First Amateur Image of the K-corona

The solar corona provides a gorgeous view during a total solar eclipse, but it is an elusive amateur target outside of an eclipse because it is about a million times dimmer than the photosphere right next to it. Bernard Lyot first viewed and imaged the corona outside of an eclipse during the 1930’s and was followed by professionally equipped astronomers over the years, but amateurs did not succeed until 2020.

The corona has several elements. The corona you mainly see during a total solar eclipse is the K-corona, made of photospheric light reflected off free electrons. About 100 times dimmer and not normally noticeable, the E-corona consists of light emitted by highly-ionized elements. The F-corona is more of a diffuse glow seen further from the Sun, consisting of light reflected off dust.

The general approach invented by Lyot is to focus the Sun on a disc that blocks the photosphere, creating an artificial eclipse, and to employ lenses and stops to reduce light that is scattered by the instrument; the instrument is called a coronagraph. A coronagraph combined with clear skies, especially from a mountaintop, can reach the corona. While the E-corona is dimmer, it is emitted in narrow bands that benefit from a narrowband filter. The filter can improve the signal-to-noise ratio by 3000 times, resulting in an overall 30 times gain compared to the brighter K-corona. The first amateur image was taken in 2020 at 530nm for light emitted by iron ions (Fe XIV). Two other amateurs followed, and another amateur used a spectrograph to detect the E-corona. Amateur techniques have improved to the point that E-corona images from sea-level skies are straightforward.

The K-corona has been more elusive, and multiple amateur attempts have failed over recent years. Fortunately, the K-corona is largely polarized so that images at different polarizations can be subtracted from each other to pull out the coronal details. On September 16, 2024, I collected some images of the Sun in an orange filter (to reduce scattered blue sky light) and a linear polarizer made for cameras. By subtracting polarizations 90 degrees apart, I detected a signal consistent with polarized light surrounding the Sun. I learned a lot from that outing, that I needed longer exposures and I needed four polarizer orientations at 45-degree increments. On March 23, 2025, from Utsayantha Mountain, NY, I took a total of 20 minutes of video or about half a terabyte of data at the four orientations. After stacking the videos and subtracting the orientations, a clear image of the K-corona emerged. The features of the K-corona matched those of the E-corona taken about the same time.

Here is a timeline of amateur coronal observations outside of an eclipse, which demonstrates how recent amateur activity has been despite the availability of the technology for almost a century:

2020-11-16: Klaus Hartkorn first amateur to view and image the E-corona in Fe XIV = 530nm

2021-03-10: Christian Buil uses a spectrograph and processing to detect the E-corona in Fe XIV

2021-09-11: George Hripcsak views and images the E-corona in Fe XIV

2023-09-23: Jim Daley images the E-corona in Fe X = 637nm

2024-09-16: George Hripcsak first amateur to detect the K-corona in polarized orange light

2025-03-23: George Hripcsak first amateur to image the K-corona in polarized 690nm light

My coronagraph consists of a 50mm f/20 uncoated singlet objective lens in an old Celestron Firstscope optical tube. Light focuses on a 9.4mm cone for the artificial eclipse, followed by a field lens, a round stop that blocks light diffracted by the objective lens, a tiny spot that blocks reflected light, and a pair of relay lenses to create a telecentric beam. A 10nm-wide, 690nm filter passes deep red light to a rotatable linear polarizer. A 0.5x reducer precedes a ASI1600MM Pro camera. I used 100msec frames at 0 gain in one-minute videos, taking several at each of four polarizer orientations. I stacked the videos, subtracted alternate polarization orientations, and assembled a composite photo.

The first image is the composite K-corona, and the second is an image of the E-corona taken minutes later. The details are similar but not identical. The K-corona shows a jet at 2 o’clock not seen in the E-corona image. A far-UV satellite image (not shown) matches the E-corona image perfectly.

George Hripcsak has been an AAA member for 30 years. He enjoys nighttime and solar astronomy and building things.