct for primary beam
  log=rings.1313	Write the results to log file