[Aavso-photometry] Cosmic Rays

[Ben Davies]

I spent some time looking into the cosmic rays a bit more.

Using a ccd  indoors at sea level, I made 30 min dark frames with the 
chip in both horizontal and vertical orientations, and what I found was 
not quite what I expected.

I confess that I have previously only paid attention to cosmic rays when 
they appeared loudly in an image, which I will then usually discard.  
The reality is that almost none of the cosmic ray hits are readily 
visible in an image, even one with no sky noise. 

I made a series of 30 minute dark frames.  Then I blinked these darks 
against a median dark.  The median cuts out the cosmic ray artifacts 
that the images contain.   It is fairly ugly. 
<http://ben.davies.net/cosmicrays1.htm>  Disregarding single pixel 
discrepancies, I typically found around 65 hits in a vertical frame and 
110 in a horizontal.  Most pixel values in the hits were only 100-600 
above the background of 150.  In any actual image, they would not be 
noticed.

Now the count rate is not that high - .015 hits / sq mm / min for a 
vertical orientation and .024 for horizontal.  For my camera, that 
amounts to 2.2 hits a minute.  Given short integration times, the hit 
rate at sea level is not onerous.  It is just that when they do hit, you 
probably will not know they are there in the image.

Ben Davies.