Image Processing - Analyzing Images
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# Analyzing a Single Picture

Before we make color images, we will start by looking at one image to learn a little more about Iris.

Click on File and Load. Select the green image and open it. You will probably see a black image.

You should see a small box at the bottom of the Iris screen - this is the Threshold box. If the threshold box is not present, activate it by clicking on this button (). Click on the Auto button in the threshold box.

Each pixel on the screen has a brightness associated with it. The top bar in the Threshold box represents the value of the brightest pixel that will be displayed, also known as the maximum brightness. All pixels brighter than the maximum brightness will have the same color. The lower value represents the value of the faintest pixel that will be displayed, also known as the minimum brightness. All pixels fainter than the minimum brightness will have the same color.

You should now see an image of the sky. This is the area around NGC 1087. Use the scrollbars to scroll through the image until you see NGC 1087 itself - it is the largest galaxy in the image. When you have NGC 1087 in your viewing area, you should see a screen like this:

 Exercise 1. Increase and decrease the maximum brightness. Notice the effect it has on the image. Increase and decrease the minimum brightness. Observe the effect on the image.

 Question 1. How would you adjust the maximum and minimum brightness to clearly see the galaxy's faint spiral arms?

 Question 2. How would you adjust the maximum and minimum brightness to clearly see the galaxy's bright center?

Note: As you change the max and min brightness, you are not changing the information contained in the picture - you are merely changing the way it is displayed.

Tip: You get a nice dark background if you set the minimum value to the background value of the image. To find the background value, use the mouse to draw a rectangle around an area where there are no visible stars or galaxies. Right-click on the area and select statistics. Set the minimum value for the threshold to the mean or median (if you choose an area with no objects, they should be almost exactly the same. If they are substantially different, you probably have some stars or a galaxy in the box!)