R. W. O'CONNELL
December 2001

In doing Part II of the Exercises, you should have copied to your
local directory all the test data files (*.FITS) from the 
IDL exercises data directory.  Also, as described in Part II,
check to see that your windows system is properly configured before
starting exercise (1).


0)  Start IDL.  

    [On most UNIX systems, just type "idl".

       This will start IDL running in the command-line 
       mode using the window from which you called it.  IDL V.5
       also offers a widgit-based interface, the IDL Development
       Environment, which is invoked by the command "idlde".
       This offers convenience features to the experienced user,
       but it is correspondingly more complicated and not
       recommended for learning the basics.] 


    Before starting the exercises, give the IDL command:

                 ON_ERROR,1

       This will return control to the main program level if
       an error occurs in a called routine.
				    

1)  CRYPTO-IMAGE

    The image file "mystery-image.fits" contains an encrypted,
    low-contrast, 2-D message.  See if you can determine what it is by
    using any of the tools you have tried so far: plotting, IMLIST,
    CTVSCL, CONTRAST, CURVAL, different color tables, histograms, and
    so forth.  If you try more than one, which method is most
    efficient?  Once you find the message, develop a method for making
    it stand out (i.e. maximize the signal-to-noise) in a TVLASER
    hardcopy. 


2)  IMAGE COMPARISONS

    Read in the two images of M87 (courtesy of Brian McNamara).
      These are roughly co-centered.  One was taken in the B band, the
      other in the I band.  

    Compare the general statistics for the two images: mean, min, max,
      variance.  One image contains some flagged bad pixels; which is
      it?

    Load a black-on-white color table.  (See the IDL Guide
      for a sample script.  Alternatively, you can LOADCT,0 and then use
      CONTRAST to flip the color table.)

    Using a 0.3 power law display to preserve the dynamic range,
      display the images in two windows, with the [MIN,MAX] for each
      adjusted to show as much of the range of intensity values as
      possible.  The central density of the two displays should be
      roughly the same.  What differences between the two images are
      apparent?

    Now use the AstUseLib BLINK command to blink between the two
      windows.  Are differences easier to see?  Increase the rate of
      the blink to 2 per second.  [Stop the blink by putting the mouse
      on the command window and pressing any key.]

    Now load one of the following standard IDL color tables:  11, 25,
      31, or 33.  Use the MOUSSE routine REFSCL to place a color
      table reference over one of the two image windows.  Now use the
      MOUSSE routine CROLL to "roll" the color table and note how
      different color table settings affect the appearance of the
      images.  You can repeat with any or all of the other suggested
      tables.


3)  UNSHARP MASKING

    Reload the black-on-white color tables.

    M87 represents a modestly complicated scene with a diffuse
      background, a structured extended source (the famous nonthermal
      jet), and a set of unresolved globular star clusters, the
      brightest of which can be seen in the outer parts of the image.
      There are many image processing techniques available to extract
      information on the structure of a scene like this by enhancing
      lower contrast features.  Here, we'll illustrate two simple ones
      which suppress low-frequency structure (the diffuse background).

      Use the M87 I band image for these: 

      i)  Edge detection:  this is fast, if crude.  Results also 
          depend on the orientation of structures in the image.

            Simply display IMAGE - SHIFT(IMAGE,K,K) , where K is a
	    small integer (2 is a good starting value).  For best
	    appearance, adjust the display to show a small range of
	    values around zero (e.g. -3 to +3).  

            When properly adjusted, your display should readily show
	    both the jet (in the bright central parts of the galaxy)
	    and the faint globular clusters scattered throughout the
	    frame.  You can try various color tables to adjust the
	    appearance, although B&W may actually be the most useful.
	    Make a hardcopy.  


     ii) Unsharp masking:  this is a similar differencing technique,
	    except that the subtracted image is a smoothed version of
	    the original.  This removes low-frequency structure and is
	    especially useful where there is a bright background, as
	    in the case of M87.  The fastest approach with IDL is to
	    use the SMOOTH(IMAGE,K) function.  However, the smoothed
	    image would include the effects of compact structures
	    (e.g. the jet here, or stars in the case of a star
	    cluster).  A better approach is to use the MEDIAN(IMAGE,K)
	    function, which is resistant to a minority of bright
	    pixels within the KxK smoothing box.  The disadvantage is
	    that the MEDIAN routine can be slow if K is large.

            Make displays for several values of K.  Good trial values
	    are in the range 9 to 21.  Displays are best for a small
	    positive range of values, e.g. 0 to 2.  Make a hardcopy of
	    a good display.


4)  SURFACE PHOTOMETRY

    Use the SKY routine to obtain an estimate of the sky background on
       the M87 I-band frame.  What is the result?  Does it agree with
       the approximate sky value you can infer by, for example, sampling
       blank regions of the frame with CURVAL or other techniques?

    Determine in which pixel the galaxy nucleus lies on the M87 I band
       frame as follows.  Adjust the display until you can easily see
       the starlike nucleus.  Measure an approximate location
       interactively with the cursor.  To do this, type
       CURSOR,XX,YY,/DEVICE, then place the cursor on the chosen point
       on the active window and press the left hand mouse button.
       Then, take the [xx,yy] location of the cursor output and use it
       as input to the AstUseLib CNTRD centroiding routine.  What are
       the centroid values for the nucleus?  [Note:  The behavior of
       CURSOR depends on your hardware.  See the IDL hyperhelp pages
       if you have trouble.]

