Learn how ambient temperature affects the image
This is a big one. The ambient (air) temperature surrounding your camera plays a vital role in the overall quality of the image when it comes to astrophotography. The hotter it is outside, the warmer your sensor is, and the worse (noisier) a long exposure will look. A hot camera sensor vs. a cold sensor makes a huge difference in image quality and can ruin an otherwise good photo. This is even more important when creating images to stack — if you shoot too hot it can result in a final image that looks like animal fur…trust me on this one.
Why do you think cooled-CCD cameras for astrophotography exist? It’s not just because they are awesome. It’s because you can get extremely low-noise images with extremely long exposures and high gain (effectively ISO). I’m very curious why none of the major camera manufacturers have not brought out a cooled-CMOS DSLR (hint hint…!). Because I’d buy that in an instant, after I sold my car.
What does heat effect, exactly?
ISO: The hotter it is around the camera, and therefore the hotter the sensor inside, the noisier your image will be. We know noise also increases along with increasing ISO (gain), so that means your highest effective ISO is going to drop when it’s warm.
Exposure length: As it is with ISO, the longer your exposures are, the more heat-based noise your sensor is going to produce in the image. You will have to shorten your exposures as the ambient temperature increases to decrease the effect of heat noise, and each camera responds differently, to boot.
How to mitigate heat issues
There are a few things you can do to help keep heat-based noise at bay, at least a little, so you have to take it on a case-by-case basis. If you notice an abnormal amount of noise in your image captures, try the following:
Take a short break between exposures: Take a short break between exposures to allow the sensor to cool down. Maybe 30s to a minute or more, depending. It takes testing to find out how much will help.
Lower the ISO: Consider dropping the ISO a little. This reduces the gain on the sensor and heat will have a smaller affect on the sensor’s ability to accurately gather the light.
Shorten exposure length: The longer the sensor is exposing, the hotter it gets, even in cold temperatures. So, when it’s excessively warm outside, shortening the exposure length a bit can help.
Cool your camera (advanced!): Some crazy-cool hardware hackers have made custom modifications to their cameras to add a Peltier cooling device, or put their cameras inside a custom-fit camera cooling box. That is extreme, but tends to work quite well! Camera manufacturers take note!