Colour presets

Why each preset recommends what it does:

• H-alpha (ha): H-alpha is deep red, so a colour camera records it almost only in the red pixels; the green and blue pixels add noise, not detail.
• Ca II K (cak): Ca K is at the violet edge of vision and a sensor sees it in its blue pixels. The line is faint, so a few more frames (30%) are kept to average the noise down.
• Ca II H (cah): Ca H sits 3.4 nm from Ca K, so it takes the same channel and frame share. It is drawn a touch redder so the two can be told apart in a pair.
• Mg I b (mg): 517.3 nm is green, which is where the green pixels of a Bayer sensor respond. The line is dim, so 30% of the frames are kept.
• Na I D (na): 589 nm is yellow, which a Bayer sensor splits between its red and green pixels, and the sodium filter passes almost nothing to the blue ones. Averaging all three channels mixes those empty blue pixels into the stack and prints the mosaic as a fine diagonal hatch over the granulation, so the green channel (two pixels in four) is stacked. No real sodium capture has been measured yet: the captures this was first tuned on are H-alpha. It shows granulation, so only the best 12% of frames are kept and the alignment points are a few granules wide.
• He I D3 (he-d3): 587.6 nm is yellow-green, shared by the red and green pixels, so the luminance of all three is used. It is drawn yellower than sodium.
• H-beta (hbeta): 486.1 nm is blue-cyan and a sensor sees it in its blue pixels. It is faint, so 30% of the frames are kept.
• White light (white): A white-light capture already carries all three colours, so the luminance is stacked, and a slight warm tint stands in for the solar spectrum. It shows granulation, so only the best 12% of frames are kept and the alignment points are a few granules wide.

The numbers behind each colour, for anyone who wants to reproduce it:

The recommended sharpening of each filter (see Sharpening and adjustments). Wavelet gains and noise cuts are for layers 1 to 6, finest first; the unsharp radius comes from the image scale, and the deconvolution blur is measured from the disc.

Why each filter is sharpened the way it is:

• H-alpha (ha): H-alpha has plenty of signal and its fibrils, plage and filaments sit on layers 2 to 4, so those layers are lifted most; the finest layer is lifted least and keeps a high noise threshold. Measured on four real captures, this adds 3 to 5 points more fine detail and 10 points more fibril-scale contrast than the old 1.5, 1.45, 1.25 for 14 points less pixel noise.
• Ca II K (cak): Ca K is faint and noisy, so the gains are gentler, the noise thresholds higher and the deconvolution short: sharpening noise only makes a prettier mess.
• Ca II H (cah): Ca H is as faint as Ca K and takes the same careful settings.
• Mg I b (mg): Mg b2 is dim, so the noise thresholds are a little higher than for H-alpha and the deconvolution shorter.
• Na I D (na): Sodium D is photospheric: granulation is its finest low-contrast detail. Layers 2 and 3 (granule size) are lifted most and carry no noise threshold, because a threshold there removes granulation with the noise; the finest layer keeps its threshold.
• He I D3 (he-d3): He D3 shows only in faint prominences and flares, so it is sharpened with the most restraint.
• H-beta (hbeta): H-beta is faint, like the calcium lines, and takes the same gentle settings.
• White light (white): White light has the most signal and the finest structure (granulation), so it takes the strongest gains and the longest deconvolution. Layers 2 and 3 (granule size) are lifted most and carry no noise threshold, because a threshold there removes granulation with the noise.