<p>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.</p>

<p>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 <a href="http://archive.stsci.edu/mast_faq.php?mission=KEPLER" rel="noreferrer">http://archive.stsci.edu/mast_faq.php?mission=KEPLER</a></p>

CC BY-SA 3.0
2014-08-07T15:34:20.403Z
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
CC BY-SA 3.0
2014-08-07T22:04:33.343Z
this is called "smear". see "Smear " section in http://keplerscience.arc.nasa.gov/PipelineCAL.shtml
Asterion
CC BY-SA 3.0
2014-08-07T15:07:10.683Z
2014-08-07T15:07:10.683Z
Asterion
<p>There are two kinds of cadence time for Kepler, 5 minutes and 30 minutes.<br/>
I just know there is no shutter and the CCDs of Kepler can be locked after exposure. <a href="https://archive.stsci.edu/kepler/manuals/KSCI-19033-001.pdf" rel="nofollow">handbook</a><br/></p>

<p>But how to realize that? Remember we have to not affect the photometry.<br/></p>

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

<kepler><exposure>
how does Kepler stop exposure after a 5 or 30 minute cadence time?
questionhang
# how does Kepler stop exposure after a 5 or 30 minute cadence time?

(Asked by: username_0 on 2014-08-07)

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](https://archive.stsci.edu/kepler/manuals/KSCI-19033-001.pdf)

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?

**Tags:** kepler, exposure


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## 1 answers

### Answer by username_1 on 2014-08-07. Score: 5

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>

> **Comments:**
> **username_0**: 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.  
> **username_1**: this is called "smear". see "Smear " section in http://keplerscience.arc.nasa.gov/PipelineCAL.shtml  

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