The ZWO ASI6200MM Pro uses Sony’s latest back-illuminated IMX455 Full Frame format sensor which has a native 16-bit ADC. It has offers 3.76µm pixels, an ultra-low 1.2e readout noise and an innovative technological breakthrough resulting in zero amp-glow, a common issue with some CMOS sensors. In a further attempt to reduce amp glow and optimise download speed without dropped frames, the ASI6200 featues a 256MB Buffer.
The IMX455 represents a step change in large format imaging. Its small, efficient pixels when combined with the 36mm x 24mm format (classic 35mm / FX format) result in a sensor that offer 62MP resolution!!
Sony’s back-illuminated CMOS image sensor improves sensitivity and noise reduction – the key factors to enhancing image quality, while radically realigning their fundamental pixel structure from front-illumination to back-illumination. It has retained the advantages of CMOS image sensors such as low power consumption and high-speed operation.
With a conventional front-illumination structure, the metal wiring and transistors on the surface of the silicon substrate that form the sensor’s light-sensitive area (photo-diode) impede photon gathering carried out by the on-chip lens. A back-illuminated structure minimizes the degradation of sensitivity to optical angle response, while also increasing the amount of light that enters each pixel due to the lack of obstacles such as metal wiring and transistors that have been moved to the reverse of the silicon substrate.
Sony has newly developed a unique photo-diode structure and on-chip lens optimized for back-illuminated structures, that achieves a higher sensitivity and a lower random noise without light by reducing noise, dark current and defect pixels compared to the conventional front-illuminated structure.
The ZWO ASI6200MM Pro also features a 16bit ADC. This 16bit ADC is not a CCD 16bit ADC, and achieves a dynamic range output of 14 stops. This significantly improves the image sharpness and contrast, and the colour changes are smoother and more natural.
This extended dynamic range also leads to greatly reduced noise almost irrespective of gain. When the gain value is 100, the magical HCG high gain mode is turned on, the readout noise is greatly reduced, and the dynamic range is basically unchanged. It is recommended to set the gain to 0 or gain 100 in deep space.
Traditional CMOS sensors produce a weak infrared light source during operation quite often seen in the corner of uncalibrated images. This is the tell tale sign of ‘amp glow’. As the ASI6200MM Pro uses zero amp glow circuitry, you won’t have to worry about amp glow even when using high gain, long exposure imaging. See above example images showing the difference between normal amp glow and that when combined with the new circuitry.
The Sony sensor is not the only feature of the new ASI6200. As befits a camera of this performance class, it also features a comprehensive anti-dew system for the sensor window. The ASI6200MC Pro is fitted with a Polyimide heater that avoids any dew problems. The anti-dew heater power consumption is around 5W and can be turned off in software to save power if not required.
To reduce noise still further, the ASI6200MM Pro uses two stage TEC cooling that can lower the CMOS sensor temperature to more that 35C° below the ambient. This can greatly reduce dark current generation and sensor noise even with extended exposure times. Due to the high sensitivity of the latest CMOS sensors, extreme cooling is not as critical as it once was with CCD. This is because exposure times can be reduced and therefore noise has less opportunity to become visible in an image.
In addition a unique dark current suppression technology is employed that can further reduce dark current noise.
At a cooling temperature of 0 °, the dark current noise is only 0.003e/s/pix. This means 300s exposure can only cause 0.9e dark current noise, which is completely negligible!
