Observing Himalia (the brightest outer moon of Jupiter)

[harrym]

Been a while since I've been on here, but I made an observation last night with the 12" Northumberland refractor in Cambridge that I'd like to share.

As everyone knows, the usual answer to "how many moons of Jupiter can you see with a telescope" is 4. The 5th brightest moon, Amalthea, is 14th magnitude and orbits even closer in than the Galilean moons so it's totally lost in the glare even with very large scopes. The easiest non-Galilean moon to observe is actually the 6th brightest moon, Himalia, which is around 15th magnitude but orbits so far out that it can get almost a degree away from Jupiter, so glare isn't much of an issue.

Now that Jupiter's getting higher in the sky again, I decided to give Himalia a go this year. Jupiter is very close to perihelion which helps. The first challenge is finding out where Himalia actually is: Stellarium gets its position very wrong, and SkySafari is closer but still off by a couple of arcminutes. Occult 4 gets the position correct, as confirmed by checking against JPL Horizons. The next step is to plot the position on a star chart that shows stars fainter than Himalia - SkySafari will do for this purpose, and I used DSS images to confirm there were no missing stars that could masquerade as the faint moon.

My first attempt was a few weeks ago, but it was made at the start of morning twilight and that may have put paid to my chances of seeing Himalia. Last night was a better opportunity. Himalia's magnitude was listed at 14.7, which is within the range of the 12" refractor, but the light pollution dome from Cambridge in the east brings the limiting magnitude down a bit on that side of the sky. I used a 28mm eyepiece giving 218x magnification; Himalia was 55' away from Jupiter, or more than two FOV widths. It made a small triangle with stars of magnitude 12.3 and 13.6 which made pinpointing the position easy. Sure enough, with averted vision a faint "star" kept popping in and out of view at the right position. Himalia appeared a little fainter than a star listed as magnitude 14.8 by UCAC4, so maybe the listed magnitude of 14.7 is slightly optimistic.

Serendipitously, there was a galaxy in the field of view too: NGC 132, a magnitude 13.7 spiral, which unsurprisingly was a new object for me.

For anyone who'd like to try this out themselves: this new moon is a particularly good chance, as Himalia will be getting closer to Jupiter until it's only 19' away on 6 October. Visual observation will require at least a 12" scope unless you're in very dark skies, but imagers should be able to catch it with smaller scopes.

[josefk]

Fantastic!

[JeremyS]

Impressive result! 👍🏻
Great tribute to you and to the Northumberland!

[Orlando Gonzalez]

Excellent!