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Today, I was taking some shots of night sky. For each, I had 20 sec exposure. During picture review, I found a red dot which is referred as "Hot Pixel" according to some people on the Internet. Because I found this red dot with 20 sec exposures (which I took for the first time to capture the night sky) and not with the normal shots, I thought to test if this was a result of long exposure. To test this, I took two shots with lens cap on. One was with 20 sec exposure and another one with 1 sec exposure. I found the red dot again with 20 sec exposure but none with 1 sec exposure.

Additional info: Camera : Canon 600D, ISO : 200 - 800, White Balance : Tungsten, Auto and Fluorescent.

Hot pixel with long exposure, 20 sec (crop out from the night sky shot):

Hot pixel with long exposure (20 sec)

My question is, does this red dot/hot pixel is a result of long exposure or something else (sensor processing etc)?

Also, how to reduce or remove this effect as it seems very annoying for the astro-photography (initially, I got confused with it as a star with red shift)?

Thanks in advance!

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this problem looks similar to dead pixels reported for laptop screens (specially Toshiba), but it used to be blue not red. Perhaps your image sensor has a dead pixel –  akram Aug 5 '12 at 1:05
@AkramMellice: Is this a serious issue or acceptable? This is a new camera, just 1 month old. Also, there isn't anything at short exposure. –  Nitin Kumar Aug 5 '12 at 1:18
I wouldn't know that for cameras, but I had a friend with a Toshiba laptop with 1 dead pixels and he called the tech center and they told him that they can't do anything for him unless he has at least 3 dead pixels, so it depends on the manufacture's policy –  akram Aug 5 '12 at 4:37
If your camera supports raw mode, record a sequence of shots of varying time periods in raw mode. See if the linear intensity of the hot pixel varies with the time. You will need some software tool that can extract images or pixes from the raw format. –  Skaperen Aug 5 '12 at 4:56
I had the exact same problem with Nikon D5100. I thought I found a red giant? –  photo101 Oct 19 '13 at 10:43

3 Answers 3

up vote 5 down vote accepted

Yes. This is a hot pixel.

Basically, there is something wrong with one pixel. It can be more sensitive than others or captures some loose electrons in the sensor. In either case, the charge goes up faster than it should and while 1s is not enough, 20s is sufficient for it to accumulate a noticeable charge. In your particular case, the pixel happens to fall behind a red color-filter (that's a 1 in 4 chance), so it appears red but hot pixels (technically photosites) can cause the appearance of red, green or blue dots.

The camera can be calibrated to take into account such anomalies and usually is. It will then apply compensation for it. On some models it is possible to do it yourself using reference images but it is normally done by a service technician. If you do not have a local service center (for Canon in your case), then you have to call and get a reference number. Your send you camera there and get it back a few weeks later without the problem hopefully (and without a new problem as it sometimes happens unfortunately).

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By the way, the pixel mapping works impeccably. I had one hot pixel on one camera and never seen it after it was remapped. –  Itai Aug 5 '12 at 14:21
Can you give any link on this remapping technique? And, can I remap this hot pixel for long exposure? –  Nitin Kumar Aug 5 '12 at 15:06
Remapping is built-into most if not all cameras. The service technician does it when it is not user accessible, otherwise you just select the Remap Pixel function in the camera's menu and wait a minute or so. I believe the 600D does not have this user-accessible. –  Itai Aug 5 '12 at 16:20
Searched for 600d and found that selecting "manual sensor cleaning" and leaving the camera for 30 sec- 1 min. has remapped hot pixels for many people. Strange trick. I don't know if I should use it as only long exposure seems to create this hot pixel and the camera is purchased only 1 month ago. –  Nitin Kumar Aug 5 '12 at 17:21

This is a known problem with every digital camera and one of the biggest advantages of film in astrophotography. You can take several exposures and postprocess in software to remove ISO noise by stacking images, there's also a technique called dark frame subtraction.

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I think, you meant "one of the biggest disadvantage over film in astrophotography", right? –  Nitin Kumar Aug 6 '12 at 22:48
yeah, i guess... not a native speaker –  Baczek Aug 7 '12 at 11:10

In short: yes, hot pixels can and do increase with longer exposure, as the sensor warms up from operation. More on this in this related question: Should I be concerned about sensor heat and the development of hot pixels?

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