Saturday, August 1, 2009

Planets and moons

One of the interesting astronomical events of the year 2009 is the Triple Conjunction between Jupiter and Neptune in Capricornus. A triple conjunction between two planets occurs when they meet each other three times in the sky. In this context "meet each other" simply indicates that the two planets are in close proximity on the celestial sphere, usually less than a degree apart. In the case of the Jupiter-Neptune 2009 conjunction, the first two encounters (in Right Ascension) occurred on May 25 and July 13; the third and last rendez-vous will occur on December 20.

Around the July 13 conjunction, I took a closer look at the pair using my favourite planetarium software (Cartes du Ciel). While I was fiddling around with dates and times I noticed that the separation between the two planets was about 30 arc-minutes, which is well within the Field Of View of my Canon XSi at prime focus of my 10" f/4.7 Newtonian (about a degree). μ Capricornii (+5.08) made a nice addition right in the middle of the two planets . A couple of days of miserable weather forced me to wait, but on the night of the 17th the sky was perfect.

I took 60 frames 15sec each at ISO1600. With the two planets so low in the sky I figured that 15sec would be a good compromise between having the final image overwhelmed by light pollution and bagging Neptune that at +7.8mag is not exactly a bright object. Not to mention that the extinction coefficient at 14 degrees high is probably significant, so +7.8 was an optimistic estimate of the actual brightness of the Icy Giant.

When I looked at the final image and zoomed in to Neptune to check the colour, I noticed the presence of an object at 2 o'clock, very close to the planet. A possible explanation was that the camera had been able to capture Triton, the main moon of Neptune. I run a couple of checks in Cartes du Ciel and Starry Night, but eventually I used the Ephemeris Generator offered by JPL . The response was that yes, the little orb beside Neptune was indeed Triton! I was actually quite surprised as you can imagine. I was working at prime focus (62.4'x42.8') and the separation between Neptune and Triton is only about 15"! Not to mention that Triton's magnitude is +13.5, like Pluto. I did another check and realized that the limit magnitude my rig was able to achieve in just 15 minutes was +15.2/+15.4 (again, these are optimistic estimates given how low Neptune was).

Here is the image.

This is the second time that I am able to image Triton, but the first time was using a MallinCam HYPER Plus through a 16" SCT.().

All in all I am very happy with this image: Jupiter blazingly bright with the four Galileian moons (from left to right: Callisto, Io, Europa and Ganymede) and then blue Neptune with Triton. Not bad for a 15' image!

P.S. If anybody is aware of an image showing two planets with moons in the same Field Of View please let me know...I could not find any!

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