Ever since Google launched its Night Sight feature on the Pixel 3 series the low light photography feature has been very popular with users. On the new Pixel 4 Google has updated Night Sight with a specific mode for astrophotography. The team behind it has now authored a blog post to explained the function in more detail.

In order to capture as much light as possible without using shutter speeds that would require a tripod and/or lead to blur on any moving subject, Night Sight splits the exposure across multiple frames that are aligned to compensate for camera shake and in-scene motion. In a second step the frames are averaged to reduce noise and increase image detail.

The astrophotography feature uses the same approach in principle but uses longer exposure times for individual frames and therefore relies on tripod use or some other kind of support.

Image with hot pixels (left) and the corrected version (right)

The team decided exposure times of individual frames should not be longer than 16 seconds to make the stars look like points of light rather than streaks. The team also found that most users were not patient enough to wait longer than four minutes for a full exposure. So the feature uses a maximum of 15 frames with up to 16 seconds exposure time per frame.

At such long exposure times hot pixel can become a problem. The system identifies them by comparing neighboring pixels within the same frame as well as across a sequence of frames recorded for a Night Sight image. If an outlier is detected its value is replaced by an average.

In addition the feature uses AI to identify the sky in night images and selectively darken it for image results that are closer to the real scene than what you would achieve with a conventional long exposure.

This image was captured under the lighting of a full moon. The left half shows the version without any sky processing applied. On the right the sky has been slightly darkened for a more realistic result, without affecting the landscape elements in the frame.

Night Sight is not only about capture, though, it also includes a special viewfinder that is optimized for shooting in ultra-low light. When the shutter is pressed each individual long-exposure frame is displayed as it is captured, showing much more detail than the standard preview image. The composition can then be corrected and a new Night Sight shot triggered.

Some of the results we have seen have been impressive. For more more technical detail head over to the original post on the Google blog. A n album of full-size sample images can be found here. The team has also put together a helpful guide for using the feature in pdf format.