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I'm starting in astrophotography with Canon 1100d and 75-300mm lens. My first goal is Milky Way. I read in a article that long exposures cause overheating sensor and even may damage it. Is it true? If yes, how long exposure can I use safely.

I have autotracker and want use exposures at least for several minutes, and maybe more than 10 minutes for very deep sky objects.

Vasya Petrova
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Unless it is pretty cold out, you will want to use exposure stacking for such a shot. Not so much because it would cause damage, but rather because of the noise that your camera will pick up from a long exposure.

As a sensor is operating, it is consuming power and this causes heat to build up. That heat results in mistaken readings where the sensor things there is light when there is not. The longer the sensor is active, the more heat builds up and the more rapidly the noise level will rise until eventually it will overpower the signal you are getting. The colder it is, the more heat will dissipate and the less the noise accumulation rate will rise.

LENR (long exposure noise reduction) can take another image of the same exposure length, but with no light being captured and then subtract the dark image from the actual exposure to help compensate somewhat, but this is still of limited benefit.

I wouldn't worry too much about damage though, most cameras have safety shutdowns built in if heat gets too high on the sensor. It is something to be aware of, but is not so much an issue as it once was (unless you are using legacy hardware that might lack modern advances and safe guards.)

AJ Henderson
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  • I read in DeepSkyStacker manual that exposure is preferable in the sense that stacking 10 images with 10 minutes exposure is better 100 shots with 1 minute exposure. Yoy say that too long exposures cause extra noise because of heat. So, what is optimal exposure time? I want milky way and deepsky objects only. – Vasya Petrova Jul 28 '14 at 16:52
  • @VasyaPetrova - yes, longer exposure is preferable as long as noise doesn't become an issue. Stacking can't detect anything that doesn't show up at least a little in the shorter exposures. In general, I recommend using the longest exposure you can without having noise issues. Unfortunately, the only way to determine that is trial and error based on your current conditions. The maximum usable exposure time depends on the ambient temperature, brightness and the particular sensor. There is no real meaningful rule of thumb. – AJ Henderson Jul 28 '14 at 16:58
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    @VasyaPetrova, you'll also want to consider focal length and sky rotation when calculating exposure times. See Rule of 600. Consider using the 18-55 kit lens or getting a tracking head. – inkista Jul 28 '14 at 17:12
  • @inkista - good point, worth noting that will also impact stacking to an extent as well, particularly if you don't first apply any lens corrections for restoring stuff to rectilinear. – AJ Henderson Jul 28 '14 at 17:16