Kepler Mission – Mr. Ian Bradley you have an answer

A question made by Ian Bradley arrived my e-mail via Stuart Atkinson, since there might be other Kepler fans with the same doubt I decided to bring it here:

Regarding the Kepler 1st light image…
 
The imaged area is well away from the ecliptic to allow 24/7 viewing of the image area without Sun being a problem. On a quick glance at the high res 23Mb image from the Kepler website, there are several diagonal tracks [not the vertical or horizontal blooming from bright stars]. One is particularly bright (see below for position). Are these earth orbit satellites (given that Kepler is in an Earth trailing heliocentric orbit, this seems unlikely) or are they other solar system bodies e.g. minor planets, asteroids, Keuper Belt objects etc, etc. I guess it could be crap from the spacecraft itself…
 
Clearly they cannot be in the ecliptic! Few surveys have been done at large angles away from the ecliptic, so have we got a way to discover other members of the Solar System? 
 
Bright diagonal object position – treat ccd array as 5 rows x 5 columns, each row, column element consists of 2 ccds of either horizontal or vertical alignment! Object is in 2nd row, 2nd column in the top ccd of the pair towards the bottom left of image.

Jon Jenkins, Kepler Co-Investigator, was, once more, kind enough to provide us an answer:

Ian is most likely seeing cosmic ray tracks which are the result of energetic particles whizzing through the CCD and depositing energy along the way. (I couldn’t tell from Ian’s email which direction he was counting CCD modules, although I found a bright cosmic ray track 2nd row, 2nd col from the top left, near the middle and low on the upper CCD in this pair.) We estimated a hit rate of 5 per square cm per second pre launch from counts reported by analysis of the LASCO CCDs aboard SOHO, which agrees well with the rates we’ve estimated from the dark frames we took prior to Dust Cover Ejection. The physics of our CCDs indicates that most cosmic rays deposit about 2500 electrons but some can deposit a whopping amount of charge. We identify and remove the “brightest” cosmic ray events from the data in ground-based processing. Most cosmic rays deposit too little energy for us  to detect them in or near the cores of our relatively bright target stars. The Field Of View (FOV) is tipped up 60 degrees above the ecliptic plane, so the chances of seeing solar system bodies is rather small, and Kuiper belt objects, for example, would not be bright enough to be seen as a bright streak in our images, and travel fast enough that to see them eclipsing stars in the FOV would require much shorter exposures than Kepler is capable of. The first light image pair consisted of one 6-sec integration (not including the 0.51 sec readout time) and one 65 sec integration (10 co-adds of individual 6-sec integrations with 0.51 sec readout times). During nominal science operations the CCD images are co-added for 30 minutes (a sum of 270 individual 6 sec exposures).

Cheers,
Jon

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  1. Thank you,
    very interesting article

  2. As I understand it, the mission only downloads the pixels near the 100k stars of interest. Does this preclude a microlens event search, since the whole pixel array will not be downloaded? Perhaps the whole array could be downloaded once a month, and dimmer stars further away evaluated for any rising trends that could indicate a potential microlens event, and the relevant pixels added to the download stream. Or would the the integration time for Kepler be too small to detect the change in brightness for these dim, distant stars? (This sounds like an excellent project where amateurs could contribute!)
    Thanks for your time!

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