Arcsinh Stretch

Step: Stretch

Brightens a linear astro image with the inverse hyperbolic sine. Faint structures are lifted strongly, bright stars burn out less than with a plain gain, and on color images the stars keep their hue.

Parameters

Parameter	Range	Default	Effect
Black point	0 … 1	0	Everything at or below this value becomes black. Raise it to darken the sky background. Must be below 1. The histogram shows it as a marker.
Stretch	1 … 1000	100	How strongly the image is brightened. Higher values lift fainter signal and compress the bright end more. The slider is logarithmic (middle ≈ 32); the number field takes the exact value.

How it works

1. The black point is subtracted and the rest is rescaled to 0…1: x = max(v − black point, 0) / (1 − black point).
2. Mono images are mapped with the arcsinh curve, which is almost linear near black and logarithmic towards white. It maps 0 to 0 and 1 to 1 (β is the Stretch value):

y = asinh(β · x) / asinh(β)

3. Color images are stretched through their brightness I, the mean of the three channels. All three channels are multiplied by the same factor, so their ratios, and with them the hue, stay intact:

k = asinh(β · I) / (asinh(β) · I)
R' = k · R    G' = k · G    B' = k · B

If a channel would end up above 1, all three channels are divided by the largest one instead of being clipped separately. Clipping each channel on its own would turn bright stars white-ish and shift their color.

The method follows R. Lupton et al., "Preparing Red-Green-Blue Images from CCD Data", PASP 116, 133 (2004), which builds on R. Lupton, J. E. Gunn and A. S. Szalay, AJ 118, 1406 (1999).

When to use it

• Use it instead of Histogram Stretch when bright stars burn out or lose their color.
• Start with the default Stretch and set the Black point just left of where the histogram starts to rise. Clipping too much destroys faint signal for good.
• Stretch in several small steps rather than one large one. Every step is in the history and can be undone.