III. Ximtool and Displaying Images

A) How To Display Images

IRAF requires a program which it can interact with to display images. There are several available: ximtool, saoimage, ds9, and perhaps more that I'm not aware of. I use ds9 most of the time, though I actually prefer ximtool because of it's (in my opinion) better user interface. Ximtool, however, requires an 8-bit display environment. Most displays today are 24-bit, so this is a huge obstacle.

To start ds9, run the command from the UNIX environment (not the IRAF cl):

This will bring up a new ds9 window.

To display an image in ds9, we'll use the IRAF display task. If you take a look at the help page for display, you'll see that display requires two inputs, the image and frame. ds9 has several image frames, meaning you can load up to 16 (or possibly only 4, depending on your software version) images and use ds9 to blink between them. The format to display in frame 1 is:

To display an image in a frame other than 1, simply replace 1 with the frame number (up to 16).

Notice that display prints out two parameters, z1 and z2. We'll discuss these in the next section.

B) Understanding Your Image Display: Display Range

You might think that once you display your image, you are seeing all of the data, this is not the case. It's important to understand what the image display is doing and what its limitations are.

The display range is set by the z1 and z2 parameters in display. Think of the image as a matrix of numbers, each number represents the brightness in that pixel. The display range, set by z1 and z2, represent the minimum and maximum values displayed. If the transfer function (to be discussed later) assigns black to low values and white to high values, then z1 is the pixel value corresponding to black, and z2 corresponds to white. Any pixel with a value lower than z1 is black, and values higher than z2 are white, anything in between z1 and z2 is some shade of gray. Thus it is important to note what your z1 and z2 values are. For example, if there is detail in your picture above the z2 value, it will look washed out until you adjust the z1 and z2 values.

Using the default parameters, the display task will automatically set the z1 and z2 values based on it's own internal algorithm. The parameter contrast will change the automatically selected contrast (though I find the default value to be a good compromise).

The parameter zscale will turn the automatic scaling on and off. If zscale is turned off and the parameter zrange is turned on then z1 and z2 are set to the minimum and maximum values in the image (usually a terrible choice). I find it convenient to epar display and turn zrange off permanently since I almost never want z1 and z2 set to the min and max. Go ahead and run epar display and turn off zrange.

To set z1 and z2 manually, you must turn off zscale and set z1 and z2 values like thisL

Note that this will only work if I've turned off zrange using epar. The z1 and z2 values printed out by display should match the ones you input.

C) Understanding Your Image Display: The Transfer Function (Brightness and Contrast)

Once you've displayed an image and have set z1 and z2, you can still alter what is displayed by changing the brightness and contrast (the transfer function). This is the mapping between pixel value and screen brightness.

Changing the transfer function is done in ximtool, not IRAF. Click on the ximtool window and while your cursor is somewhere in the image area, click and hold the right mouse button and move the mouse around within the image display area. Essentially the left and right position of the mouse controls the contrast and the up and down position controls the brightness. This can really help bring out faint details, so when inspecting and image be sure to adjust the transfer function to one which shows the faint detail if that's what you're looking at.

NOTE: The combination of the settings of z1, z2, brightness, and contrast is sometimes called the image stretch. After you play around with it a bit, you will see that different stretches of the image will reveal different information in either the faint or bright regions of the image.