Detecting stars with Digital Detectors

You will use 4 CCD simulations below to "observe" a star. Each simulation mimics 4 kinds of observing conditions. These conditions can represent bright, light polluted skies, lots of atmospheric turbulence, Hubble-like perfect conditions etc. You will first determine if your detector for each of the 4 cases can find the star based on certain requirements listed below, and then you will try to determine the conditions of the sky.

Case 1

Case 2

Case 3

Case 4

Procedure:

1. Open the first simulation

2. Click Measure

3. Slide the Exposure time until you see a star

4. Create a box of dimension 10x10 pixels. This is an aperture to measure the photons from the star and/or sky. To make a box place the cursor on the gray background (this is the sky) and use the left mouse button to draw a square. Notice that two squares, a red and green one, come up. Change the size of the square to 10x10 by typing that into the sample size text field.

5. Place the green box over the star. Place the red box on any other part of the sky (not including the star). Look at the numbers to the right in the red and green cells.
   • The green box is measuring the total number of photons from star + sky
   • The red box is measuring the total number of photons in the sky
   • The total number of photons from star = counts from green box - counts from red box
   • Mean is the average pixel value in each box
   • Standard deviation is like the random error
   • Total counts are the total number of photons in each box.

   Determine the minimum exposure time required to believe that you can even "see" the star, using just your eye, with your virtual detector.

   Determine the minimum exposure time so that the difference between the numbers in the green box labelled mean and red box labelled mean is equal or greater to the sky standard deviation (error) in the red box Remember, the star is centered in the green box while the detector background (e.g. the brightness of the virtual night sky in each case of the simulation) is registered in the red box.

   What happens to the difference between the green and red mean values when you increase the exposure time for each detector? Is there a significant change?

From this exercise it should be apparent to you that for any given exposure of the sky with any given telescope plus detector there will be many stars that are simply too faint to register on the detector and different detectors will require different amounts of exposure time to produce similar quality data.

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