How do Digital Detectors Work? Meauring the apparent brightnesses of stars.

Apparent brightness means the amount of energy in that a detector receives on the earth from a distant star.

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.

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.

We will go through the detectors one by one and focus our attention on measuring star 6 for each detector case.

Each one of the 5 cases below represents a different combination of observing conditions. The goal here is to see what kind of observing conditions adversely effect our ability to detect faint stars. Note that the first case, which is the best and simulates data taken with the Hubble Space Telescope, shows 8 distinct stars. None of the other cases will show all 8 stars and you should try to think about what combination of detector + observing conditions which is simulated precludes the detection of the fainter stars.

Observations with the Hubble have two primary advantages over those from the ground:

• Because your not looking through the aberrating effects of the atmosphere, the image size on the detector is smaller. Most of the light from a star is in just one pixel and not spread out over many pixels.

• The background is darker.

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)

When you observe a star, whether with your naked eye or a modern digital detector, there are really only two attributes that you can measure for that star:

Its apparent brightness. This is basically the amount of energy that the star deposits on a detector, whether its your eyeball or some pixel in a CCD detector. The reason you shouldn't look directly at the sun is not because the sun is "too bright" but rather the amount of energy received by your eye is very harmful to it.

Its apparent color. Even with your naked eye you can see that some stars are blue while others are red. Some of this color diversity is seen in the image below:

We will later learn that blue stars are hot (and usually young), with relatively short lifetimes and red stars are cool (usually old) and have long lifetimes. For now, however, all we care about is that stars do come in different colors.

Apparent brightness:

The apparent brightness of a star imaged with some detector (i.e. the amount of energy which it registers on the detector) is due to 3 things:

• The intrinsic energy output of the star (we will call this the Luminosity)

• The distance of the star from the detector (e.g. the distance between the earth and the star)

• The quality of the detector and/or the observing conditions.

While the first two items are the most important, it nevertheless is important to remember that different detectors can yield different information.