Specifications

Section 2 - Introduction to CCD Cameras
Page 29
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 Research Series cameras
to achieve readout noises below 20 electrons rms for two reads (light - dark). Typically the read
noise is 15 electrons or less. 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 6° C of additional cooling, the dark current in the CCD is reduced to half. Each
Research Series camera has a two-stage TE cooler, efficient heat exchanger and water
circulation capability. A temperature sensing thermistor on the CCD mount monitors the
temperature. 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.
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 Research Series cameras utilize double correlated sampling to produce the lowest