What is Exposure Tracking?

As camera technology advances, photographing the night sky becomes possible for photographers of all levels. Low-light performance continues to improve, allowing us to photograph the stars at higher and higher ISOs. However, digital noise continues to be one of the biggest challenges for astrophotographers.

Technically, some amount of noise will always be in every photo. There is nothing you can do to prevent this; it is a physical property of light and photography. There are two broad types of noise in your photographs: shot noise and digital noise. Shot noise and digital noise are typically hard to distinguish from one another when you look at the final photo, since they generally lead to the same result: pixels that are randomly too bright, too dark, or discolored. Digital noise, or electronic noise, is randomness caused by the camera sensor and internal electronics, which introduce imperfections to an image. 

A photo is made up of signal and noise. Signal is the light we want that makes the image we are capturing. Noise is the stuff we don’t want, the grainy veil across the face of our image. The best benefits about stacking multiple exposures is the dramatic increase in the image quality.

This happens by removing noise and increasing the image signal. Noise, is randomness due to photons in light in the scene you are photographing, which are discreet and random. There are a number of different approaches to dealing with digital noise  Noise can be affected by changing camera settings to the way an image is processed in post-production. 

Digital noise is caused by a couple of things. Firstly, the camera sensor heats up as it exposes an image, causing an increase in noise. Secondly, an increase in sensor sensitivity, or ISO, can lead to more digital noise in your images. Both high ISO values and long exposures will lead to more digital noise, stacking is the best procedure to maintain signal strength and remove noise.

Stacking reduces the differences in the noise created by the camera sensor. Each time an image is shot, the electrical characteristics of the sensor cause it to do its best at representing the photons it “sees. From shot to shot, there are 

slight brightness and color variations on each pixel for the exact same image. By stacking 10 Images. that is 10 layers of the same camera position and settings. Then do a "Median Stack", an intelligent average of each pixel of all

exposures, detail for detail, instead of trusting just one exposure to have a good signal to noise ratio. Exposure stacking then becomes very effective in reducing the digital noise in a photo. The final image will show the parts of the image that is consistent through each layer, like the stars. Digital noise being random, will cancel each other out. This is due to the changes from one exposure to the next, it will not be visible in the final stacked image.

Eight of the top images were stacked in Photoshop for the ground and Five of the lower image was stacked in Sequator for the sky. The sky and land are then blended to produce the final image. This is called a stack/blend because the the camera and tripod stayed in a fixed position while taking the image. This is the final result.