forum.xda-developers.com/showpost.php?p=80974497&postcount=1888

Requirements:
OS: Android 9 or 10
Device: Google Pixel series (some other devices may work, but are not supported in the traditional sense - please do not post in this thread about non Pixel devices)

Download: PXv4.0_GoogleCamera_7.2.014.apk

Astrophotography instructions: forum.xda-developers.com/showpost.php?p=80678229&postcount=1507

Changelog: Initial release based off Google Camera 7.2.014. Supported features are shown in the image below. "Default" means your stock app already has this feature; "Y"/"OK" means this was added to the listed device; "N/A"/"Disabled" means the feature could not be enabled for the listed device, either because of unsolved crashes, missing hardware, or messing up a fundamental function.
An extra shout out to marcello for the initial Lib Patcher, further implemented and tweaked by johngalt1, and Urnyx05 for some Android 9 fixes.

Some notable features:

Hold the camera switch icon for quick access to settings.

Lib Patcher: this can be used to modify certain parameters like sharpness and noise removal amount. The settings here will depend on what you like, there is no "best" setting. Please read the info in "Learn More" within the app under the enable button.

The max exposure setting can be adjusted but does not mean it will always choose this shutter speed. GCam will automatically detect certain parameters and choose the most optimal shutter speed. A stock Pixel 1 supports up to 2s, Pixel 2 up to 4s, Pixel 3/3a up to 12s, Pixel 4 up to 30s. Using Magisk Modules in post #2 can make this longer, but Google decided on 16s due to star trails and the perception of blur being about the limit around 16s. By default, this APK selects 12s for Pixel 3 and 3a as those devices support higher exposure times than the stock APK allows for Astro.

There are 2 buttons that can be added to Night Sight. Forced Astro mode, and Light Painting Mode. Use a tripod or tripod-like structure for Light Painting Mode or if the scene is really dark. Handheld will not be good.

www.celsoazevedo.com/files/android/google-camera/f/changelog856/