The observation and its solar-cycle context
Spektrum's image data gives the observation time as 12:09 UT. Meyer's setup paired the spectroheliograph with a ZWO ASI 178MM camera, an 80/560 mm ED refractor and a Skywatcher EQ6-R mount. He used JSol'Ex 5.5.0 for post-processing.
The observation falls in the declining phase of Solar Cycle 25. In a September 30, 2026, report in The Boston Globe, NOAA space-weather expert Shawn Dahl said sunspot numbers appeared to have peaked around 2024–2025 and had since fallen. The count had dropped from more than 200 sunspots visible at once at the cycle's height to fewer than 100 by 2026. Dahl cautioned that some of the most impactful geomagnetic storms can still occur one to three years after the peak.
Building a solar image from a scan
Rather than taking a direct photograph of the Sun, a spectroheliograph records narrow slices that software later assembles into a full-disk image. As the mount slowly slews, the solar image moves across a slit and the camera records video.
Inside the instrument, a collimator makes the light passing through the slit into parallel rays. A diffraction grating separates those rays into a spectrum, which a lens refocuses onto the camera sensor. Tilting the grating selects the wavelength to record: H-alpha in this case, with sodium D, helium D3 and Ca K among the other possibilities.
The SHG 700's optical components include a holographic grating with 2,400 lines/mm, a quartz slit 7 mm long and 7 microns wide, and two 72 mm flat-field compound lenses. Its bandpass is approximately 0.182 Å FWHM. Retailer Testar says the instrument can cover the full solar disk with telescopes having focal lengths up to about 700 mm, a range that includes Meyer's 560 mm refractor.
JSol'Ex handles the reconstruction by processing SER video files into solar-disk images. The free, open-source Java application uses the Apache 2 license and supports Christian Buil's Sol'Ex as well as other spectroheliographs, including the SHG 700. Its current documentation describes version 5.5.2, slightly newer than the 5.5.0 credited for Meyer's image.
Narrow-band imaging and its trade-offs
Manufacturer MLAstro describes a narrower effective bandpass as an advantage of spectroheliographs over conventional etalon H-alpha filters. It puts a top-grade etalon filter at about 0.5 Å FWHM, while an SHG resolves roughly 0.06–0.1 Å per pixel. According to the company, the approach provides very high contrast without the photospheric light leakage associated with single-stack etalons.
That spectral performance comes with limitations. MLAstro says spectroheliographs generally deliver lower spatial resolution than etalon systems of the same aperture and are unsuitable for very long focal lengths.
From a 19th-century instrument to digital research
The digital workflow is recent, but the underlying instrument dates to the 1890s, when George Ellery Hale and Henri-Alexandre Deslandres independently developed the spectroheliograph. Robert R. McMath improved it in 1932 to record motion pictures. Advances in hardware and software later enabled the digital, software-reconstructed approach used for Meyer's image.
Amateur instruments in this class have also contributed to published scientific work. A 2025 paper by Váradi Nagy and Pietrow presents an atlas of solar spectroheliograms spanning 3641–6600 Å, with spectral resolution R of approximately 20,000–40,000 and average spatial resolution of about 2.5 arcseconds. The team collected the observations over three months during the 2025 solar maximum using Sol'Ex and ML Astro SHG 700 spectroheliographs.
