If you have shot all of your sub images at the same ISO and exposure time, you can skip this entire page and go directly to the next section.

If you have Images Plus v3.75, you can do this automatically by checking the Normalize box under the Process Tab in the section at the bottom called Perform After Calibration and Color Conversion.

Normalizing - Stacking Images Shot at the Same Exposure but Different ISOs

If images are to be stacked that are shot at different ISO settings, they must first be "normalized" before being stacked. This process modifies the images so each will contribute equally to the final image.

Say we have a 5 minute exposure at ISO 800 and a 5 minute exposure at ISO 1600. Even though the same amount of photons were collected in the same amount of time, the pixel values in each image will be different because of the ISO gain factor that is applied in the camera (ISO 800 vs ISO 1600).

To normalize these two images, the pixel values in the ISO 800 image must be multiplied by 2 to equal the pixel values in the ISO 1600 image. This is done with the "pixel math" function in an astronomical image processing program like Images Plus.

The Images Plus Pixel Math Dialog

After the ISO 800 image is normalized to the ISO 1600 image, the individual sub-frames are then added or averaged together as described in the previous section on stacking.

Weighting - Stacking Images Shot at Different Exposures but with the Same ISO

Images shot at different exposure times, but at the same ISO, can be directly added or averaged together. No normalization or weighting is necessary.

For example, a 5 minute exposure at ISO 800 and a 10 minute exposure at ISO 800 can be added directly to produce the equivalent of a 15 minute exposure. The problem that can crop up here is clipping. If the program you use does not work in a high enough bit depth, pixel values for bright objects may become clipped and contain no detail. In this case, it is better to use the averaging method of combining the images.

Stacking Images Shot in Combinations of Different ISOs and Exposures

If images are shot in combinations of different ISOs and exposures at the same time, the ISOs must be normalized before stacking.

After normalization, all of the images can then simply be added or averaged together.

Averaging and adding multiple images are best done in an astronomical image processing program.