Alternate Views of Images
Isophotes
Another interesting way to view an image is through the use of
isophotes. Isophotes are lines drawn through areas of constant
brightness. Isophote images are very similar to contour maps, which show
lines through areas of constant elevation.
Click on View and then Isophotes. The following window will
appear:
X-Dim and Y-Dim are the size of the picture. High and low are the
highest intensity and lowest intensity for which you want isophotes drawn.
Step is how far in between isophotes. For the values shown above, Iris
will draw isophotes where the intensity is 100, 200, 300, 400, 500, etc,
up to 32,700. It is important to choose these numbers wisely.
Exercise 10.
Open the green image of NGC 1087, the galaxy you used earlier. Click
on View and then Isophotes. For the high value, you want to
enter the intensity of the brightest pixel. To find this intensity, draw
a box around
the brightest area of the diagram. Right-click on the box and
select Statistics. Enter the number for the maximum value in the
high box (you may round to the nearest 100). Enter the image's minimum brightness
value in the Low box. Select a step size that will give you 15 to
20 isophotes. Click OK. |
Question 6.
Where are the isophotes widely spaced? Where are the closely
spaced? What does the distance between isophotes tell you? |
3-D Views
Another way to view an image is with a 3-D view - a 3-D picture
where the height represents the brightness of each pixel. Below you
can see a 3-D view of two stars.
The dialog box for 3D view is shown below.
X Dim and Y Dim are the size of the picture and will usually not need
to be changed. High and Low are the counts of the brightest and dimmest
pixel you want to observe. Azimuth and elevation determine the
viewing angle of the plot. Step is the size of the bins, in pixels,
on the x and y axis used to make the 3-D graph. A smaller step size
will give you a smoother, more detailed graph, but the graph will
take longer to make.
Exercise 11.
Once again, open the green image of NGC 1087.
Click on View and 3D. You may adjust the High and Low to
correspond to your max and min values. Click OK. You
will see the 3D view. You may experiment with the azimuth and
elevation. These values change the angle at which you view the
display. The defaults will give you a good view, but you may find
different angles better for a particular image. |
Question 7.
What parts of the image correspond to the highest peaks?
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