Solar activity on July 16 & 17, 2022 ‐ A new filtering method

The cycle 25 in the rising phase, with sunspot number at 141 and 153 on those days.

Considering the appropriate filtering method is always a challenge for solar photography. First of all, the filter has to be safe for the observer and the equipment. Obtaining high-resoluting images with good contrast is the goal of any observer. Depending on the optics (telescope or refractor), using always the same filter would not systematically lead to the optimal images. On the market, the main front filters like the famous AstroSolar, the metallic filters or the less known Marumi (DHG ND-100000) are available. They all offer a safe solution. However, in term of resolution and contrast, they are not always compatible with the full potential of the optics. A Herschel wedge is also a very good solution, but quite expensive and almost only compatible with a refractor. By the way, this solution has been tested on April 21 & 24, 2021.

The proposed setup here is very simple and highly efficient. The solution is based on two filters. As main filter, which is put in the front of the optics, is a classical ND1000 (or ND3.0) photographic absorber. As an example available in 77mm diameter: Hoya ProND1000. It is mandatory to consider a secondary filter like a narrowband oxygen III (OIII) with a bandwidth of about 12nm or less. Such filters are proposed in 1.25″ or 2″ diameter through different vendors. For a visual use of the setup and not only with image sensors, adding and IR-cut filter would be highly recommended.

Compared to an AstroSolar foil or a conventional metallic filter, the solution presented here is offering the following additional advantages:
• Selection of a relatively narrow band of the spectrum (« continuum » or other) for a more detailed image of the structures at the solar surface. The atmospheric dispersion is drastically reduced!
• Possibility of tuning the percentage of transmission for better freezing of the seeing effect.
• The contrast of the image is also better, compared to all « metallic » filters, like AstroSolar foil.
• Possibility of use with small reflectors (full aperture) or with bigger reflectors (off-axis), which is not recommended with a Herschel wedge.
• The Stehl ratio is generally better.

 · Solar images taken with this new setup

Setup: Vixen 80M achromat refractor (910mm, f/11), with the Hoya ProND1000, and the Celestron OIII 12nm. First image: Planetary ZWO ASI462MC on July 17, 2022, UTC 12:51. Exp. 48x 150μs, Gain 100. Second image: Canon EOS M6 on July 16, 2022, UTC 17:20. Exp 1/1600s (single black & white shot), ISO 100. Third image: Planetary ZWO ASI462MC on July 17, 2022, UTC 12:48. Exp. 114x 150μs, Gain 100. The raw images are monochrome and the color of the images has been added through a post processing, which has no scientific value. The purpose is only of esthetic nature.

Remark regarding the resolution:
For detecting reasonnably the solar granulation, the literature recommends the use of an optics of 100mm aperture or more. Knowing that the sun was at a distance of 152 millions km that day, a solar granule having an average size of about 1500km, will have an apparent size of about 2.0″. A solar images presnented here are taken with a real aperture of 72mm, leading to a theoretical resolution of 1.9″ However, the solar granulation is clearly visible with reasonnable contrast, in spite working at the limit of the resolution of the optics.

 · Setup details

Photographic setup based on a small achromat Vixen 80M and two different cameras Canon EOS M6 and ZWO ASI462MC. The new solar filtering methods is extremely efficient against the atmospheric dispersion. A SkyWatcher HEQ5 Pro mount and a small PC tent are very convenient for a reliable image acquisition.

© Michel Willemin

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