Damian Peach: Talk to Newbury Astronomical Society, 8th September 2017

Jupiter is approximately ten times the size of the Earth. It is believed to have a small rocky core, which is surrounded by liquid hydrogen occupying most of the volume of the planet, and an atmosphere composed primarily of hydrogen. It has a powerful magnetic field and belts of intense radiation consisting of trapped particles from the solar wind. The visible atmosphere shows dark belts and light zones, which are regions of strong jet streams moving at different speeds. The interactions between these jet streams cause the phenomena that we see in Jupiter’s atmosphere.

The earliest known sketches of Jupiter were made by Cassini between 1670 and 1690. Some of these show evidence for a large red spot. Later observers such as Antoniadi (1928) used telescopes of 10 to 12 inches aperture; their drawings show increasing amounts of detail. Drawing at the telescope was the main observing technique used until the 1980s, and there are still some observers who continue the tradition in order to maintain an unbroken record of visual observations.

Film photography has had limited success at recording details, even with large apertures. A Lick Observatory photo from 1879 showed a large, elongated Great Red Spot (GRS) quite different from its present-day appearance, which has led to speculation that the today’s GRS may be a different storm. The best photographs of Jupiter were taken with the 1.5 metre telescope at the Catalina Observatory in 1967, but amateur photos of the time, even with large apertures, are of poor quality.

Amateurs began to use CCD cameras in the early 1990s, and this led to a step increase in the quality of images. Donald Parker in the USA used a 16-inch Newtonian with a CCD camera to capture the most detailed terrestrial images of the impact of comet Shoemaker-Levy 9 on Jupiter in 1994. His results were only surpassed by those from the Hubble Space Telescope.

The advent of webcam imaging revolutionised amateur planetary observing. Using a cheap camera to record multiple images and stacking them with free software gave results that were as good as or better than professional images. As the camera technology developed, increasing sensitivity of the image sensors allowed capture frame rates to increase from 5 to 30 or 60 frames per second, which made it possible to isolate the moments of best seeing, and capture exceptionally fine details. With telescopes of 10 to 14 inches it is possible to capture detail on the Galilean moons.

There is now a worldwide observing community which gives more or less 24-hour coverage of Jupiter when it is visible. The WinJupos software allows observers to measure the positions of features and track them over time, measure wind speeds and follow changes in the belts as they occur. The amateur community has also detected five meteorite impacts on Jupiter, seen as flashes in a few video frames, or in one case a dark spot similar to those seen after the SL-9 impact. Data from amateur observers is quite commonly used by professional planetary scientists in their work.

NASA’s Juno probe is now sending back superb data, and one of the instruments, JunoCam, is used to obtain images of regions suggested by the amateur community. The raw data is available on the Juno website, so that amateurs who wish to can download it and process the images for themselves.

The future for amateur imaging of Jupiter will probably come from the use of more sensitive cameras that are currently under development, and the use of larger apertures to capture more detail from locations where the seeing is good enough.

Notes and summary by Chris Hooker.

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