Section 2 - Introduction to CCD Cameras
using passive radiators and a small fan, making the design and operation of the heads simple and not inconvenienced by requirements for liquid recirculation cooling. The ST-9E includes SBIG's secondary TE/Liquid cooling booster.
Since the CCD is cooled below 0°C, some provision must be made to prevent frost from forming on the CCD. The ST-7E, ST-8E and ST-9E cameras have the CCD/TE Cooler mounted in a windowed hermetic chamber sealed with an O-Ring. The hermetic chamber does not need to be evacuated, another "ease of use" feature we employ in the design of our cameras. Using a rechargeable desiccant in the chamber keeps the humidity low, forcing the dew point below the cold stage temperature.
Other elements in the self contained ST-7E, ST-8E and ST-9E include the preamplifier and an electromechanical shutter. The shutter makes taking dark frames a simple matter of pushing a button on the computer and provides streak-free readout. Timing of exposures in ST-7E, ST-8E and ST-9E cameras is controlled by this shutter.
The Clock Drivers and Analog to Digital Converter interface to the CCD. The Clock Drivers convert the logic-level signals from the microcontroller to the voltage levels and sequences required by the CCD. Clocking the CCD transfers charge in the array and is used to clear the array or read it out. The Analog to Digital Converter (A/D) digitizes the data in the CCD for storage in the Host Computer.
The microcontroller is used to regulate the CCD's temperature by varying the drive to the TE cooler. The external Power Supply provides +5V and ±12V to the cameras. Finally, the cameras contain a TTL level telescope interface port to control the telescope and the optional CFW-6A motorized color filter wheel.
Although not part of the CCD Camera itself, the Host Computer and Software are an integral part of the system. SBIG provides software for the ST-7E, ST-8E and ST-9E cameras for the IBM PC and Compatible computers (all cameras, DOS and Windows) and the ST-7E/8E are also supported by the Macintosh. The software allows image acquisition, image processing, and auto guiding with ease of use and professional quality. Many man-years and much customer feedback have gone into the SBIG software and it is unmatched in its capabilities.
2.4.CCD Special Requirements
This section describes the unique features of CCD cameras and the special requirements that CCD systems impose.
2.4.1. Cooling
Random readout noise and noise due to dark current combine to place a lower limit on the ability of the CCD to detect faint light sources. SBIG has optimized the ST-7E, ST-8E and ST-9E to achieve readout noises below 20 electrons rms for two reads (light - dark). This will not limit most users. The noise due to the dark current is equal to the square root of the number of electrons accumulated during the integration time. For these cameras, the dark current is not significant until it accumulates to more than 280 electrons. Dark current is thermally generated in the device itself, and can be reduced by cooling. All CCDs have dark current, which can cause each pixel to fill with electrons in only a few seconds at room temperature even in the absence of light. By cooling the CCD, the dark current and corresponding noise is reduced, and longer exposures are possible. In fact, for roughly every 5°C of additional cooling, the dark current in the CCD is reduced to half. The ST-7E and ST-8E