Thermal signal looks like noise, but it really isn't.

Thermal signal looks like bright pixels and small red, green and blue colored spots or blotches in the image background.

More thermal signal is recorded when the ambient environmental temperature is higher, such as in the summertime, because the inside of the camera is also at a higher temperature. Thermal signal is usually not as much of a problem on cold winter nights.

Thermal signal increases in longer exposures and at higher ambient temperatures. The temperature inside the camera will also heat up as the camera is in continuous use, and if the LCD display on the back of the camera is used a lot.

Luckily, since this "noise" is really signal, and it repeats consistently in the same place in the image, we can remove it. We do this by taking a picture of just the thermal signal, and then subtracting it from our normal picture of the deep-sky object.

We can take a picture of the thermal signal by taking a dark frame. A dark frame is a time exposure that is taken where no light reaches the sensor and the thermal signal is recorded.

A light frame is a time exposure where light from the celestial object reaches the sensor. A light frame records both the object signal as well as the thermal signal.

If we take a dark frame at the same exposure length and temperature and ISO as the light frame, the thermal signal will be the same. It can then be removed by subtracting the dark frame from the light frame. This is what in-camera noise reduction does, as we will see in the next section.

Mouse-Over

Hold your mouse cursor over the image to see the thermal signal in a dark frame compared to a light frame. The colored blotches and hot pixels are in exactly the same place. This is a 100 percent enlargement from the original JPEG images to make the noise more visible.

In this example, the thermal signal is visible in the dark frame as hot pixels and colored blotches. We can see that it is in exactly the same place in the light frame. While it is a problem if it is not removed, it can easily be fixed by subtracting a dark frame of the same exposure and same ISO taken at the same temperature.

Mouse-Over

Hold your mouse cursor over the image to see how the thermal signal in a dark frame increases over time. This is a 100 percent enlargement from the original JPEG images.

The mouse-over comparison image immediately above shows how the thermal signal increases as the camera is used continuously over time. In this case, the camera was shooting dark frames with the body cap on, and the eyepiece covered so no light could enter the camera. This is pure thermal signal from electrons released by heat in the camera.

As the camera is used continuously, it heats up internally. As it heats up, the thermal signal increases. In this example, five-minute exposures at ISO 1600 were shot continuously in my garage where the ambient temperature stayed constant at 70-degrees Farenheit (21 degrees C).

Five-minute exposures were shot one after another for 3 hours and 20 minutes, for a total of 40 individual dark frames. The mouse-over example shows how the thermal signal increases from the first frame to the last frame.

This thermal signal is also present in light frames, and if the camera is used continuously, it will continue to increase in intensity. Note that while the intensity increased, the location of the signal did not change. Hot pixels and colored blotches stayed in exactly the same place.

Some people ask if it might not be a good idea to pause between pictures to give the camera a chance to cool off. This will work, but it is a waste of clear dark sky time. It is much better to spend this time gathering more photons.

If thermal signal is a problem, it can be removed later with separate dark frames. We will discuss this in more detail in section 803, Image Calibration, in the chapter on advanced techniques.

Thermal Signal - The Bottom Line Thermal signal is caused by electrons from heat recorded by the sensor in DSLR cameras. Thermal signal looks like single bright pixels and colored red, green and blue blotches scattered throughout the image. It can be removed with in-camera long-exposure noise reduction if you are just shooting a couple of JPEG frames, or with more advanced techniques if you are shooting Raw frames.