How do Digital Detectors Work?
In 1905 Einstein showed that incident light (photons - we don't know what those are yet) upon certain metals would produce a current flowing through the material. Thus light energy can be converted to electrical energy.

This is called the Photoelectric Effect

Imagine the surface of our digital detector is like the graph paper shown above. Each square can contain a certain amount of information - that is, receive a certain amount of incident light.

The more light that is received, the more electrical charge that is built up on that square. That charge is converted to a number so you computer might render the graph paper like what is shown below:

Once we have numbers to represent the light that hit the detector we can reconstruct a picture on the computer screen from the brightness values, as we see on the left. On this scale, 0 is completely black while 255 is completely white.

Note that because the light sensitive elements are so small, we usually don't see the individual picture elements ("pixels"). Reduced in size, we see what this star looks like at the usual level of detail. Individual pixels can't be seen by the eye at this scale, but they're there.

And Voila, we have Digital Imaging:




So with that as an introduction, today we want to understand how one actually can make quantitative measurments from these kinds of detectors and learn about the important role that different observing conditions play in determining the kinds of stars that can be detected.


To do this we will use the CCD simulator JAVA applet. A snapshot of this is shown below

The features of this applet are:

  1. An exposure time slider. In this example the exposure time was 80 seconds. The green box is placed around the star and the red box is placed on the image background (anywhere).

  2. Clicking the measure button will allow a "measurement" to be made. Each time you adjust the exposure time you then need to click on measure to re-run the simulator.

  3. A green and a red box. The size of the boxes are specified by typing a number in the Sample Size window. In this case, the box size is 20x20 pixels (which is what we will be using). Note to make an intial box, place the cursor on the gray background and use the left mouse button to draw a square. Then change the size of the square to 20x20.

  4. The red and green readout boxes show the total number of "counts" (think of these as photons) in the respective red and green boxes. In this example the green box is placed over a star, and the red box is placed over the "background".

    The mean counts in the green box are 2260, those counts include the background counts since the star sits on the background level of the detector which is determined by the brightness of the night sky at the time of the observation.

    The mean background count is 1999.

    So the actual brightness of the star is then 2260 - 1999 = 261 counts. You have to subtract the background to get the true brightness of the star on the detector

  5. To move the green box to another star, grab it with the mouse button by putting the pointer INSIDE the box.

There are five cases below to do. Fill out your worksheet accordingly and hand them in to Rachel when your done. The "reference" stars referred to on the work sheet are Star A - in the upper right hand corner of the detector and Star B - the brightest star in the field near the right center of the detector. (These are also shown in yellow in the image above). On the applet simulation they will be labelled star 3 and star 1.

Record on your worksheet the values reported under the label "Mean" in the applet. The values in the green readout box refer to the values in the green box (which should be around a star), the values in the red readout box are for the red box, which should be placed on the background (i.e. no star should be in that box)

After you have completed this, we will have a discussion.

Use an exposure time of 20 seconds for each case, except case 1 where you use an exposure time of 80 seconds

In all five cases, the detector is "imaging" the same field of stars so in each case there are 8 stars on the detector. But not all detectors will detect 8 stars or even Star A. All detectors will record star B. You should think about what aspects of the various virtual observations are precluding detection of the fainter stars as you can also learn, increasing the exposure time still won't cause star A to be detected in most cases.

Case 1 (80 seconds exposure)

Case 2 (20 seconds exposure)

Case 3 (20 seconds exposure)

Case 4 (20 seconds exposure)

Case 5 (20 seconds exposure)