Moravian instruments, Inc., source: https://www.gxccd.com/art?id=603&lang=409, printed: 27.09.2020 21:42:28
|New cooled CMOS cameras C1+7000 and C2-7000 offer quite large pixels of 4.5 μm, especially compared to majority of other CMOS based cameras available. As CCD sensors, often employing much bigger pixels, are no longer available, even relatively small increase in pixel size of a CMOS sensor significantly increases some key parameters like dynamic range (pixel area and thus also a number of electrons each pixel can accommodate corresponds to the square of pixel dimension).|
Small pixel size and the resulting limited dynamic range (number of electrons every pixel can hold before it saturates) of the CMOS sensors concern especially astronomers focused to research applications. Very low read noise of CMOS sensors offsets this problem when the camera read noise determines the S/N ratio — CMOS cameras often offer better theoretical S/N than the CCD ones. Such situation is typical e.g. for narrow-band imaging in astronomical photography. But when the noise introduced by sky background overgrows the other noise sources and becomes dominant (this is typical for photometry and other research applications), small pixels can become the limit.
When the new C1+7000 and C2-7000 cameras are used in 2 × 2 binned mode, resulting pixel size 9 μm equals to the pixel size of the G2-1600 CCD camera, widely popular and beloved by astronomers focused to scientific applications. The original resolution of 7 MPx is reduced to approx. 1.75 MPx by 2 × 2 binning, which is still slightly more than 1.6 MPx of G2 camera thanks to greater IMX428 area compared to KAF-1603ME CCD. So, the C2-7000 looks like the very promising replacement of the research workhorse G2-1600 camera.
One of the key requirements for the scientific camera is linear response to light. Only then we can rely on the count values of pixels (called ADU or DN) correspond to the amount of light each pixel received. This is crucial for precise measuring of brightness of celestial objects. While the linearity of the cameras equipped with 3.45 μm pixels is nearly perfect, linearity of the IMX428 CMOS sensor with 4.5 μm is excellent without any compromises.
Especially medium size and large amateur telescopes with focal length a meter or greater cannot utilize very small pixels of many CMOS sensors and images become over-sampled, at last for longer exposure times (planetary imagers, employing very short exposures in some form of lucky-imaging techniques, prefer small pixels even for very long focal lengths).