The CCD camera

The CB245 camera mounted on my telescope.

1. Camera body
2. CCD chip
3. Electronics

1. Camera body
   The camera body is the mechanical part of the camera. It houses the CCD chip and protects it against unwanted light from outside. It can also be mounted on the telescope.
   
   

   The black tube is the camera body.




2. CCD chip
   

   A CCD (Charge Coupled Device) chip is a light sensitive device, made of silicon. It is an array of light sensitive pixels. When light falls on a pixel, it will be converted to a charge. This charge is captured in the pixel. It can’t go to other pixels. The more light falls on the pixel, the more charge will accumulate in the pixel. The amount of charge is a measure of the amount of light that felt on the pixel.

   The ccd chip seen in the camera.

   
   
   

   The left part shows an image of a TC245 ccd chip. The right part is a representation of a small part of the light sensitive surface of the ccd chip. You can see it's made out of pixels.

   
   
   How registers a CCD chip an image?
   
   

   The telescope projects an image on the CCD chip. High intensity areas will produce more charge in the pixels than low intensity areas. The amount of charge in every pixel makes the image.The computer can read this charge and convert it to an image.
   	The value in every pixel gives the charge, which is related to the light intensity.

   
   
   How is an image made with a CCD chip?
   
   There are three main phases:
   
   1. Clearing phase
      Mostly you don’t have a shutter in your camera, this means that there is always light falling on the CCD chip. When you want to make an image, all existing charge has to be removed from the pixels. The CCD chip can shift lines downward with electrical pulses. All charge in a line will move to the line below of it. This happens with all lines of the chip on the same moment. The lowest line will move the charge to a clearing line, where it will be removed. When all lines are shifted away, there is no charge left anymore. This phase takes a few ms.
      
      

      Charge clearing.

   
   
   2. Exposure or integration phase
      The telescope projects the image on the CCD chip, so charge will accumulate in the pixels. The longer you wait, the more light will be registered by the chip. This phase controls the exposure time, also called: integration time, because the chip integrates the amount of light in a certain period. With lunar and planetary photography it usually takes a few tenths of a second, but it can take minutes or even hours.
      
      

      The CCD array after 1 and 2 seconds exposure. The charge in the pixels accumulated with a factor of two.

   
   
   3. Readout phase
      To get the image in your computer, it will have to read the amount of charge of every pixel. There is a device, the charge detection node, in the CCD chip that is able to measure the charge of a pixel. Every pixel must be shifted in this device. The lowest line is able to shift pixels to the left in the direction of the device. So these pixels can be measured one by one. When all pixels of the line are measured, all lines are shifted one line down. Then again the lowest line can be measured. This will be repeated until all lines are measured. The computer can read the measurements through the camera electronics. This phase can take a few seconds.
      
      When the computer has collected all measurements, it can show an image of it.
      
      
   
   
   The most used types of CCD chips
   
   
   1. Full frame device
      This is the most simple device as described above. It is used in the CB211 camera. This type can’t be used with lunar and planetary photography without a shutter. Making images of this kind of objects typically requires an integration time of a few tenths of a second. The problem is the readout time, which can take a few seconds. During readout, the chip is still receiving light, which will destroy the original image, before it’s completely read by the computer.
      
   
   
   2. Frame transfer device
      The CCD chip in the CB245 camera has a TC245 CCD chip from Texas Instruments, this is a frame transfer device. With this kind of chip, the lowest half of the chip is protected against light. The image is only made on the upper half of the chip. When you are finished making your image, the image will be transferred from the upper half to the lower half, where it will be protected from light. This will take only a few ms. There is enough time, in this area, to read the image. A frame transfer device can be used with lunar and planetary photography.
      
      

      The TC245 is divided in 2 parts.

   
   
   3. Interline transfer device
      One line of every two lines is protected from light. During integration, only one line receives light. At the end, this line is shifted to the protected line, where it will be read by the computer. An interline transfer device can be used with planetary photography, Lunar photography will be more difficult, because of it’s brightness. The bright light can influence the protected line. The protected area of a frame transfer device is safer.
      



3. Electronics
   The electronics of the camera are the intermediate between the CCD chip and the computer. The computer must be able to control the camera through its electronics. To shift the lines on the CCD chip, it needs clock signals or pulses. They must be generated by the electronics or, as is the case with the CB245 camera, by the computer. In the latter case they need to be converted by the electronics to values accepted by the CCD chip. Another important task is the conversion of the analog output signal of the charge detection node to a digital format the computer is able to read. The conversion is done by an ADC (Analog to Digital Converter) and takes most time of readout phase.
   

   The CB245 camera needs two electronical units.

   1. The preamplifier
      This unit is mounted on the camera body. It amplifies the analog signal from the CCD chip and converts the clock signals.
      
   2. The interface card
      This unit connects the computer to the preamplifier. It converts the computer signals and also the analog signal from the CCD chip to a digital format.
      
   
   

   The camera and preamplifier are on the left, and the interface card is on the right.

   
   

   The electronics from inside.