Solar activity on July 3, 2021

Beginning of the cycle 25 – ISES Solar Cycle Sunspot Number Progression from the National Oceanic and Atmospheric Administration (NOAA)

The solar cycle 25 is at its beginning. Good news for the fans of our sun: Large sunspots can be observed more and more often. The example below has a size of about 50″ which is just below the naked-eye resolution (typically 1′ or 60″). This can be used as a resolution test. The following solar images have been taken with a very simple setup based on a standard telephoto 400mm f/5.6 lens with a 2x extender and appropriate solar filters.

 · Solar images taken on July 3, 2021, @ 7:27 UTC and 7:36 UTC respectively

A Marumi filter (DHG ND-100000) was considered for the first image. The settings are: Exp 1/1600s, f/13, ISO 200. For the second solar image, a much older filtering system based on a thin layer of chrome combined with an IR-blocker B+W F-Pro 486 was used. The density of this filter is slightly higher, leading to the following settings: Exp 1/1600s, f/11, ISO 400. On both solar discs, the limb darkening is evident and is due to the solar atmosphere. In both cases, the optical diameter is 72mm, giving a theoretical resolution of 1.9″. The solar granules with an average size of 1500km are at the limit of the resolution of the setup, knowing that the sun was at a distance of 152 millions km that day, which leads to an average apparent granulation size of 2.0″.

 · Setup details

Photographic setup based on a Canon EOS M6 + EF 400mm f/5.6 L + Extender 2x III. Two different solar filters were considered: Marumi DHG ND-100000 and a combination of a thin layer of chrome with the IR-blocker B+W 486. Both systems are considered as white-light filters. A SkyWatcher AZ5 alt-azimuth mount with slow-motion control was well suited for this rapid shooting. Some serious rules have to be followed for a safe solar observation and photography. To know more on this topic, please consult the publication written in French: Observation solaire ‐ Qualité de protection oculaire.

© Michel Willemin

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