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Surface Brightness When you define a region, the net counts you get is obviously going to change as you change the region size. Thus, if the source was of constant intensity, doubling the region size would double the counts. But what you really want to compare things with (usually), is the counts/unit area. Typically, we define the unit area as a square arc-second in the sky. Doubling the region size would double the number of counts, but it would also double the area, so the surface brightness, or counts/(square arc second) would stay the same, as it should. Try using ds9 on an arbitrary region in Cas-A. Do a "counts in region". You will see a net counts result, and an area result (which is the size of the region you selected in terms of square arc seconds). If you divide the counts by the area, you get the surface brightness result. The surf_bri program does this for you. Since different sources, or different regions of interest in a single source like Cas-A, are of different sizes, the net counts for each is of limited use, since it may only reflect the size of the object, and not its (surface) brightness. But by normalizing the counts by the area from which the counts originated we get a truer picture of the "brightness" of each area or source. We can see this in Cas-A. We could define a large region near the center of the remnant, which has more counts than near one of the knots. But the area near the knot would be "brighter". Set up a region centered in the area at about physical X=4246, Y=4298. Extend the region out to about double its default size (radius = 76 pixels). Run Counts in Region. You should get about 43900 counts, with an area of about 4350 sq. arc-sec. So the surface brightness is about 10 counts/sq. arc-sec. Now, near the bright knot, set up a region at physical X=4218,Y=4124, radius = 28 (about 3/4 of the default region radius). You should get about 25000 counts (about half the amount in the first region), but because the area is smaller, about 600 sq. arc-sec, the surface brightness is about 40 counts/sq. arc-sec. |