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.

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

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

Miriad manager
2015-09-14