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Assuming you use a CCD image sensor, would it be possible for you to take images with exposure time of 1s without stacking image?

I'm wondering how you can decrease data rate of the CCD. CCD camera has ADC and ADC has its own clock frequency. Let's assume that the clock frequency of your ADC is 20Mhz and its bitdepth is 10-bits . Then it outputs 10-bits data every 50ns. Therefore during 1s you would have lots of meaningless data. So in order to take long exposure image, you always need to process data(remove meaningless data) and stack images. Could anyone please let me know if I'm correct?

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Sorry, but you're not correct, and in general taking long exposures with a CCD is straightforward.

In general, CCDs have two sets of storage cells, the photoactive cells and transfer cells. During exposure, charge accumulates in the photo cells, and at the end of the exposure is pushed into the transfer cells. The transfer cells are organized as one or more analog shift registers, and after a transfer the individual charges are stepped out of the transfer cells to an A/D converter.

So, if you had a 1M CCD array, and a 20 MHz clock rate, the minimum exposure time can be very short, consisting of two successive transfer pulses, but the minimum readout rate would be 20 Hz.

Start with Wikipedia

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