Polaroid CCD Camera - Manual (Page 12)

Section 2 - Introduction to CCD Cameras
Page 8
The CCD is cooled with a solid-state a thermoelectric (TE) cooler. The TE cooler pumps heat
out of the CCD and dissipates it into a heat sink which forms part of the optical head's
mechanical housing. In the ST-7E and ST-8E cameras this waste heat is dumped into the air
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 and ST-
10E include 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, ST-9E, ST-10E and ST-1001E 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, ST-9E, ST-10E and ST-1001E 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, ST-9E, ST-10E and ST-1001E 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, ST-9E, ST-10E and ST-
1001E 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, ST-
9E, ST-10E and ST-1001E 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