To CMOS or not to CMOS

QHY163MCMOS type sensors currently rule the market. At home, when traveling, under water, in the studio or when making movies – everywhere when photons needs to be collected, CMOS sensors can be found. Maybe astronomy amateurs also should pay more attention to the CMOS equipped cameras? CCD cameras were number one in this area for many years. So let’s take a closer look what are the differences and similarities between these two technologies.

In both detector types the same photoelectric effect is responsible for photon counting, so there is not much difference at this point. Both sensor types can be front illuminated (FSI) or back side illuminated (BSI). Also peak quantum efficiency for both kinds is similar in amateur cameras at level of 40-60%. There are few differences however, and they can be noticed when read out process starts. In the CCD sensors charge from one pixel is transferred across many pixels to the output register, and then to amplifier and analog to digital converter. In the CMOS chips each pixel is surrounded with its own electronic cirquit and read out process happens there. Due to this fact read out noise  in the CMOS matrices is much lower (level of 1-2e) than in CCD sensors (5-10e). It follows pretty important fact – that CMOS chips will handle much better short exposures (these are they “natural” working conditions at the end). So they can be used with exposures of single seconds attached to large instruments with large image scale, but not necessarily equipped with precise tracking or guiding (for example dobson newtonian with tracking platform). Stacking large number of such short frames can yield pretty impressive results, that you can admire for example at http://www.astrokraai.nl/ page.

Next part will be about AD converters plus gain and offset settings in the CMOS cameras.

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