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Big increase in hot pixels! Is this due to cosmic ray or normal?

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#1 George P Dunham

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Posted 16 February 2019 - 02:14 PM

I've had several CCD cameras and over the years I've noticed that increases in hot pixels usually happens periodically and not gradually.  I would assume that this is a cosmic ray event.  Am I correct or am I mistaken?



#2 Jim Davis

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Posted 16 February 2019 - 02:29 PM

If it corresponds to this, it is possible.

 

https://upload.wikim...om_19830101.jpg



#3 jfrech14

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Posted 16 February 2019 - 02:31 PM

Cosmic rays do over time damage the photosites they hit and semiconductors naturally degrade over time. Cosmic rays damage should only happen on long timescales because cosmic ray flux that can damage silicon isn't too awfully high at sea level and gets quite a lot worse if you live at a high altitude. The Hubble instruments actually heat up their sensors every now and then to anneal them to reverse some of the cosmic ray damage.not recommending it, just saying it's normal to see them get worse if you track it over time


Edited by jfrech14, 16 February 2019 - 02:33 PM.

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#4 pyrasanth

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Posted 16 February 2019 - 03:36 PM

I'm surprised at the number of trails I'm seeing on my long frame captures. I'm capturing a set of 1 hour dark frames and the tell tale worm trails are all over the images. The longer your exposures the more your going to get!



#5 jfrech14

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Posted 16 February 2019 - 03:38 PM

Yeah, a fun fact is you can identify the particle with fairly reasonable accuracy (not energy though) based on the type of path it takes. My favorite are when you get decays.



#6 George P Dunham

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Posted 16 February 2019 - 04:59 PM

So noticing a large number of new hot pixels on last nights imaging session would not necessarily correspond to a cosmic ray event?  Would it correspond to something local?  I've been using the same bad pixel map for a year and this morning I have ~20,000 new hot pixel and the BPM has to be remade.



#7 jfrech14

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Posted 16 February 2019 - 05:05 PM

When did you last use it? Hot pixels will be left in the master dark. Cosmic rays will not be in the master dark and should have no effect on the bad pixel mask assuming you make the bad pixel mask using cuts from your master dark if you use enough dark frames in the master. I would recommend making sure the temperature was at the right point and make sure your cuts were made identically. I can think of nothing electronically that would cause that in a short period of time.



#8 George P Dunham

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Posted 28 February 2019 - 12:36 PM

I believe that I have misconceived what causes bad pixels in general.  Other than just random failure on the sensor, I thought that cosmic rays could leave pixels dead or show up as a lit up pixel during an exposure.  My bad.  My question remains though, what would cause a sudden discrete increase in bad pixels.  Might it be something local like a voltage spike or static discharge or something?  Sorry I think this was answered above.  Also, yes random slow degradation of the sensor is obviously happening, I just noticed I didn't acknowledge this in my original post.

 

For the question above, my sensor temperatures are consistent from session to session (-25c) in the winter and (-20) in the

summer. 

 

"Cuts" is a new term for me.  My sessions are usually very consistent.  I am working remotely and don't fiddle with anything during an imaging session.  My darks and biases are made on a night when nothing else is happening.  Usually 151 biases followed by 30-70 darks depending on the exposure time.  Lights are done when ever the camera is moved or filter is changed or any other change in the image path.  My electronics are passed through a good APC and so no spikes should get through but static discharge may happen.  I'm at 8200feet.

 

I have had this happen twice over the past 5 years or so.  A sudden "overnight" increase in bad pixels requiring a new masterdark/BPM.  It is a curiosity at this point but I still would like to know what I am dealing with.  Based on some of the replies above, I am not sure I am describing the issue or asking the question clearly.


Edited by George P Dunham, 28 February 2019 - 12:49 PM.


#9 jfrech14

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Posted 28 February 2019 - 03:49 PM

When I say cuts, I'm referring to how you "cut" your histogram while making your bpm. If you have a bias frame, your histogram should be very Gaussian with maybe a small bump on the left of dead pixels and a small bump on the right with really hot pixels. As you expose longer, your histogram will form a tail on the higher counts side and the lower end side should stay fairly Gaussian with some stragglers which relate to cold or dead pixels. In order to have consistent bpm, you have to consistently decide which level on the histogram are you going to declare the pixels bad. The best way to do it consistently is probably using your rejected pixels when making the master. If you made cuts manually and didn't look carefully it can be easy to include wayyyy more pixels than you meant to.

When you say there are way more hot pixels, is it visually in your darks or just looking at your bpm? More importantly, are your hot pixels consistent? The new hot pixels from 5 years ago.. are they still hot now, or are these new hot pixels original and don't really include the older ones? Real hot pixels should only get worse. If you are working remotely and it isn't in a really dry environment, moisture can do a lot of damage to readout electronics by causing charge buildup in some of the components. Is there a way to give us access to archival data?

#10 George P Dunham

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Posted 01 March 2019 - 11:05 AM

At this point, I realize my original questions should have been "why do I get periodic, significant increases in bad pixels?" and it has hit a wall for me.  I understand the replies and their significance and I REALLY appreciate all of your replies.  Some light has been shed on a misunderstanding I have had on the effects of cosmic rays on my sensor.  I would like to follow up on all the questions and suggestions but I need to put efforts into other things.   I think that if I want to get an answer to my question, I will need a dialogue with someone in real time to avoid misunderstandings.  Thanks... G




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