When photographing faint deep-sky objects, taking a single exposure that lasts for several hours is not practical. Instead, astrophotography often uses a method called image stacking. For example, 60 exposures of 5 minutes each can be combined to create a result similar to a single 5-hour exposure.

Each exposure is a measurement of light, and during this process, different kinds of errors appear. Equipment-related errors can be reduced using image calibration methods such as bias, dark, and flat frames. However, random noise still remains.

This remaining noise comes from the random arrival of light particles, known as photons. This randomness follows as Poisson distribution. For instance, even if the average number of photons is expected to be 10, an individual exposure might record 8, 9, or 12. By taking many exposures and combining them, this randomness is averaged out. As more images are stacked, noise decreases and fine details become clearer.

Image stacking does more than simply average brightness. In some cases, the median value is used, or unusual data points are removed before averaging. When exposure time, equipment settings, or sky brightness differ between images, additional steps such as normalization and weighting are needed.

In real practice, there is no single best choice between taking 10 images at 30 minutes each or 100 images at 3 minutes each. Several factors must be considered:

a. Tracking accuracy of the equatorial mount: Very long exposures can cause stars to stretch into lines.

b. Camera saturation limit: Once a pixel is full, it can no longer record more light.

c. Read noise: Very short exposures increase the effect of electronic noise.

d. Storage space and processing time: Large numbers of images require more data storage and longer processing.

Image stacking is a key technique in astrophotography. By patiently collecting light over time, faint signals from deep space slowly rise above the noise, revealing the true beauty of the night sky.