Mouse-Over

Hold your mouse cursor over the image to see the thermal signal in a light frame compared to a frame with the thermal signal removed with in-camera long-exposure noise reduction. This is a 100 percent enlargement from the original to make the noise more visible.

The sensors in DSLR cameras are so sensitive that they also record electrons released by the heat in the sensor, even when it is not exposed to light. This is called thermal signal. At higher temperatures, and in longer exposures, more of this thermal signal is recorded.

In a long-exposure astrophoto, this thermal signal produces hot pixels and color blotches. This thermal signal looks like noise in an image. Luckily for us though, it is not noise, it is signal.

By definition, noise is random. Signal however, repeats in exactly the same place. Because of this characteristic, we can remove the thermal signal in our images. To do this, we must take an image of just the thermal signal. We do this by taking a dark frame.

We call the image of the deep-sky object a light frame, because it is exposed to the light from the subject.

A dark frame is a photo taken at the same exposure, ISO and temperature as a light frame, but with no light reaching the sensor. It is a picture of the camera's thermal signal. By subtracting a dark frame from a light frame, thermal current signal is removed. This process is called calibration, and we will discuss it in the chapter on advanced techniques.

Canon 1000D (Digital Rebel XS) Custom Function 3, long-exposure noise reduction.

In-Camera Long-Exposure Noise Reduction

Modern DSLR cameras will automatically shoot a dark frame and subtract it after you shoot a light frame in the camera. This is called long-exposure noise reduction. This is a setting that is usually located in the camera's custom function menu. Read your camera's manual to find out exactly where.

If you are shooting individual light frames, one at a time, at long-exposures, at relatively high ambient temperatures, your images will probably have a lot of thermal signal. If you only plan on shooting one or two frames, using long-exposure noise reduction can help improve your images.

When you get to a more advanced level in your astrophotography, you will not want to use in-camera long-exposure noise reduction. You will want to shoot separate dark frames and subtract them yourself later. This will allow you to spend more time gathering photons from the object you are interested in. We will discuss how to do this in Section 803, Image Calibration, in the chapter on advanced techniques.

Using long-exposure noise reduction will double the amount of time required to take a picture. Say you expose for 2 minutes. After the shutter closes, the camera will continue to take a dark frame for another two minutes and then subtract it internally. Some people who turn on long-exposure noise reduction forget that they have turned it on, and then can't figure out why they can't take another picture immediately after the shutter closes. This is because the camera is busy taking the dark frame.

In the latest generation of DSLR cameras, in-camera noise reduction may not be needed at all if the ambient temperature is low.

In-Camera Noise Reduction - The Bottom Line If you see a lot of thermal signal in your images, and you are only going to shoot a couple of frames, then you can turn on in-camera long-exposure noise reduction. This function will take a dark frame in the camera after the exposure ends, and automatically remove the thermal signal. If you are going to shoot more than a few light frames and stack them, it is better to shoot separate dark frames and subtract them later to remove the thermal signal.