The moons of the Outer Planets have always fascinated me. I was fourteen years old when the images of the Jovian system started pouring down on Earth as Voyager 1 and 2 passed Jupiter on their way to the rest of the Solar System. What captured my imagination was the variety of sizes, colours and surface features of the moons of the gas giant. It seemed to me that each one of them was a completely different world with a long story to tell. Think of Io and Europa, for example: one the most volcanic body in the Solar System, the other encased in a crust of ice probably hiding an ocean! As the years passed, the probes flew by Saturn and then started following diverging trajectories: probe #1 towards the interstellar space which is about to enter 30 years later, probe #2 towards Uranus and Neptune, never explored before. Like Jupiter, each of the Outer Planets came with a variety of moons, many of which revealed features that are still keeping astronomers busy these days.
Three decades after the Voyagers flew by Jupiter my interest in the moons of the Outer Planets is still alive. I guess there's a reason why three of the seven (so far) posts on this blog are about the satellites of Saturn, Jupiter and Neptune...As I described in a previous post, imaging the moons of the outer planets is well within reach of amateur astronomers. A mid-size telescope like mine (10" reflector f/4.7) and a digital SLR camera at prime focus are more than adequate for the task. As a point in case, I was able to image Triton (14 degrees above the horizon) with only a 15 minute total exposure from my urban location (a slower focal ratio will need a longer imaging session).
After the success with Triton, I decided to go for the moons of Uranus. The Uranian system presents a different set of challenges when compared to the Neptunian system. First of all, Uranus is brighter than Neptune by 2 magnitudes, +5.8 against +7.8, which implies that the glare from the planet affects a larger area around it assuming the same optics, camera sensor, camera settings (exposure and gain in particular) and image processing steps. Because of that a faint moon too close to the planet might not be detected by the camera sensor simply because the signal from the moon is below the threshold of the signal caused by the planet's glare. So if there were a moon exactly like Triton orbiting Uranus, it would probably be a little more difficult to image than the actual Triton in orbit around Neptune. Second, the satellites of Uranus are dimmer than Triton (+13.46). The brightest is Titania at +13.97, followed by Oberon (+14.18), Ariel (+14.4), Umbriel (+15.05) and Miranda (+16.55). The others are too dim to be of interest for the amateur astronomer. Third, at the moment (2009), while the inclination of Triton's orbit as seen from Earth is very high, the inclination of the Uranian moons is almost zero, which means that, as seen from Earth, Triton moves on a low eccentricity ellipse around Neptune of radius about 15 arc seconds while the moons of Uranus moves on an almost straight line from one side of the planet to the other:
The implication is that while any time is good for observing Triton (provided it is night!), the moons of Uranus required a little bit of planning to make sure that they are not too close to the planet during an imaging session. On the evening of July 20th, the weather looked promising, so I decided to check the arrangements of the Uranian moons in Cartes du Ciel. On July 21th at 2:45am they looked like this:
The arrangement didn't look very favourable, with Umbriel and Titania only 4.5" and 4.8" away from Uranus. The reason why I am saying that is because the image scale, IS, measured in arc-second/pixel of my setup is:
IS = 206265 x (pixel size) / (focal length) =
206265 x 5.2 microns / 1200 mm = 0.89 arcsecond/pixel
Given that Uranus is 3.8" in diameter, the distance of Umbriel (say) from the limb of the planet was 2.6". The seeing on that night was around 3", so it would have been extremely difficult for my scope and camera to resolve Umbriel and Titania. The other two moons, Ariel and Oberon were more accessible, being 13.4" and 24.4" away from the planet. Miranda, being so faint and so close, wouldn't have a chance, but that would be OK for my first attempt. So I waited for Uranus to clear the roofs of my neighbourhood and climb to about 25 degrees above the SE horizon. Based on the experience acquired with Neptune and Triton, I decided to go for 20sec individual exposures instead of 15 and bump up the total exposure to 30min instead of 15. The fact that the moons of Uranus are fainter than Triton and that Triton was barely visible in my Jupiter-Neptune image, convinced me to go for longer individual and total exposures. The gain was set to 1600ISO. The camera (Canon XSi) was connected to the scope in the prime focus configuration. A Baader Coma Corrector was used to contain coma. The reason for the coma corrector was because the entire field of view around Uranus is quite pretty, with star 20 Psc of magnitude +5.5 (very similar to Uranus) less than 35 arc-minutes away. 20 Psc is a giant star of spectroscopic class G8, so yellow-orange in colour. That would make a nice contrast with the light blue cast of Uranus. To eliminate trailing (even with short subframes) I used an autoguider (Orion Starshoot). After 30 minutes I packed up and left the processing for the following day, but not before launching DeepSkyStacker to take care of the lengthy aligning and stacking of the 100 individual frames.
Here's the final image.
Ariel and Oberon are clearly visible. I checked against the arrangement provided by Cartes du Ciel and then double-checked using the JPL Ephemeris Generator (HORIZONS), just to be sure of which one is which. No trace of Umbriel and Titania, as expected. I am quite pleased with the result: with some planning it should be possible to see all four main moons. I doubt that Miranda, at magnitude +16.6 and less than 9 arc-seconds maximum elongation will be detected by my setup, but who knows? Perhaps under excellent conditions it might be possible. It certainly won't be easy!
Here's the final image without the inset.
The colour contrast between Uranus and 20 Psc is quite nice.
As I was studying the image in detail, I noticed the pretty asterism to the right of Uranus. What also caught my attention was that some of the "stars" of the asterism looked fuzzy. Upon closer inspection it turned out that those faint fuzzies were indeed galaxies! A quick search confirmed that on that night Uranus was just 12 arc-minutes away from the Compact Group of galaxies Hickson 97. I also noticed other fuzzies disseminated across the image.
The fact that there were a bunch of galaxies in my image convinced me to take a longer exposure of the same field of view. That opportunity presented itself few days later, on July 24. That will be the subject of the next post.