Calcium K

Goal: a violet picture of the Ca II K network and plage from a calcium K capture.

Not tested on a real capture. Helia's Ca K settings come from its filter preset; the project has no Ca K capture of its own yet, so this page has no real output to show. The advice follows what the preset does and why.

You need: a capture through a Ca II K filter (393.4 nm), with CaK in the file name or folder (CaK_session/capture.ser works). Ca K is at the violet edge of what a sensor sees and the line is faint, so the signal is weak and the noise is the problem.

Settings

Press Ca II K on the Quick stack card, or use --preset cak. The preset recommends:

• the blue channel for a colour camera, because the blue pixels are the ones that see 393 nm (a mono camera has only one channel);
• 30 per cent of the frames, a few more than for H-alpha, to average the noise down;
• gentler sharpening: wavelet gains 1.30, 1.40, 1.25, 1.05 with higher noise cuts (2.5, 1.5, 0.8, 0.3) and a short deconvolution of 8 iterations. Sharpening noise only makes a prettier mess.

helia stack cak_001.ser -o cak.tif --preset cak --sharpen wavelet
helia stack cak_001.ser -o cak-negative.png --preset cak --sharpen wavelet --invert

The second command inverts before colouring, which makes a negative; it shows filaments and prominences well, and some people prefer it for calcium. Try both.

Tips

• Exposure and gain: the line is faint, so light is the constraint. Keep the exposure short enough to freeze the seeing and raise the gain until the histogram sits at about 50 to 70 per cent of full scale; many frames beat few long ones. See Exposure, gain and bit depth.
• Do not push the wavelets. If the picture looks processed, lower layer 1 and 2 gains first.
• Ca H (--preset cah) takes the same settings and a slightly redder violet, so a K and H pair can be told apart.

Related: Colour presets.