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Part 1 Section Contents
Part 1: First Look Qualitative Analysis using ds9

Learning to Use the ds9 Imaging System

Part 1: First Look Qualitative Analysis using ds9

A quick list of the basic techniques of X-ray analysis:


Counting Photons

Recall that an incoming X-ray photon is focused by the X-ray telescope and then hits the X-ray detector that is currently in the focal plane of the telescope. This can be the HRC or ACIS (with some gratings options thrown in for good measure). The X-ray detector then records information about that X-ray photon for eventual transmission back to earth. If the detector is the HRC, an accurate position and arrival time are recorded. If it is the ACIS, an accurate position and energy are recorded, along with some timing information. To record an accurate position, both detectors act like a super-fine mesh or grid, recording the grid element (or "pixel") that the X-ray strikes. As more and more X-rays strike the detector grid, each pixel in the grid can be hit by zero, one, or more X-rays. When we display this initial image of Cas-A, what we are displaying is the number of X-rays that were recorded to hit each pixel during a given observation. Try this:
  • In the ds9 window there is a text table at the upper left. One entry is "file" and to the right of this is a box with the file name, in this case, "casa" (Cas-A). Directly below that is another entry called "value" which has a single box to its right. Still below are two entries called "physical" and "image". They each have two boxes to the right, labelled "x" and "y". Watch those boxes while you move your mouse around the Cas A image.
  • You will see both rows of "x" and "y" values change as you move your mouse. The differences between these two sets of values do not matter for this exercise, and so for clarity, all subsequent x,y values will refer to the numbers appearing in the "physical" row. That pair of numbers together are the x,y coordinates that identify each individual pixel of the detector. As you move the mouse, the "physical" table is telling you which pixel of the detector you are located in.
    Cas A
  • The "value" shows the number of X-ray photons that were recorded at that pixel position.
  • Note that you can use the arrow keys on your computer to move the mouse one pixel at a time in any direction, so that you can see the values change slowly over small regions of the image. There is a smaller window in the upper right side that shows a magnified view of a portion of the image around your pointer location. You can use this window to "zone in" on a specific area of the image.
  • Pick a particular pixel by its x,y coordinates (image value), for instance, the pixel for which the physical x,y value is 4158,4398. Move your cursor around until you figure out which way the grid coordinates are oriented. Hint: This pixel is located in a nice bright spot in the upper left part of Cas-A. Note that the "value" at that pixel is 92. Ninety-two is the numbers of X-ray photons that were recorded to strike that pixel.


Next: Looking at an X-ray image: Image Manipulation & Pixel "Value"



Last updated: 05/11/05



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