# Which kind of properties can we get for cosmic ray particles hitting on an optical ccd?

It is very common that we meet cosmic ray particles in optical images recorded by CCDs. You can see the "snowflakes" in the hubble images below:

Generally we should remove them in order to get photometry or spectroscopic info.

However, there may be useful info we can derive, for example, can we get their directions or energies statistically before they arrive at a CCD?

There is some correlation between the ADU value of a cosmic ray point and its real energy?

• Please clarify your question, ask one question at time. The topic is very confused and too broad in my opinion. – Py-ser Sep 16 '14 at 5:11
• Are you talking about background issues? Could you give an example of a CCD hit by a cosmic ray? Usually CCD detectors are "protected" against such a radiation. – Py-ser Sep 16 '14 at 6:40
• @Py-ser If they are, I'd hate to use one that isn't protected. All CCDs that I have ever used suffer from cosmic ray hits, which dependent on the purpose, make long exposures useless. – ProfRob Jul 13 '15 at 13:58
• @RobJeffries, yes, it is true, detectors usually suffer of cosmic rays noise. What I meant is that images are usually cleaned from such events. Instead of "protected" I should have used "veto shielded". – Py-ser Jul 13 '15 at 20:38
• @Py-ser I don't think this technique works for optical astronomical CCDs. The HST picture shown is quite typical for a say 1 hour exposure with an optical CCD. – ProfRob Jul 13 '15 at 21:52

They are a tremendous nuisance that can be removed to a certain extent by median stacking images - though this also fails if you get into a situation where cosmic ray hits are found in the same pixels of $>1$ images in your stack. This limits the lengths of exposures that can safely be made with astronomical CCDs. Depending on exact usage, this limit is usually between 30 minutes and an hour.