Properties of CCDs
The most common detector used on telescopes today is called the CCD (Charge Coupled Device). It is basically a computer chip that functions as an electronic camera and those used in astronomy are very similar to those used in digital cameras. It consists of a 2-D array of highly efficient light detectors that count incoming photons at each location (pixel) on the chip and output a 2-D array of numbers that can easily be stored on a computer. Thus, astronomers are fond of saying “pictures are numbers”. CCDs have been getting larger as technology advances. Many amateur CCDs today are 2048 x 2048 having over 5 million pixels.
The simulation to the right allows one to view the output of a star on a CCD and vary the CCD properties. All CCDs “count” incident light by trapping electric charge that is knocked loose from a semi-conductor material by the light. CCDs only count up to a certain maximum level. If the number of counts for a certain pixel exceed this value it is said to be saturated and the excess charge would flow over into neighboring pixels and create long streaks in the image known as blooming. This maximum value is determined by the CCD’s bit depth b. This is the number of computer bits (0 or 1) that can be used to store the value of each pixel. A pixel’s value ranges from 0 to a maximum of (2b - 1). Use the adjacent simulator to determine the maximum pixel value for a CCD with a bit depth of 12. If a CCD pixel saturates at a value of 16383, what is the CCD's bit depth?
You can see in the Observations Plot that CCD pixels are linear up to the maximum value (unlike photographic plates). If you double the exposure time you should get twice the number of counts for each pixel within random variations (noise). Configure the simulator for a bit depth of 8 and an exposure time of 10 s. Use the cursor to explore the star image and determine the peak value at the central pixel. What is the peak value if the exposure time is increased to 20 seconds? At what exposure time does saturation begin to occur? Verify your answers using the graph in the Observations Plot.
Random fluctuations in the arrival of photons keep CCDs from being exactly linear. The effect can be included in the simulator using the include noise option. Use the cursor to sample the sky background around the star with the include noise option both on and off. Can you detect any differences in the two images?
The light from a star is typically also spread out over many pixels due to diffraction and atmospheric turbulence. Astronomers usually add up all of the pixel values within some radius of the central peak when calculating the brightness of a star in what is known as aperture photometry. Configure a CCD with a bit depth of 10 and take a 20 second exposure. Use the cursor to probe the star image and keep a running total of all of the pixel values making up the star image. What is the total number of counts due to the star? Use the total counts due to star feature of the Observations Plot to verify your estimate.