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RHEF

The Radial Histogram Equalizing Filter. Solar images usually show a bright ball in a dark void; RHEF shows the corona that is actually there, on the disk and off the limb, with no parameters to tune.

pip install sunkit-image

import sunpy.map
from sunkit_image.radial import rhef

rhef(sunpy.map.Map("aia_171.fits")).peek()

The paper (Solar Physics, 2025) Worked example API reference Try it in your browser

Works frame by frame, so movies need no dataset-wide statistics.
Treats the disk and the off-limb corona the same way across the whole field of view.
Optional Upsilon tone curve, a symmetric cousin of gamma correction, for finer control.
Part of sunkit-image, SunPy's image-processing package. An IDL version is described in the paper.

How it works

RHEF splits the image into rings around the Sun's centre and replaces each value by its rank among the values in its ring. A faint ring and a bright ring then both span the whole range from dark to light.

A teaching visual, not a figure from the paper: a synthetic Sun is unrolled so each row is one ring, every row is ranked, and the strip rolls back up. The rings are equal width and the Upsilon curve is off. Nothing plays until you press play. Made with tools/make_rhef_unroll.py.

Every implementation

The same filter ships in several places. Each one names the convention it follows and lists, by id, where its code still differs; the ids are defined in the RHEF conventions.

Implementation	Where	Convention	Differs by
sunkit-image 0.7.0 (Python)	PyPI	`sunkit-0.7`	none: it defines the convention
sunback (the live Sun on this site)	GitHub	`sunkit-0.7`	none
My Heliogram (prints)	myheliograph.com	`sunkit-0.7`	none
HelioFITS (Swift)	Mac App Store	targets `oRHEF-2.0`	`UPS-MEAN` `TIES-TOL` `KEY-QUANT` `GEOM-GRID` `DTYPE-IN`
HelioFITS Studio, ranking (Java)	GitHub releases	targets `oRHEF-2.0`	`RANK-N1` `POS-ONLY` `FP16` `MIN-BIN` `GEOM-PX` `UPS-EXT` `DTYPE-IN`
HelioFITS Studio, Upsilon (GLSL)	GitHub releases	targets `oRHEF-2.0`	`RANK-EXT` `UPS-CLAMP` `UPS-DEFAULT`
IDL	described in the paper	targets `oRHEF-2.0`	`RANK-N1` `TIES-INTERP` `GEOM-CRPIX` `GEOM-PX` `NO-NAN-GUARD` `DTYPE-IN` `UPS-DEFAULT`

Rows from the fastRHEF conventions registry, read 2026-10-15. RHEF output is a visualization, not a calibrated radiance.

Cite it

Gilly & Cranmer (2025), Visualization of High Dynamic Range Solar Imagery and the Radial Histogram Equalizing Filter, Solar Physics.

@article{GillyCranmer2025,
  author  = {Gilly, C. R. and Cranmer, S. R.},
  title   = {Visualization of High Dynamic Range Solar Imagery and the Radial Histogram Equalizing Filter},
  journal = {Solar Physics},
  year    = {2025},
  volume  = {300},
  pages   = {174},
  doi     = {10.1007/s11207-025-02578-x},
  eprint  = {2511.02798},
  archivePrefix = {arXiv}
}

arXiv