Two windows should be open. This one in which the instructions are given and a smaller one which contains the actual applet that we will be using.

Tutorial:

The purpose of this applet is to get the student to understand the relation between the distance from a star and the received flux. The detectors (both a point and an area detector) can be set up to deal with noise so that this applet can be used to show what detection limits are, what signal to noise is, etc.

In the point application, the user moves the red point around at different distances from the star to produce a plot of flux (in units of counts per square pixel) versus distance (in units of pixels). In the area application, the user draws a box around the star. Each right click of the mouse button makes a new measurement of the total flux of the star inside that box. This is reported as a pseudo magnitude on the Y-axis of the plot. The distance button can be used to represent the size of the star on the detector if we move farther away. Of course, stars are in reality point sources, but when we image them from the earth the light is spread out over many pixels on the a CCD detector due to atmospheric smearing as well as intstrumental focus (e.g. even on the Hubble Space Telescope the light from stars doesn't fall in just one pixel).

It is easy to setup a situation via the parameter tags listed below that will show how detector noise swallows up a stellar image at some limiting distance. The goal here is to get the students to understand that detector noise, not the "faintness" of the object, is what limits our ability to detect stellar objects in the universe.

Suggested Activities:

• Measuring two stars of the same brightness but with different values of detector noise. Use the point function for this and just move the red dot around and click. In the upper graph there is very little noise. In the lower of the two stars, the detector has more noise. The idea is to take multiple data at the same radius (position the red dot and then click multiple times). Since you can interrogate the graph with the cursor and left mouse click, you should be able to measure the "range of values" reported at a given distance. That range, of course, will be smaller for either detectors with lower noise or at positions closer to the star (e.g. Signal-to-noise).

  Two stars of same brightness

• Set up a situation where stars of different luminosity are measured using the same detector. Determine the faintest star which can be detected by that detector or equivalently the limiting distance of detection. Again, use the point mode for this.

  Three stars of different brightness but same detector

• Now we use the area mode. This will make a similar set of measurements on the graph side as the point mode only the y-axis will be in magnitudes. In this application, a box is placed around the star. Distances from the star can then be selected by the distance in the lower left of the applet. The star starts off at unit distance but distances of 0.25, 0.5, 2.0,4.0, and 8.0 are available. The image size of the star on the detector will decrease as you go to larger distances (fainter objects to have smaller image sizes on detectors) and, with suitable noise, the desired effect of seeing the star disappear into detector noise can be achieved. In the example below, by unit distance = 4, the star will no longer be visible in the box. After you set the box, right click the mouse inside the box to make a measurement. For distance = 1, (the default) the magnitude will be about 12.4. Make a few measurments at this distance to see the spread in the data. A measurement in this case consists of differencing the sum of the pixel values inside the box with a box of similar size randomly placed on the background. Try the same set of measurements at distance = 0.5 and distance =2.0. By distance =4 the star will be gone and the measurement will be meaningless.

  Star disappears into detector noise

  By way of these examples hopefully you can see how to change the parameter tags for different situations.

  Have the students work together in teams to measure a common set of stars but give each team a different parameter tag that controls noise. That is, give some teams good detectors and some teams bad detectors and ask them to measure the same stars and pool their data. Have a classoom discussion on why different teams get different answers.