Acorn by Flying Meat Inc. is a powerful image editor for macOS that supports nondestructive levels, curves and filters, layer masks, batch editing and many tools that work on pixels, text and shapes. It supports images with 8, 16 and 32 bits per channel. And you don’t need a subscription. It is a cost-effective solution for post-processing astronomical images.
After installing the plugins, you can open FITS, XISF and SBIG images with Acorn as easy as a JPEG or TIFF. The plugins are free, but you do need Observatory and Acorn to use them. They are a great addition to Observatory’s Quick Look and Spotlight plugins for the same image formats.
Read more about it here.