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Thu, 07 Sep 2017 - Observing the Ice-Giants in Autumn

Uranus and Neptune are rather over looked by most planetary observers. While relatively large planets, this pair of frigid gas balls are also relatively remote members of the solar system and will never appear large at the telescope eyepiece.  Most observers seek them out once or twice and then, having placed a mental tick against a list of targets, never bother to observe them again. This is rather a shame as these distant worlds have, in recent years, proved to be rather dynamic with more going on in their atmospheres than was ever thought possible; especially given their remoteness from the heat of the Sun.

Modern equipment, particularly large diameter amateur telescopes and sensitive planetary cameras, have revealed huge storm features rotating in the atmospheres of both planets along with subtle shading and banding from equator to pole. This gives the amateur the opportunity to make observations that are of genuine scientific interest and value and help with understanding of planetary meteorology in extreme conditions.

Visually it can be hard to make out more than subtle variations in colour and even this will take an aperture of at least 8 inches (200 mm) and more is always preferable.  Both planets are largely made up of hydrogen and helium gas but each contains a similar amount of methane and this gas readily absorbs various frequencies of light, especially red and yellow. As a result both appear distinctly blue-green at the eyepiece with Neptune usually seen as bluer and Uranus as more turquoise in colour. It will take a good eye and careful observation under very steady skies to make out atmospheric variations on either planet.

The situation improves photographically. Modern planetary cameras can be very sensitive to infrared frequencies and amateurs have made good use of filters that pass only infrared or near infrared light to capture banding and storm features on both planets. This is challenging work at the frontiers of what amateurs can achieve but apertures down to 10 inches (250 mm) in size have caught these features and plenty of amateur planetary observers use larger instruments. If you want to try this then choose the steadiest nights and don’t be afraid of high levels of gain, and consequent noise, in your captured video stream. Where for closer and brighter targets we are used to capturing relatively short video streams to avoid rotation of the target blurring surface features in the final image, this is not an issue with the Ice Giants. Imaging runs can be over many minutes, even tens of minutes, with a large number captured files helping to average out the large amounts of noise in individual image frames. While both planets will have rotated during the imaging run we are only able to capture the largest atmospheric features and these will not blur much on these time scales.





Another possibility is to try and capture the rings of Uranus. These are very faint compared with the planet so normal long exposure, guided images of Uranus will have the rings hidden by the glare of the planet; however the amount of methane in the planet’s atmosphere make it very dark when viewed through a methane absorption band filter and the rings reflect brightly at the wavelengths passed by the filter. This is rather like observing Jupiter with a methane filter. Where that gas is visible it absorbs light and features appear very dark. Where methane is not present, or is covered by clouds of ice and other gas, features reflect sunlight and appear very bright in comparison. Capturing the rings of Uranus will not be so simple but is possible and presents an interesting challenge for the amateur observer.

Whether you want to try to capture meaningful scientific data or to merely tick these two planets off from your observing checklist, seek out Uranus and Neptune this autumn with the help of these finder charts prepared using the excellent software ‘Stellarium’. They show both planets in wide and narrow star fields for early September through to late October. Neptune is best observed from dark-fall towards midnight, then move on to Uranus and follow into the early hours. To see each chart in more detail, right-click the picture and select 'open in new tab' to open a full-screen view. Good luck.


Neptune wide view.






And Narrow:-







And now Uranus wide view:-










And finally, Uranus narrow view.





Added by: Alan Clitherow