Popular Astronomy

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Neptune and Uranus

The outer gas giants:

These two images (above) taken by David Finnigan on 28th & 29th September 2011 are a good example of what you can expect to see visually on Uranus. David used an 8” LX 200 SCT telescope. The left hand image he’s taken at F10, it’s reasonably bright but very, very tiny. The right hand image is at F30, it’s larger but very dim.

 Uranus is quite easy to observe visually. It can become as bright as mag 5.7 and I have personally seen it naked eye from the Kelling Heath star party (very dark skies) a couple of years ago. But visually through a telescope it does not offer much in the way of detail other than a tiny blue-green disc although it is very satisfying to observe this remote gas giant and consider the elation Sir William Herschel must have had when he discovered it. Uranus is quite tiny and never exceeds 3.7” in diameter, it is also faint compared to the known planets prior to its discovery. Also unlike Jupiter or even Saturn it is quite bland, presumably (and I’m not a planetary geologist here) because it is further from the sun and therefore cooler and less active. However a lighter equatorial band has occasionally been observed visually through large telescopes. The best I’ve ever seen was limb darkening through my 7” F8 Maksutov-Newtonian with Quartz Optics, a purpose built planetary telescope.

 On a planet with a lot of detail such as Jupiter I would normally argue that for visual observations from the UK an 8” aperture telescope optimised for planetary work (such as an Apochromatic refractor or an F8 or longer Newtonian (with a tiny central obstruction) is ideal (possibly up to 10” aperture with the Newtonian). Don’t misunderstand me, you can’t beat the laws of Physics and larger aperture means more resolution and a brighter image. But, one’s observing through the turbulent atmosphere which moves around in cells 6”-8” in diameter. An 8” telescope will more readily ‘punch’ through this providing steadier and sharper views than a much larger telescope on most occasions. But when the seeing steadies the larger telescope wins. This is always apparent in imaging whereby the camera freezes moments of good seeing. When imaging the planets a larger aperture invariably wins.

 Uranus is a different object from Jupiter, it’s just too small and faint for say an 8” telescope to pull out any detail that may be there. On Uranus you really do need a large aperture telescope (of the order of 16” and the seeing to go with it) to get the resolution and brightness to have any chance of visually catching any subtle detail that may be there. F.W. Price in his book, ‘The Planet Observer’s Handbook’ gives a good account of this.

Andrew Robertson 28/2/2012

What I said about Uranus above applies doubly so to Neptune which being of a similar size to Uranus but much further away is a good couple of magnitudes fainter (at best mag 7.7) and much smaller, only reaching 2.2" diameter at opposition. One probably needs at least a 6" reflector and a power of about x200 under good seeing to discern its tiny bluish disc. But it is satisfying to identify the outermost planet of our solar system. Cliff Meredith took the picture below and used star charts to identify its position. But you can identify it with binoculars plotting stars in its approximate position and noting which one has moved over the course of several nights.