CMOS cameras of the C3 series
The C3 cameras use the latest generation of Sony IMX CMOS sensors, which offer exceptional quantum efficiency thanks to their back-illuminated design and very low dark current. Despite relatively small pixels, the full well capacity is over 50 ke-. Combined with full 16-bit digitisation, perfectly linear response to light and exceptionally low readout noise, these cameras are ideal for both aesthetic astrophotography and astronomical research. Sensor formats ranging from APS to full photographic format (24 × 36 mm) ensure a wide field of view and make optimal use of the capabilities of the optical systems most commonly used by amateur astronomers.
The mechanical design of the C3 series of astronomical CMOS cameras has been carried over from the earlier CCD-based G3 Mark II cameras, making the C3 camera series fully compatible with a wide range of telescope adaptors, off-axis-guider adaptors, filter wheels, Ethernet adaptors, guiding cameras, etc.
Extensive software and driver support allows the C3 camera to be used without the need to invest in a third-party software package, thanks to the free SIPS software package included. ASCOM (for Windows) and INDI drivers (for Linux) as well as Linux driver libraries are supplied with the camera, offering the possibility of integrating the C3 camera into a wide range of camera control programs.
The C3 cameras are designed to work with a personal computer (PC). Unlike digital still cameras, which operate independently of a computer, scientific cameras require a computer for operation control, image download, processing and storage, etc.
C3 cameras are designed to connect to the host PC via a very fast USB 3.0 connection. While C3 cameras remain compatible with the older (and slower) USB 2.0 interface, image download times are significantly longer.
C3 cameras are equipped with Sony IMX rolling shutter CMOS detectors with back-illuminated, square pixels measuring 3.76 × 3.76 µm. Despite the relatively small pixel size, the full well capacity exceeds 50 ke-, which is higher than the full well capacity of competing CMOS sensors with much larger pixels and even exceeds the full well capacity of CCD sensors with comparable pixel sizes.
The Sony sensors used are equipped with 16-bit ADCs (analogue-to-digital converters). The 16-bit digitisation ensures sufficient resolution to fully cover the sensor's exceptional dynamic range.
exposure control The shortest exposure time of the C3 cameras depends on the sensor type used:
C3-26000 shortest exposure is 17.3 µs
C3-61000 shortest exposure time is 19.5 µs
There is no practical limit to the maximum exposure time, but in practice the longest exposure times are limited by the saturation of the sensor either by incident light or by dark current (see the following chapter on sensor cooling).
Mechanical shutter
The C3 cameras are equipped with a mechanical shutter, which is very important to enable unattended observations (fully robotic or only remote-controlled installations). Without the mechanical shutter, it is not possible to automatically capture dark images, which are necessary for correct image calibration, etc.
The mechanical shutter in the C3 cameras is as reliable as possible, the number of opening/closing cycles is practically unlimited, as there are no surfaces rubbing against each other. The price for high reliability is slow shutter movement. Fortunately, the mechanical shutter is not required for exposure control, but only for capturing dark frames and possibly bias frames – all CMOS sensors used are equipped with an electronic shutter.
The camera firmware optimises shutter operation to avoid unnecessary movements. When a series of images is taken in quick succession, the shutter remains open to avoid introducing any significant delay in the close/open cycle between each pair of consecutive images. If the next image is to be a dark image or a bias image, the shutter closes before the dark image is taken and vice versa – the shutter remains closed when a series of dark images is taken and only opens before the next bright image. If no image is taken for a few seconds while the shutter is open (i.e. after an exposure with a bright image), the camera firmware closes the shutter to protect the sensor from incident light.
Cooling and power supply
The regulated thermoelectric cooling system is capable of cooling the CMOS sensor to 40 to 45 °C below the ambient temperature, depending on the camera type. The hot Peltier side is cooled by fans. The sensor temperature is controlled with an accuracy of ±0.1 °C. The high temperature drop and precise control ensure very low dark current during long exposure times and enable correct image calibration.
C3 cameras are available in two versions, which differ in their cooling performance:
Cameras with standard cooling achieve a controlled temperature difference of up to 40 °C below the ambient temperature.
Cameras with extended cooling can regulate the temperature to up to 45 °C below the ambient temperature. Compared to the standard version, cameras with enhanced cooling are slightly larger, slightly heavier and slightly louder due to the larger heat sink and more powerful fans.
Good storage is an important preventive measure against condensation and also extends the service life.
After use, place the camera immediately in the TS Protect Case and add some silica gel. During storage, the silica gel absorbs moisture from the camera. At the same time, it prevents moist air from entering the case. Dry storage allows even the small desiccant tablets in cooled cameras to regenerate partially. Your camera or accessories will always be ready for you in optimal condition. You can find the silica gel and case in our product recommendations.
The cameras are not airtight, so if the camera remains on the telescope, it will be exposed to moisture. The small amount of desiccant in the camera can protect the sensor and the inside of the protective glass from moisture for the duration of the recording, but not for days on end when left on the telescope. This can result in moisture problems. A simple trick is to stretch a plastic bag around the focuser to which the camera is mounted, making sure that no air can escape. Place some silica gel in a small cloth bag inside the plastic bag. This will create a "dry climate zone" for the camera, even on the telescope. This allows you to leave the camera on the telescope for a few days for a multi-day recording session.
In the medium and long term, however, this cannot replace proper storage in an airtight case with silica gel.
Colour or mono? Black and white cameras have the advantage of higher sensitivity and resolution than colour cameras. However, more effort is required to obtain a colour image. You also need colour filters and a filter wheel
