taking long exposure image with CCD sensor
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.

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