[Aavso-photometry] Cosmic Rays

[Ben Davies]

Hi Arne,


Thank you for the help.  I'll try and get my hands on a copy of that book.


I searched for undercorrected hot pixels, both by following some of the 
'hits' back and also by following hot pixels forward.  I could not 
locate any, but it is a big image with a lot of artifacts and I only 
looked at a small percentage.


I believe that I really do have wide track cosmic ray hits.  At 4.5 
hours, 1.5 cm, and 100 hits/, that would be 675 hits expected in that 
image.  So at least some of the events must have been muons.  Some 
certainly have that straight line look about them.  Yet there are no 
instances of extended objects that are not fat.  Perhaps this the result 
of smallish pixels, 6.8 microns?  And maybe you only see single pixel 
wide artifacts because you are using a thinned ccd?
<http://www.stsci.edu/stsci/meetings/irw/proceedings/birettaj.dir/section3_5.html> 
next to last paragraph.


Anyway, thanks to everyone who replied on this subject!  Particularly to 
Wolfgang and Justin for the bit of software.  I intend to keep using 
it.  From a purely pragmatic point of view it doesn't matter what is the 
cause of the artifacts.  It will be ok to avoid them in any case.


Thanks again,

Ben Davies


arne wrote:

> I've looked at thousands of CRs in my days.  The overriding feature
> is that they are pointlike, affecting only one pixel, or form a line
> that is one pixel wide and so easily distinguished from the seeing
> profile of stellar objects.  They are usually, though not always, very
> bright. With a TEC-cooled CCD, I would believe Michael's
> observation that undercorrected hot pixels would be a "contaminant"
> in searching for CRs, and would be very hesitant on accepting
> the CR-theory for events that are only marginally above the background
> noise level.
>
> Your best source of information is Jim Janesick's book,
> "Scientific Charge Coupled CCDs", especially the discussion in
> section 7.1.7 (page 670 and following).  The usual rate is about
> 100 events per cm2 per hour at sea level.
> Arne
>
> Ben Davies wrote:
>
>> I looked at your mexdev image. ... there are a lot of low angle CR's. 
>> Not every bright thing you see is a CR. The vast majority of those 
>> single points look to me like shot noise from under-corrected hot 
>> pixels.... So you may be sitting near something radioactive---that's 
>> no joke. Are you in Colorado? Got any other ideas?
>>  >>>>>>>>>>>>>>>>>
>>
>> Hello Michael,
>>
>> I'm in San Francisco, but who knows?
>>
>> The single, seemingly symmetrical, points you see on the jpeg are 
>> really not symmetrical.  If you trace them back to the originating 
>> image, they are linelike, about 3 pixels thick, with pixel values (to 
>> take one example) like this
>> 125(noise)   176   1066   506   421   283   288   134(noise)
>>
>> What I suspect, is that I am seeing low level cosmic rays that go 
>> unnoticed in an image just because they are not big and bright.  I 
>> expect my flux levels to be what anyone will see.
>>
>> The idea that I am sitting near something radioactive needs to be 
>> tested.
>> I will go down to San Jose soon and make the same test at the same 
>> time, 7-12pm pacific std time.  That of course will not eliminate the 
>> camera itself as a source.
>>
>> Ben
>> http://ben.davies.net/cosmicrays1.htm
>>
>> send me the ftp location and I'll send you the whole lot to look at.  
>> Your experienced eye will probably be able to sort something out.
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