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Tue, 06 Sep 2016 - Clouds observed on Neptune by SPA member in UK

Both of the outer “ice-giant” planets, Uranus and Neptune, are very well placed for observation at the moment and planetary section member Martin Lewis has recently managed something of a coup by capturing images of Neptune showing multiple pale clouds in the southern hemisphere of that planet.  

Neptune has just passed ‘opposition’, that is the point where the Earth lies directly between it and the Sun, therefore the planet can be observed throughout the available hours of darkness against the background stars of Aquarius, just west of the star Lambda Aquarii. It is below the naked-eye limit in terms of brightness so can only be found with good binoculars or a telescope and, from the Earth, this distant blue planet appears very tiny. Imaging such a target is a real challenge and although we have known of cloud features at Neptune from images taken by space probes and from the Hubble space telescope it was only in 1997 that the first observations of atmospheric features were made with professional ground-based telescopes.

In the last two years amateurs have started to achieve similar results with modern cameras and filters that pass near infra-red light but this is still difficult and challenging work requiring skill and very good ‘seeing’ conditions; clear and steady air at the observation site. Following recent professional observations of large cloud features and of darker spots on Neptune the amateur imaging community has been trying to catch these features in order to help understand the weather patterns on this far-flung but surprisingly active world.

Martin has succeeded by using a planetary imaging camera which has excellent sensitivity in the near infra-red, matched to a filter that passes light from 610 nm wavelength and upwards to the camera sensor. He used a large Dobsonian telescope to capture a video stream lasting over 6 minutes in length then used software to extract the best individual frames from this video, stacking them together in large numbers to ‘average-out’ any noise and to reinforce actual features in the final image. His false-colour image shows extensive clouds in the southern hemisphere and he has also captured the position of Neptune’s moon Triton. I have inserted a box into his image showing a simulation of the system at the time of Martin’s image just to help show the correct orientation.

This is challenging work for any amateur and Martin is to be congratulated on his results. In theory these features could be captured with a telescope of around 10 inches, 250 mm, aperture and upwards if given very steady seeing conditions and I would like to challenge other suitably equipped UK amateurs to  try and duplicate Martin’s work; I know I’m going to!


Added by: Alan Clitherow