Moravian instruments, Inc., source: https://www.gxccd.com/art?id=465&lang=409, printed: 18.04.2024 11:37:03

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18.1.2018
Enhanced cooling models of G3 and G4 cameras
 Dark current, injecting electrons into CCD pixels regardless if the sensor is illuminated or not, is the most important source of the unwanted signal. As the dark current is caused by random processes, it exponentially depends of the sensor temperature — typically the dark current is halved with temperature drop by every 5 or 7 degrees Celsius, depending on the particular sensor. While dark current can be to some extend removed from images during calibration (subtraction of the so-called “dark frame”), the random portion of the signal always remains in the image. This is why the CCD sensors in cameras intended for very low light imaging (and thus allowing for many minutes long exposures) are always cooled. Despite the inherent dark current of the OnSemi detectors, used in the Gx series of CCD cameras, is relatively small, sometimes the camera is used in hot environments and more effective cooling is required to lower the dark current.

It is more difficult to cool down rather big sensors in the G3 cameras compared to smaller detectors used in the G2 series. Big sensors require greater CCD cold chamber with larger area front optical window, which naturally leads to higher thermal leaks and the necessity to use much higher cooling power to achieve similar temperature drop. To cool down sensors in the G4 cameras is even more difficult, as the G4 cold chamber is even bigger and the optical front window has even greater area.

Still, the G3 cameras can reach up to 48°C below the ambient temperature and the G4 camera are able to cool down the sensor up to 45°C below the ambient.

SIPS Imager tool showing actual temperature drop of the standard G3 (left) and G4 (right) cameras at room temperature 25°C

However, these maximal values provide very little room for regulation and it is not recommended to use such high delta T for regular observations, as the camera may not be able to maintain the absolute CCD temperature if the environment temperature rises. Also please keep on mind that the maximum temperature drop may vary among individual cameras, as the Peltier coolers are also manufactured with some variations in their parameters.

As the thermal leaks cannot be easily lowered (the CCD cold chamber dimensions, the front optical window area and also the sensor size are fixed parameters), the only way how to achieve lower absolute temperatures is to enhance cooling power. However, one of the design goals of the EC cameras was keeping the total power consumption within the 60W range of the standard power adapter. Higher power consumption would require not only using of different power supply, but also a different (less common) power plug connector capable to carry higher currents. Also power cable is typically shorter, which bring another problems with power adapter mounting close to the camera etc.

New variants of both G3 and G4 cameras with the “EC” suffix are similar to the standard versions, but the camera head back shell is thicker to accommodate larger heat sink.

Front and back outlines of EC cameras are identical to standard models

Side view reveals bigger heat sink of the EC variant

The thicker camera head back shell is not the only difference, EC variant cameras also use more powerful fans. This is why EC cameras are also a bit louder than the standard models, but this is the price for increased amount of heat removed from the Peltier coolers hot side.

Camera plugs on the bottom side are identical (left), top side reveals bigger heat sink of EC cameras (right)

SIPS Imager tool showing actual temperature drop of the Enhanced Cooling variants of the G3 (left) and G4 (right) cameras at room temperature 25°C

Otherwise EC variants are identical to standard cameras, they have the same BFD and use the same telescope adapters, software and drivers etc. The absolute sensor temperature of the EC variant can achieve up to 5°C lower temperature compared to standard models. This may not look like big difference, but as previously explained, the dark current depends exponentially on the temperature, so 5°C temperature drop can result into close to halved dark current, depending on the particular sensor.

  G3 standard G3 Enhanced Cooling G4 standard G4 Enhanced Cooling
Maximum cooling 48°C 53°C 45°C 50°C
Typical cooling 45°C 50°C 42°C 47°C

CCD sensor temperature drop below ambient temperature for standard and EC variants of the G3 and G4 cameras