6. Sharpen¶

A stack is clean but soft. This step brings the fine detail back. It works on the mono stack, before any colour, and is non-destructive: your choices are a list of operations applied to the stack for the preview and again, at full size, when you export.

Sharpening can create artefacts. Halos beside bright edges, ringing at the limb and round sunspots, and noise that turns into a texture that looks like detail. None of the methods can add detail that the telescope and the seeing did not record. Ease off as soon as the picture looks different in kind rather than just crisper.

Limb darkening¶

The first control is Limb darkening: Off, Model or Profile, and the law to fit (Linear, Quadratic, Neckel). The line under the chips is the fit made on your stack, for example quadratic: u1 = 0.472, u2 = 0.046; limb at 48% of the centre; rings differ from the law by 0.66% rms. Model divides the disc by that law and Profile by the measured radial profile, so the sharpening below sees flat contrast from the centre to the limb. Only the disc changes. It is off by default; try it with Before / after, and see Limb darkening for what the fit measures on real captures.

The Sharpen step with Model chosen under Limb darkening, split before and after

The method chips¶

The Sharpen step with Wavelets selected: scheme, filter, and an amount, sharpen and denoise for each of the six layers

The row of chips at the top picks the method: Off, Wavelets, Unsharp or Deconvolution. Helia has already switched on the recommended wavelets for your filter, so the picture you see when you arrive is sharpened. Press Off to see the plain stack. Every slider updates the picture as you drag it, and a line says how long the last preview took (a few tens of milliseconds in the picture).

Three chips above the picture help you judge: Before / after splits the picture at a line you can drag (the stack as it was on the left, with your edits on the right), Full size keeps the preview at full size while you drag (otherwise it is shrunk while you drag and made again at full size when you let go), and 1:1 shows the picture at 100%. A preview is a close look, not a proof: what is written is always made from the full-size stack.

Wavelets¶

The image is split into six layers by the size of detail. Layer 1 holds the finest detail and most of the noise, layer 2 fine detail such as granulation and fibrils, and higher layers bigger structure. The controls are those of RegiStax 6, and a user of that program will find the same things in the same order:

Dyadic or Linear, an Initial layer and (for Linear) a Step: how the layer sizes grow.
Default or Gaussian: the smoothing kernel.
Linked: each layer works on the result of the finer ones.
For each layer an Amount % slider (the RegiStax layer slider, 0 to 200), a Sharpen value (gain 1 + amount x sharpen squared: 0.14 is 3x, 0.5 is 26x, 1 is 101x at 100%) and a Denoise value (detail weaker than ten times it, in noise sigmas, is removed).

Under the grid a line gives the gain of each layer. The recommended values for a filter are in Sharpening and adjustments. Reset to recommended puts them back. The mapping from RegiStax, and a worked example, are in Coming from RegiStax and Sharpening like RegiStax.

Here is the effect of the recommended wavelets on a real H-alpha stack (250 of 1,000 frames of 15_32_09.avi). This is a 120 pixel crop at 3x, the plain stack on the left and the recommended wavelets on the right:

A crop of the stack: before sharpening on the left, after the recommended wavelets on the right

The mottling and the filament at the left are much crisper. Measured over the disc-centre crop, the band of 1 to 4 pixel detail rises from 0.96% to 2.17% of the mean, about 2.3 times, and the pixel-scale band (mostly noise) from 0.30% to 0.62%, about 2 times. That is the point of the numbers: the detail goes up faster than the noise rises, and the noise cut on layer 1 keeps the grain from taking over. If the picture looks processed rather than just crisp, lower the sharpen of layer 2 or raise the denoise of layer 1.

Dumbbells of contrast, before then after sharpening: 1 to 4 px detail 0.96 then 2.17 per cent of the mean; pixel scale 0.30 then 0.62.

Band-pass contrast of the disc-centre crop before and after the recommended wavelets, from helia measure.

The Sharpen step with RegiStax-style settings: Gaussian filter, layer 1 at 0.14 and 0.20, layer 2 at 0.13 and 0.15, layer 3 at 50%

Unsharp mask¶

The Sharpen step with the unsharp mask: radius, amount and threshold

The radius is the size of the blur that is subtracted, which picks the size of detail that is boosted. The amount is how much of the difference is added back (0.6 is gentle, 2 strong). The threshold leaves weaker detail alone so noise is not boosted. It is a light, predictable touch:

A crop of the stack: before on the left, after the unsharp mask on the right

Deconvolution¶

The Sharpen step with Richardson-Lucy deconvolution: blur, iterations, noise control and the limb guard

Richardson-Lucy deconvolution tries to undo the blur itself, by repeated correction. It has the blur to undo (the PSF), the number of iterations, a noise control and a limb and sunspot guard that stops the bright halo that a bright edge would otherwise grow. It is the strongest and the riskiest method.

A crop of the stack: before on the left, after twelve iterations of deconvolution on the right

On this capture it gains little and makes the grain coarser, which is why wavelets are the default: stop when noise starts to look like texture. The page Sharpening and adjustments has the measurements on four real captures.

My presets¶

My presets, below the controls, keeps the whole set of sharpening and adjustments under a name. Type a name and press Save; press a preset to use it. The command line takes the same presets with --edits.

Next¶

Adjust.