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EXPERIMENTAL CCD CAMERA Camera description A simple colour CCD camera equipped with a motorised shutter for amateur astronomy use.
Choice and sourcing of components The Sony ICX224AK Super HAD CCD ® sensor used in this camera was obtained from an old digital camera. This 1/2" sensor has a square pixel array with a total of more than 2 million effective pixels. The YeCyMgG complementary colour Bayer filter used in this CCD ensures high sensitivity by allowing more light to pass than a RGGB primary colour filter.
From the outset the decision was made to interface to the camera through an USB port since parallel and serial ports on PC's and laptops are becoming a rarity. An inexpensive solution is Microchip's PIC18F4550 microcontroller. This device also comes with an onboard analogue to digital converter. Minimal programming effort is required on the PC side if the USB CDC (Communications Device Class) specification route is followed, which permits a maximum data transfer speed of about 1Mbit/s. In this application data transfer speed is limited by the relatively slow PIC ADC throughput. The rest of the components were sourced from a local electronic components supplier and the junk box. Knowledge and experience gained from this project will lead to a more ambitious design sometime in the future. Significant advances can be made by using a faster analogue to digital converter and moving to full speed USB communication.
Acquisition software The camera data acquisition programme is in effect a standard serial communications application with the added ability to display received data on the screen as an image. As mentioned before, little programming is required if the USB CDC specification is adhered to. Full speed USB communication requires substantially more development and is earmarked for a future project. The picture below shows an acquired screen with a mid level DC voltage applied to the input of the analogue to digital converter. 
Power supply unit A +12V switching power supply unit generates the +15V, -15V and -7V supplies required for the camera. The +12V and +5V voltage regulators are mounted on a piece of aluminium sheet away from the CCD to minimise heat transfer to the sensor.
Vertical driver circuit The CCD sensor requires two 3-level (+15V, 0V and -7V) and two 2-level (0V and -7V) driver circuits to deliver the transfer clock signals to its vertical register. The design for a 3-level driver is shown below. For a 2-level driver only Q3, R4, R5 and R6 are required.
Analogue to digital conversion process The dynamic range of the camera is increased by setting the analogue to digital converter reference voltage through a digital to analogue converter under software control. The PIC18F4550 ADC has a resolution of 10 bits which amounts to 1024 shades of grey. The performance of the 10-bit ADC is further enhanced by oversampling the input and manipulating the result with software. Oversampling also improves the signal to noise ratio. The price paid for this enhancement is an increase in the time it takes to do a conversion. Below is the result of oversampling a 250mV input signal to the ADC with the reference voltage set to 10% (500mV). At such low voltage levels noise is very evident in the input signal. The reduction in noise through oversampling can clearly be seen. The pixel size in this image was magnified by a factor of two.
Notes PIC18F4550 code written in C and compiled with Microchip C18 Toolsuite.
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