FITS n’ Finish
Gradient removal for astrophotography that starts from the sky itself, on macOS and iOS. The glow of the atmosphere is modeled and subtracted first, so faint nebulosity is left alone.
Download for macOSStart from the real sky
Most background tools treat your image as just numbers: place enough sample boxes, fit a flexible surface, and hope it doesn’t erase the faint nebula along with the glow. FITS n’ Finish starts from the physics instead. It knows where the telescope pointed, when, and through how much air, and from that predicts the shape of the sky glow across the frame. That glow is matched to your image and removed first. What is left is smooth enough for a very simple surface to finish the job — and a surface that simple cannot bend itself around faint, extended targets. Outer galaxy halos and dim nebulosity survive, and you can look at exactly what each step took away.
Pointing From Your File
Plate solutions, coordinates, capture time and optics details in the file set where the telescope was pointing and how the sky was oriented, automatically on open.
Color-Aware
Sky glow is bluer and steeper than it is red, so each color channel gets its own model instead of one shared gradient.
Moonlight
The moon’s position and phase at capture time are computed and its scattered light is included in the model.
Light Domes
The glow from a nearby city is modeled in its direction and saved with per-site presets.
Long Exposures
Over a long session the target and the moon move; the model averages the sky across the whole integration.
Star-Proof Fitting
Bright stars and their halos are masked out so they cannot tilt the background fit.
All of FITS
Color stacks, multi-image files and every common FITS compression open directly. Plate-solve data survives export.
Inspectable Stages
Switch between the original, the modeled sky, the fitted surface and the result to see exactly what was subtracted.
Fast
The whole pipeline runs on the GPU, with results identical to the reference implementation.
Fits your workflow
Use it before stretching, alongside Siril or PixInsight: open a stack (FITS or XISF), remove the gradient, export a 16-bit FITS with the plate-solve data intact, and carry on. Rig and site presets are plain JSON files you can share between machines, and a command-line tool runs the same engine for scripting. Workflow guide for Siril and PixInsight users.