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Hold the mouse-cursor over the text descriptions below the image to compare the background noise in stacks of different numbers of frames. The noise has been reduced because it has been averaged out while the signal has stayed in the same place. The overall signal-to-noise ratio is greatly improved when more frames are stacked. This is a 100 percent crop.

In the above example, we can see the noise in the background and in galaxy M110 in a single frame of 150 seconds of exposure. Most of this noise is photon noise because all of the images have been calibrated with a master dark and master bias frame.

There is only one real way to collect more signal, and that is by increasing the exposure time. This can be done by using longer exposures, or by adding or averaging a bunch of shorter exposures together.

Adding keeps the noise the same, but increases the signal. Averaging keeps the signal the same, but reduces the noise. Both methods produce exactly the same results in terms of signal-to-noise ratio.

Adding produces brighter images, but runs the risk of clipping highlight detail. Averaging produces less bright images, but gives more control over how the image is stretched to increase the contrast. Generally, averaging is a better method to use.

With digital cameras, shooting longer exposures, or stacking a bunch of shorter ones, yields pretty much the same results, within certain restrictions, of course.

The basic restrictions are these:

• Longer exposures will start to clip, or overexpose highlights to the point where no detail is recorded. So we can't shoot a ridiculously long single exposure to improve the signal-to-noise ratio. There is a practical limit, and this is when the highlight detail that we care about starts to get so overexposed there is no detail recorded. Longer exposures will also be limited by thermal signal from heat in the camera.

• Shorter exposures must be long enough so that the faintest detail that we care about in the image is up out of the camera noise.

It's easy to determine the minimum necessary exposure by simply examining the histogram on the back of the camera and exposing long enough to separate the "mountain" of the histogram away from the left side wall of the histogram. I usually aim to put the peak of the "mountain" at about 25 to 33 percent from the left side.

Once you have established the correct minimum exposure, there really is no reason to go longer in a single exposure if you are going to stack multiple frames. Going longer just risks ruining your image by making it more susceptible to tracking errors. Just keep shooting these minimum exposures, and stack them later.

It is very important that your exposure be long enough to get the faint detail up out of the camera noise.

Stacking can be accomplished very easily with a freeware image stacking program like Deepsky Stacker, or a commercial astronomical image processing program like Images Plus, or Nebulosity.

Stacking Light Frames - The Bottom Line A key to improving our images is gathering more signal. More signal means a better signal-to-noise ratio. Because we are usually limited in the length of an exposure by thermal signal and tracking considerations, the best way to gather more signal is to shoot a lot of shorter exposures and stack them by adding or averaging them together. Each individual exposure must, however, be long enough to get the important faint detail up out of the camera's noise. Stacking can greatly improve an image's signal-to-noise ratio and greatly enhance an image's final appearance.