A FIT (Flexible Image Transport System) file is a data format commonly used in astronomy for storing and sharing astronomical images, as well as other data such as spectra and tables. It was developed by NASA and the International Astronomical Union as a successor to the older FITS format, with the goal of improving its flexibility and usability. FIT files can store multiple data arrays and can be compressed to reduce their size while retaining full data fidelity. FIT files are widely used in astrophotography software, such as DeepSkyStacker, PixInsight, and AstroPixelProcessor.

In astrophotography, a FIT file will often contain raw bayer data. This is a raw image format, meaning it contains the unprocessed image data directly from the camera sensor, without any demosaicing or color correction applied. Astrophotographers often use specialized software to process Bayer data and create high-quality color images.