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Section 2 - Introduction to CCD Cameras
Page 9
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-7E and ST-
8E have a single stage TE cooler and a temperature sensing thermistor on the CCD mount to
monitor the temperature. The ST-1001E has two-stage cooling. The ST-9E and ST-10E have a
supplemental second stage cooling booster with water cooling as an option (described in
section 6.1). The same cooling booster used on the ST-9E and ST-10E may be added to the ST-
7E, ST-8E or ST-1001E. The microcontroller 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-7E, ST-8E and ST-10E, high-resolution mode, is only 60 electrons per minute! The ST-1001E
and ST-9E, with bigger pixels, have roughly 8 to 15 times this amount of dark current,
respectively, due largely to the larger pixel area but also due to the inherent higher bulk dark
current in the devices. That's why we include the cooling booster with the ST-9E and two-stage
cooling plus the option of an additional cooling booster for the ST-1001E.
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.
2.4.2. Double Correlated Sampling Readout
During readout, the charge stored in a pixel is stored temporarily on a capacitor. This capacitor
converts the optically generated charge to a voltage level for the output amplifier to sense.
When the readout process for the previous pixel is completed, the capacitor is drained and the
next charge shifted, read, and so on. However, each time the capacitor is drained, some
residual charge remains.
This residual charge is actually the dominant noise source in CCD readout electronics.
This residual charge may be measured before the next charge is shifted in, and the actual
difference calculated. This is called double correlated sampling. It produces more accurate
data at the expense of slightly longer read out times (two measurements are made instead of
one). The ST-7E, ST-8E, ST-9E, ST-10E and ST-1001E utilize double correlated sampling to
produce the lowest possible readout noise. At 11e
-
to 16e
-
rms per read these cameras are
unsurpassed in performance.
2.4.3. Dark Frames
No matter how much care is taken to reduce all sources of unwanted noise, some will remain.
Fortunately, however, due to the nature of electronic imaging and the use of computers for
storing and manipulating data, this remaining noise can be drastically reduced by the
subtraction of a dark frame from the raw light image. A dark frame is simply an image taken