As is known, the Sun is the closest star to us. It is also the only star whose disk and details in the atmosphere can be seen in an amateur telescope. Of course, for safe observation and photography of the Sun, it is necessary to use either protective light filters or a special solar telescope with built-in light filters.
At first glance, photographing the Sun is not difficult at all – just install a protective light filter on the telescope and go ahead. The Sun turns out either white or yellow (depending on the light filter). Based on my own experience, I have concluded that there are three most interesting spectral ranges for photographing the Sun:
Red region of the spectrum – H-alpha, 656.28 nm, chromosphere. In this range, prominences, active regions, solar flares are visible. A very narrow bandwidth is required (about 1 angstrom).
Green region of the spectrum – 540 nm, photosphere. In this range, the intrinsic aberrations of the telescope are best corrected. This range is good for photographing spots and granulation. A narrow bandwidth is not necessary, but desirable (about 10 nm).
Near ultraviolet, namely 393.3 nm (CaK). In this range, sunspots, granulation, active regions, facular fields, and prominences are visible. A narrow bandwidth is desirable (less than 3 nm, ideally about 1 angstrom).
At the moment, there are a number of different methods for processing and displaying the Sun – regular, pseudo-color, inversion, collages, etc. However, for myself, I have settled on the following directions:
1) Monochrome Sun. I convert photos taken even with a color camera into monochrome and leave them black and white. The main advantage is the low dependence of the display on the type of screen, since monochrome looks similar on different devices, be it a smartphone or a TV, there are no problems with setting the color balance, etc. Of course, such photos are inferior in color to color ones, but they take up less space with better quality.
2) Pseudocolor (false colors). We shoot the Sun in monochrome, but we make the color close to the real spectral range. For example, when shooting in the green region of the spectrum at a wavelength of 540 nm, even through a monochrome camera, we convert the final image into color, cut off the red and blue channels to zero, leaving only green. We get a green image of the Sun, as if through a 540 nm filter. Color variations are allowed for the red and ultraviolet range, this is up to the taste of the processor.
3) Multispectral image. To obtain such an image, it is necessary to sequentially obtain three images in different spectral ranges (H-alpha 656.28 nm, 540 nm and CaK 393.3 nm), then combine all three images into one color RGB image. As a result, you will get an approximate of each spectral range of its spectral region, as well as good visibility of prominences, sunspots and granulation. Yes, the visibility of the chromosphere will noticeably deteriorate, but no one forbids additionally publishing these images separately, because each spectral range is quite self-sufficient.
In my opinion, it is the multispectral image that is the logical result of photographing the Sun in the three most interesting spectral ranges.
To photograph the Sun in the H-alpha line, I use the Coronado PST H-alpha 40 mm chromospheric telescope, for shooting in the green spectrum – the Baader Solar Continuum filter (540 nm) + additional protective filters, and I shoot the near ultraviolet through the Antlia CaK 393.3 nm filter.
Photographs of the Sun for multispectral imaging can be taken through different cameras and telescopes, preferably over a short period of time (up to 10-15 minutes). In total, it takes me about 3 minutes to shoot the necessary material, plus time for rearranging filters, telescopes, etc. The shooting sequence can be any. It is desirable that the telescopes have the same focal length and use the same monochrome camera – for example, the Coronado PST H-alpha 40 mm and Levenhuk Ra 66ED telescopes have a focal length of 400 mm. To monitor the entire solar disk, it is desirable that the Sun be placed entirely in the field of view of a particular camera – for a focal length of 400 mm, cameras with a monochrome Sony IMX178 sensor are ideal for these tasks.
My shooting algorithm is approximately as follows:
1) I shoot the Sun through a chromospheric telescope, a 1-minute video clip, QHY5III178m astrocamera. This is the red channel.
2) I remove the chromospheric telescope from the mount, install a regular lens telescope + Herschel wedge + Baader Solar Continuum filter + ND3.0 filter + QHY5III178m astrocamera. This is the green channel.
3) I leave the same telescope, but remove the Baader Solar Continuum and ND3.0 filters on the Antlia CaK and ND96-09. This is the blue channel.
4) I stabilize and crop each of the resulting videos in PIPP, sort and stack the frames in Autostakkert, add sharpness in AstroSurface. In Adobe Photoshop, I move the levels and curves. As a result, I get three clear monochrome images of the Sun in different spectral ranges.
5) In Adobe Photoshop, I open each of the documents, layer them on top of each other, rotate them, center them and adjust the scale, focusing on the spots.
6) In Adobe Photoshop, I create a new RGB image and insert the resulting images into each of the color channels: H-alpha into the red channel, Solar Continuum into the green, Antlia CaK into the blue.
7) In the same Adobe Photoshop, I launch Adobe Camera RAW, select the tool for working with white balance, select the central area of the Sun with a frame, and get a more or less adequate color balance. If necessary, I add detail with the Texture tool. If necessary, I fill in the black fields around. Done.
I’m sure that if you are a fan of photographing the Sun, then sooner or later you will come to this technique yourself or at least try it.