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There are two kinds of cadence time for Kepler, 5 minutes and 30 minutes.
I just know there is no shutter and the CCDs of Kepler can be locked after exposure. handbook

But how to realize that? Remember we have to not affect the photometry.

The other question is so far how many unique sources has Kepler observed? 160,000?

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They don't stop. The Kepler CCDs read out the signals collected during the accumulated time of 6.02 seconds. The fixed read out time is 0.52 seconds. So each CCD gets one frame every (6.02 + 0.52) = 6.54 seconds. Then Kepler sums up every 9 frames (short cadence) and 270 frames (long cadence). The time between two short cadences is (6.54 x 9) = 58.9 seconds, and (6.54 x 270) = 1766 seconds between two long cadences, but the exposure time is (6.02 x 9) = 54.2 seconds for a short cadence, and (6.02 x 270) = 1625 seconds for a long cadence.

There are totally 95 million pixels available on Kepler CCDs, but the spacecraft can store only 5.4 million. The maximum number of targets is about 170,000. see http://archive.stsci.edu/mast_faq.php?mission=KEPLER

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How to remove the photons which arrive in that 0.52s? Every quarter we can observe 160000 unique sources and there are repeated sources between different quarters. I can not find a value that gives how many sources we have observed in the 17 quarters. –  questionhang Aug 7 at 15:34
    
this is called "smear". see "Smear " section in keplerscience.arc.nasa.gov/PipelineCAL.shtml –  Asterion Aug 7 at 22:04

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