Approximately to the middle of the histogram, because sometimes, at various characteristics of various camera sensors want to deviate away from it or want the exposure in a particular way, like the 294 chip, which -according to their users- the flat must be at least >1 second long , so in that case we need to adjust the light output to lower than usual, while other cameras don't care if it's only a 0.1 second exposure or 7 seconds to get the optimal flat.
Hardware BIN, as the name implies, is to merge the pixel with hardware, usually refers to the completion of pixel merge on the chip, due to the different chip structure, CCD can do this, but CMOS cannot. CMOS hardware BIN is more like pixel skip. The frame rate will be faster by pixel extraction, but the signal-to-noise ratio is limited. Generally, we apply it to scenes that require high frame rates, such as shooting solar system objects. For deep space shooting, we recommend binning with software on CMOS cameras.
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