Section 2 - Introduction to CCD Cameras
Page 29
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-7XE, ST-8XE, ST-9XE, ST-10XE,
ST-10XME and ST-2000XM cameras for the IBM PC and Compatible computers running
Windows 95/98/2000/Me/NT/XP. 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-7XE, ST-8XE, ST-
9XE, ST-10XE, ST-10XME and ST-2000XM 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 to 6° C of additional cooling, the dark current in the CCD is reduced to half.
The ST-7XE, ST-8XE, ST-9XE, ST-10XE, ST-10XME and ST-2000XM have a single stage TE
cooler, efficient heat exchanger and water circulation capability. A temperature sensing
thermistor on the CCD mount monitors the temperature (Earlier parallel models offered a
cooling booster which used a second TE cooler but we feel that the new design provides similar
performance without the need for a second power supply). The micro controller controls the
temperature at a user-determined value for long periods. As a result, exposures hours long are
possible, and saturation of the CCD by the sky background typically limits the exposure time.
At 0 °C the dark current in the ST-7XE, ST-8XE and ST-10XE, high-resolution mode, is only 60
electrons per minute! The ST-9XE, with bigger pixels, has roughly 8 times this amount of dark
current due largely to the larger pixel area but also due to the inherent higher bulk dark current
in the devices.
The sky background conditions also increase the noise in images, and in fact, as far as
the CCD is concerned, there is no difference between the noise caused by dark current and that
from sky background. If your sky conditions are causing photoelectrons to be generated at the
rate of 100 e
-
/pixel/sec, for example, increasing the cooling beyond the point where the dark
current is roughly half that amount will not improve the quality of the image. This very reason
is why deep sky filters are so popular with astrophotography. They reduce the sky background
level, increasing the contrast of dim objects. They will improve CCD images from very light
polluted sights.
To Next Page
Loading...
