Orwell Astronomical Society (Ipswich)

Uranus, Neptune And Pluto, 28 December 2013 - 02 June 2014

To complete my first year experimenting with spectroscopy I tried for a spectrum of Uranus. On 28 December 2013, I took a ranging shot to find the planet as it emerged from behind a conifer in my front garden! The 55 mm lens on my DSLR gave a big enough field of view to track down Uranus on the border between Pisces and Cetus.

On 30 December 2013, I zoomed in on Uranus with the 11 cm F/5 Newtonian, which gives a 2.3x1.5° field and, remarkably, picked up one of its satellites as a bonus. The satellite, Oberon, was far from the planet and just discernible at magnitude 14.2. (Magnitudes in the image below are shown without decimal points, so 112 is magnitude 11.2 for example.) Then I attached the 12° objective prism to the front of the telescope, offset the 8° deviation angle and took another shot. This gave a rather nice spectrum which shows broad bands due to methane absorption in the planet's atmosphere. The spectrum of Jupiter is shown for comparison: this has only very weak methane bands and is essentially a reflected solar spectrum. Neptune has even stronger methane absorption (hence the bluer colour) so is on my list of targets for next year...

Having glimpsed Oberon by chance with just a small Newtonian, I thought it must be possible to image some of the other satellites of Uranus with the telescope I use to routinely monitor variable nebulae. With a focal length of 2.28 m the 0.51 m reflector in New Mexico (a member of the iTelescope Network, www.itelescope.net/) would have enough resolution to separate the faint satellites from the glare of planet. So I booked a 30 second exposure (!) for the early hours of 04 January 2014 and received the image later in the morning. Three of the satellites are clearly visible with a fourth just apparent on one of the diffraction spikes from the planet. The magnitudes of the planet and its satellites are: Uranus 5.8, Oberon 14.2, Titania 14.0, Ariel 14.4 and Umbriel 15.1. Miranda, the smallest and closest of the five main satellites, is a magnitude 16.6 object and too close to Uranus to be seen.

It might be worth attempting to repeat this observation with a DSLR on the Tomline Refractor. The much longer focal length should give a greater chance of separating the satellites from the planet. But a colour filter may prove necessary to reduce the effect of chromatic aberration.

On 07 January 2014, I used the 0.81 m Mount Lemmon telescope in Arizona to image the satellites of Uranus again. The instrument is a member of the Sierra Stars observatory network http://sierrastars.com/; it is situated at an altitude of 2800 m and provides excellent performance. On the original image, the fifth of the main satellites, Miranda, is just faintly visible. At one 20,000^th the brightness of Uranus and only eight arcseconds from its centre, that's remarkable!

It's also interesting to note that Oberon has moved from north of Uranus on 30 December to south of it on 07 January. This gives an indication of the strange orientation of the planet, its equatorial plane, within which the satellites orbit, being tilted at 98° to its orbital plane. Oberon's orbital period is just over 15 days so the change in positions corresponds to about half an orbit of the planet. I confirmed the positions of the satellites using Guide 7.0 (an excellent planetarium-type software package from Project Pluto, https://www.projectpluto.com/). An image below shows the movement of the planet and its moons during the period 04-07 January 2014.

In May 2014, Neptune and Pluto were well placed for observation from the southern hemisphere. From Siding Spring Observatory in Australia, home to telescopes of iTelescope.net, both were high in the early morning sky, eminently suitable for imaging. I used the iTelescope.net 0.5 m telescope for Neptune so that I could also image its large satellite, Triton. As Neptune is blue (due to methane absorption in its atmosphere), I used a red filter to reduce its brightness relative to Triton. Neptune is about magnitude 7.8 and Triton magnitude 13.5, only 11.5 arcseconds distant from the planet, so easily lost in the glare! I took my first image on 29 May 2014 and, following several days of bad weather in Australia, a second on 02 June. In this period, Triton, which moves anti-clockwise around the planet, completed four days of its 5.8 day orbit. Note that the same star appears in both images below.

I aimed to detect only the motion of Pluto, so used a lesser iTelescope.net instrument, with aperture 31 cm. The image below shows the motion of the planet over a period of a day, a little over one arcsecond.

Mike Harlow

Original: Forum December 2013; January, May, June 2014
Updated: 26 March 2023