    Do aperture photometry on the M87 I-band frame as follows:

       Read the information header for the AstUseLib routine DIST_CIRCLE,
         and be sure you understand how it works.  You may want to try
         using it on a small test array and plotting slices across the
         array.  
       
       Devise a method, similar to that given as a sample script in the
         DIST_CIRCLE header, to obtain the integrated flux of the M87 image
         within a circular aperture (centered on the galaxy nucleus).

       Measure the total fluxes within circular apertures of 10, 20,
         30, and 40 pixel radii.

    Make up a simple table (printed to your screen and the journal file) of
       the aperture radius, total flux in aperture, and net flux in aperture
       after an appropriate correction for the sky background is made.

       By what percentage did the sky correction reduce the final flux in
       the 40 pixel aperture?

    To illustrate the general shape of the galaxy brightness profile
       and emphasize any asymmetries, make a "pixel plot" of the
       galaxy as follows:

       Extract a subarray of 71x71 pixels centered on the nuclear pixel
         you identified above.  Call this FLX.

       Use DIST_CIRCLE to make a matching array containing the distance of
         each pixel from the center.  Call this RAD.

       Now open a plotting window and make a plot of the radius of
	 each pixel in the subarray vs its flux by typing
	 PLOT,RAD,FLX,PSYM=5.  (PSYM = 5 plots points as open
	 triangles; PSYM =1 plots plus signs and is similarly
	 useful.)  Most data points should fall in a compact
	 distribution showing that the source is highly symmetrical.
	 Have you reached the sky background level at the edge of the
	 subarray?  Is there evidence of an important asymmetry?  Make
	 a hardcopy of the plot.  [A linear-log plot using PLOT_IO can
	 also be useful, but use WHERE to exclude any negative flux
	 points.]

       Test the sensitivity of this technique to asymmetries by 
         re-extracting the subarray but with the center offset by a
         couple of pixels from the true nuclear pixel.  

     Another way to visualize the source structure is to make a surface
       plot with the SURFACE routine.  Open a plotting window and give
       the command SURFACE,FLX.  You can explore the shape of the surface
       by inverting the plot (SURFACE,-FLX) or viewing it from another
       angle (e.g. SURFACE,FLX,AX=60).  Make at least one hardcopy.


5)  POINT SOURCE PHOTOMETRY

    Read in the image of the globular cluster M13 from the file
       "m13-ex8.fits".  

    Display the image.  The center of the cluster is very crowded but
       is not saturated in this exposure.  Find a set of display
       parameters which retains information in the center while still
       showing the stars in the outer part of the cluster.  Make a hardcopy.

    Extract a 100x100 subarray of the image near the cluster center.
       Open a plotting window, and make a SURFACE plot of the
       subarray.  Explore several regions at different radii.  How
       far from the center do you have to be to avoid blended point-spread
       functions? 

    Extract a 400x400 subarray of the image in its upper left hand
       quadrant, and use this for the remainder of exercise (5).

    The AstUseLib FIND routine is part of the DAOPHOT photometry package
       written by Peter Stetson.  This version has been converted to IDL
       from the original FORTRAN.  FIND locates stellar objects within
       a given image.  It is efficient and employs only a few parameters,
       the most important of which is HMIN, the minimum flux of a candidate
       source above the background for a threshold detection.

       Apply FIND to the subarray.  Use an HMIN value of 120 and a
	 convolution FWHM of 4 pixels.  Leave other parameters at their
	 default values.  In the initial call to FIND, be sure to
	 specify output variables to store the [x,y] and flux values
	 produced by the routine.

       Display the subarray on your terminal and use the AstUseLib
	 TVBOX routine to place small boxes around all the stars
	 chosen by FIND. (11 pixels is a good boxsize.)  Make a
	 hardcopy of the result.  

    APER (in AstUseLib) is the next routine used in a standard DAOPHOT
       reduction.  It obtains circular aperture photometry of stars
       selected by FIND.  It uses a small inner aperture to measure
       each star and a slightly larger annulus to measure the local
       sky.  

       Use APER to obtain brightnesses for the sources located in the
	 FIND run above.  Reasonable values for this image are 5
	 pixels for the "first" star aperture radius (press the return
	 key when you are asked for "another aperture radius") and 6
	 and 9 pixels for the sky aperture.  Set the number of "photons
	 per ADU" equal to 1.  The output will be expressed in
	 magnitudes with an arbitrary zero point.  Be sure to store
	 the magnitude values.  [Note for non-astronomers: "magnitudes"
         are related to fluxes measured in power per unit area by
         m = -2.5*alog10(f) + const. ]

       Make a histogram of the cluster luminosity function you obtained.
         Make a hardcopy of this plot.

       Note:  the standard procedure with DAOPHOT is not to use a
         "first" measuring aperture large enough to include all the light
         from a given stellar point spread function.  Instead, a composite
         PSF derived from isolated stellar images is used to extrapolate the
         light measured in a fixed aperture (say 3 pixels radius) to the
         total light.  
       

6) USING ATV
       
       Skim the documentation on the ATV widgit display program (see
       the ATV home page).

       Read the M13 image into ATV.  Experiment with the various display and
         image sampling features of ATV.

       Change the "MouseMode" to "ImExam" and measure the FWHM of
	 selected stars of various brightnesses and locations on the
	 image.  Is there a significant variation in the FWHM's?
	 Quote the approximate mean value of the FWHM for the image.  

      



END OF IDL EXERCISES Part III 

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