Simple IDL programs

Arecibo Observatory

A quick intro to IDL

IDL is the program environment we work in when we monitor, reduce and analyze ALFALFA data. IDL is also useful for many other things, so if you can learn how to use it, it can solve problems for you to.

Rather than writing code interactively but then losing it when you exit IDL, you can write your own IDL programs in a text editor, save them outside IDL, and then, inside IDL, compile and use them. Here, we will give you an example of that.

Locate yourself in an appropriate directory in your working area (e.g. /home/hatillo/galaxy/YOURNAME/idl).
Open an editor on your machine, e.g. gedit testprog.pro &

Copy and paste the text below into the textedit window:

; Convert from channel number to frequency and velocity for a2010
; based on EGG a2010fv.f
; Translated by Schuyler Smith 6-22-09
; ** NOTE THIS IS ONLY APPROXIMIATE FOR VELOCITIES!

pro testprog, channel

        restf=1420.405752D
        c=299792.458D
        centerf=1385.0D
        bw=100.0D
        nchans=4096
        deltaf=bw/nchans
        startf=centerf-deltaf*nchans/2.

        freq=startf+channel*deltaf
        z=(restf/freq)-1
        vel=c*z
        print, '    frequency = ', freq, format = '(a,d9.2)'
        print, 'velocity (cz) = ', vel, format = '(a,d9.2)'
        return
        end

Note: this program is a little one written by our friend Schuyler Smith. It's based on one Martha wrote long ago in FORTRAN. For signal in a given ALFALFA frequency "channel" (i.e. pixel), it calculates the approximate HI line velocity. Note very tough, but serving as a good example for you!

Try to understand exctly what the code does.
Save the file for yourself, with the code in it.
In a terminal in the same directory where the file testprog.pro now resides, start up IDL. We hope you remember how to do that!?
Compile the program within IDL by typing:
.compile testprog.pro
Now run the program within IDL by typing:
testprog,500

Congratulations. You have now written, compiled and executed an IDL program!

Notice what this program does. It computes for the ALFALFA spectrometer setup the frequency and velocity (defined simply here as cz) in km/s for each channel (i.e. pixel in the frequency domain). Knowing that, figure out:

What is the range of velocity covered by the ALFALFA frequency setup?
What channel corresponds to 0 velocity? This, of course, is the channel nearest to the rest frequency of the HI line, 1420.4058 MHz.
Adapt the program to write another one which calculates the frequency and channel if you input the velocity.
Read through chapters 1 and 2 of Gumley's book Practical IDL Programming. Be sure you understand what is meant by array and structure. It undoubtedly helps to follow along as the book guides you through some of the operations.

Now see if you can write some programs on your own, following this example. We hope IDL makes your life easier and your research more powerful!

Last modified: Mon Oct 4 17:41:20 EDT 2010 by martha