YAGEx

Overview

A perfect sky would be uniform. In practice every image carries a veil that varies from edge to edge: the city lights one corner of the field, the Moon another, the optics darken the corners. Removing it looks simple, except that a large diffuse nebula looks a lot like a gradient. Too aggressive a tool erases it along with the veil, and the result looks clean precisely because your signal is gone. YAGEx attacks that trade-off, and it aims at a weakness the current tools share: they reject their background samples against a global statistic that is itself distorted by the very gradient being removed. So YAGEx looks for the real sky rather than for the dark areas. It examines the texture of each region, because pure sky has a recognisable statistical signature whether it is bright or dark: a background area on the lit side of the gradient is recognised as background, and faint nebulosity, however dark, does not pass itself off as sky. It then fits its model to the bottom of the distribution, where only the sky lives, since everything that shines merely adds light. And rather than asking you to guess a degree or a smoothing factor, it puts a range of models into competition, from the rigid polynomial to relaxed splines, and keeps the one that best predicts the background in areas it has not seen. Subtraction for light pollution, division for vignetting, 32-bit internal computation, optional diagnostic images to see exactly what was removed. YAGEx is a native Process written in C++, and it is developed against a bench where the truth is known: synthetic gradients first, then real masters into which a measured gradient is injected. Two numbers are tracked on every image, the departure from the true background and the share of nebula flux preserved, because a perfectly flat background obtained by shaving the nebula off is not a success.

Features