Planetary Sharpening: The Wavelet Sliders, Dispersion, and Why 4× Looks Worse

A planetary stack straight out of the integrator is soft. That is expected: it is the average of a few thousand frames that each wobbled a little differently, and averaging blurs. The image everyone recognizes (belts, festoons, the Cassini Division as a black line) is made in the sharpening stage, and that stage is six sliders that most people set by copying someone else’s screenshot.

This post covers what those sliders do, what the two related controls (RGB alignment and restoration) do, and one mistake that is easy to make and hard to see: pushing the fine layers to their limit and getting an image that is noisier but not sharper.

What a wavelet layer is

Take the stack. Blur it slightly and subtract the blur from the original. What is left is the finest detail, single-pixel structure. Blur the blur a little more and subtract again, and you have the next scale up. Do it six times and the image is separated into six layers by size. The finest holds pixel-scale texture, the coarsest holds the disc’s overall shading, and the belts and the Cassini Division sit somewhere in the middle.

A wavelet slider multiplies one layer before the image is put back together. Set the third layer to 2× and every feature at roughly that size has twice the contrast. Set the finest layer to 4× and every single-pixel feature has four times the contrast, and on a planetary stack most single-pixel features are noise.

That is the whole tool. RegiStax made it famous. Akastroid’s Sharpening panel has the same six layers, Finest to Coarsest, each with a noise-suppression control that runs before the layer is amplified.

Where the starting values come from

They are not a preset. The tuner measures how much fine-scale structure the stack carries relative to its noise, and how deep the stack is, and sets the layers from that.

Depth matters more than you might think. The finest layer of a 60-frame stack is mostly read noise, and amplifying it produces the crunchy, boiling look that says “oversharpened” from across the room. The finest layer of a 2,000-frame stack is different. Averaging has beaten the noise down by a factor of forty-five, and at that depth the finest scale holds real detail: festoons, the fine ring gaps, small ovals. The tuner lifts it in proportion, from “leave it alone” on a thin stack to a modest gain on a deep one, and eases the noise suppression back at the same time so it does not remove what it just amplified.

The values it lands on are a starting point. They are chosen to be safe, which is not always the best for a given capture, and the sliders are there for the difference.

The 4× mistake

A real case: a Saturn capture, 17,934 frames, the best 12% stacked. A good stack from rough air. The tuner set the three fine layers to roughly 1.4, 2.3 and 2.6. The operator wanted more detail, pushed all three to 4.0 (the top of the range), and turned the finest layer’s noise suppression up to compensate.

The result looked softer. It was not obviously noisier, because the suppression was hiding that, but the Cassini edge was less clean and the belt boundaries less defined than at the starting values, and the globe had a faint grain to it.

Here is what happened. The fine layers on that stack held detail and noise in a ratio the tuner had measured. Multiplying both by 4 kept the ratio. Then the noise suppression, turned up to hide the amplified noise, removed detail at the same scale along with it, because a threshold cannot tell the two apart. The net effect was less detail and more grain than the starting values, and the suppression smoothed away the evidence.

Rule of thumb: on a deep stack the fine layers can go a little past the tuner’s values. If you find yourself raising the noise suppression to make a slider setting tolerable, the slider is too high.

Atmospheric dispersion and RGB align

A planet low in the sky is seen through a lot of atmosphere, which acts like a weak prism. Red and blue light refract by different amounts, so the red image of the disc lands slightly above the blue one, giving a red fringe on one limb and blue on the other. At 30° altitude that is a pixel or two; at 15° it is several. Sharpening cannot fix a disc whose colors are in different places.

RGB align measures the offset of the red and blue channels from the green one and shifts them into registration. It matches on the disc’s structure, after standardizing the channels so brightness differences do not fool the matcher. On the Saturn capture above, the red channel was 0.63 px off and the blue 0.87 px, in opposite directions: a 1.5 px total split that measured under 0.25 px after correction. It is on by default. You would only turn it off for a capture taken through an atmospheric dispersion corrector, where the offset is already zero.

It cannot fix a fringe that is not dispersion. A color camera with a Bayer filter has its own small channel offsets, and a refractor has chromatic aberration; both shift with focus, not with altitude. RGB align corrects the sum of all of them as one shift, which is right for dispersion and approximately right for the rest.

Restoration

This is off by default, and it is worth understanding why before you turn it on.

Restoration is deconvolution: measure how blurred the planet’s limb is, then computationally undo that blur before sharpening. On a deep-sky image the same technique has a built-in check. Stars are point sources, so if the deconvolved stars look wrong, the deconvolution is wrong. A planet has no point source in the frame. The algorithm will produce plausible-looking detail whether or not that detail was in the data, and there is nothing to compare it against.

So the slider starts at zero. Raise it a little, compare with the stack, and stop when you are no longer sure the new detail is real. That point comes sooner than people expect.

Practical order

1. Crop to the planet, so the sharpening is judged on the disc and not on the sky.
2. Leave the wavelets at the tuned values and look. That is the stack as it is.
3. Nudge the second and third layers up if the belts are soft. Back off if the limb starts to ring.
4. Touch the finest layer last, and only on a deep stack.
5. Check the limbs for color fringing. If RGB align was on and a fringe remains, it is not dispersion and the sliders will not remove it.
6. Try Restoration at a low value if the stack is deep and the seeing was good. Turn it off if you cannot tell what it changed.

The short version

• Six layers by scale, and each slider multiplies one layer. Belts live in the middle layers, noise in the finest.
• The tuned values are measured from the stack’s depth and detail, not copied from a preset.
• Pushing the fine layers to 4× and hiding it with noise suppression loses detail. If you need more suppression, lower the slider.
• RGB align corrects dispersion to a fraction of a pixel. Leave it on.
• Restoration invents detail as readily as it recovers it. Start at zero.

If you already sharpen with wavelet sliders, Akastroid vs RegiStax explains what changes when the strengths are measured from the stack, including the comparison nobody has measured yet.