Procedure:

1. Open up the simulation (it should appear in a new browser window).

2. There are two panels: Left side contains a star (a bright one) and the right side contains a graphing window. The graph plots distance (X) vs Flux received at that distance (Y). You can left click anywhere on the graph to get a cursor read out of the X and Y position.

3. In the graphing window Click on Point Mode A red dot should appear in the center of the star.

4. Click anywhere in the star window to move that red dot to the position you clicked. Note that a green dot appeared on the graph. Click at a variety of distances in the star window to see the inverse square law fill out.

Your graph that you made should look like this:

Now, in the case you just did there was a bight star and the virual detector (the red dot) that you used to measure the star had no random errors in it. For this next part, we will use a dectector with a larger random error in it on the exact same star. So go ahead and close the current detector window and then click here to open up a new window. Repeat the procedure as defined above.

The limiting distance at which an observer can detect a star is when the signal from that star is higher than the noise of that detector. In our case, once the data starts to scatter around zero then we have no longer detected the star.

You should find that the observed flux starts to scatter around zero at a distance of 160--180 units. Before (see above) with the perfect detector, you could still detect the star out to the very edge of the field (270 units). Now you can no longer detect the star at such great distances because of the intrinsic noise in the detector .

As said, detection depends upon signal to noise - in this case we have increased the noise but kept the signal the same. If we now observe a fainter star with this same detector, our limiting distance will be even less, because the signal is now down as well.

Close the curernt simulation and open up this last one - following the same measuring procedure as before with the red dot detector. You should find, in this case, a limiting distance of about 40 units.