idl examples

Here are some example commands that could be used to analyse images
and produce plots in IDL. The IDL commands that you type are in red font, Unix commands are in green font and my coments are in blue font.

You may want to work through the IDL tutorials linked on our website here:

http://www.star.bris.ac.uk/links.html#webtut

You can cut and paste from this webpage into a terminal window.

First of all, lets make a directory (folder) called
telescope_data to store our data.

mkdir telescope_data

go into this directory
cd telescope_data

copy some files to here 
cp ~rahm/junk/telescope/*.FIT .

Start IDL 

idl

 IDL Version 6.0 (linux x86 m32). (c) 2003, Research Systems, Inc.
 Installation number: 65539.
 Licensed for use by: STARLINK: University of Bristol

Read the contents of the image star1b.FIT in an
	array called a
 IDL>  a=readfits("star1b.FIT")
%   Compiled module: READFITS.
%   Compiled module: SXPAR.
%   Compiled module: GETTOK.
%   Compiled module: VALID_NUM.
%   READFITS: Now reading 765 by 510 array
%   Compiled module: SXADDPAR.
display a as an image
IDL>   tvscl,a

print information about the array a
IDL>   help,a
 A               UINT      = Array[765, 510]

Display the array a as a surface plot.
 IDL>  shade_surf,a

Bring up the IDL help manual
 IDL>  ?

 % Unable to determine the version of Acrobat Reader in use on this system.
 One
   reason for this would be if your acroread command (/usr/bin/acroread) is
   a
   locally supplied wrapper in front of the Adobe supplied script. IDL will
   therefore assume Acrobat Reader version 8. If this assumption is wrong,
   you will get an error 30 seconds after Acrobat Reader starts. To
   eliminate
   this message, and/or to explicitly specify the version of Acrobat Reader
   in use, please consult the file
   /usr/local/rsi/idl/help/idl_acrobat_plugin.
   config.

Fit a two dimensional gaussian to the array a
IDL>  z=gauss2dfit(a,params)
%   Compiled module: GAUSS2DFIT.
%   Compiled module: GAUSSFIT.
%   Compiled module: POLY_FIT.
%   Compiled module: CURVEFIT.
%   Program caused arithmetic error: Floating underflow

Print fit parameters, 

A[0] = A0= constant term
A[1] = A1= scale factor
A[2] = a = width of Gaussian in the X direction
A[3] = b = width of Gaussian in the Y direction
A[4] = h = center X location
A[5] = k = center Y location.
A[6] = T = Theta, the rotation of the ellipse from the X axis in
	radians, counterclockwise.


IDL>  print, params
       110.833      1805.78      20.4058      19.2199      214.416
       412.040       0.00000

Look at gauss2dfit manual page for full info on what
each number is, but the 3rd and 4th numbers are the Full Width Half
Maximum (FWHM) of the gaussian that is the best fit to the data.

Calculating mean and standard deviations

Calculate the mean of array "a" and store it in a
	variable called "b".
IDL> b=mean(a)
%  Compiled module: MEAN.
%  Compiled module: MOMENT.

print the mean value
IDL> print,b
      122.245

You can combine IDL commands into one line. Eg, to
calculate and print the standard deviation of array "a"
IDL print, stdev(a)
      123.504


Note, for this image, which contains an object, then the standard deviation is 
bigger than the mean! This should not be the case for dark frames. If it is, 
something is badly wrong!

Plotting

First of all, in Linux (outside of IDL) use an
editor to put the data into a two column file, eg

gedit temperature_vs_darkcurrent.dat

and the file should look like this:

15	300
10 	200
5	100
0	90

Then when you are done, you can read it into idl.


IDL> iplot, temperature_vs_darkcurrent.dat
% Compiled module: FILEPATH.
% Compiled module: PATH_SEP.
% Loaded DLM: PNG.
% Compiled module: XMANAGER.
% Loaded DLM: IPTOOL.
% Compiled module: GET_SCREEN_SIZE.
% Compiled module: REVERSE.
% Compiled module: CVTTOBM.
% Compiled module: IDENTITY.
% Compiled module: STRSPLIT.
% Compiled module: UNIQ.

The plot appears in a window. You can now edit the plot, label the 
axes and, using the "export"  item in the file menu, save it as a 
jpeg.

You can also use the command

IDL> isurface,a

to look at a two dimensional array, from lots of
      directions. 

Quit out of IDL
IDL> exit

Remember, by default, idl does not save variable
between sessions. so you will need to open and read the image in again
if you have closed idl.

If you are doing something very hard, with dozens of variables, idl
can "save state" so you can start again where you left off. Let me
know if you will need to do this.