Greetings from Palmia Observatory
Well now we can report on the observation of the Jupiter-Saturn conjunction that took place on December 21.
We have been practicing for several days before the event and here we see the latest image of December 20. Using a 600mm DSLR telephoto lens we can see both planets and moons. Seeing the rings of Saturn was a bit problematic. With some exposure settings, the rings would sometimes be quite clear, but I could never time the shutter with the good atmospheric seeing. Also a little higher magnification with longer focal length would definitely help. So, the main emphasis in selecting the exposure timing was to make sure the moons were visible in the photos.
There were two mystery objects in the image. First, it seems like there are five visible moons about Jupiter. What is going on there? Also for the first time there seems to be another mystery object next to Saturn. It turns out that it is Titan, the largest moon of Saturn with visible mag = 9.3.
The other mystery "moon" is really just a background star, HD191250, that was in the right position to look like another moon. Check out the names and locations of the other moons in this Sky Safari Pro screenshot.
|Sky Safari Pro screenshot identifies mystery moon as a star (Source: Palmia Observatory)
Whoa, so, I really didn't expect to be able to see any of the moons of Saturn. But there Titan is visible and the image was taken with just 600mm telephoto lens and 1/2 second exposure. This means we can go back and look through other conjunction images and see if by chance the right exposure was used and if the moon Titan shows up there too. It would be neat practice to make repeated measurements of the location of Titan around Saturn, just like we did several years ago for the moons around Jupiter. In this next chart, you can see the measured linear pixel distance between Saturn and its moon Titan. Hmm, the plotted curve looks a little suspect, with just four data points. The data point of December 16 was also the first time that the 600mm lens was used and its actual focal length setting is suspect.
|Plotting linear pixel distance between Saturn and Titan (Source: Palmia Observatory)
It turns out that the orbital plane of Saturn's moons is not aligned to our point of view the same as the orbital plane of Jupiter's moons. Jupiter's moons are mostly seen edge on for us and so the moons path will bring the moon both in front of Jupiter and then behind Jupiter during its periodic motion. But our view of Saturn's moons is almost face on or top down and Titan is not ever eclipsed by Saturn. So, instead of just measuring the linear separation between moon and planet, we now can measure the angular motion of the moon around Saturn. This is pretty neat and quite unexpected.
So, here we are on December 21 and we are ready to repeat the measurements and photograph the two planets when they appear the closest together. The December 21 conjunction photo is shown below.
|Jupiter-Saturn Dec 21 conjunction with DSLR, 600mm, 1/2 second (Source: Palmia Observatory)
We have watched the planets move closer and closer together for a week and now we get to watch as they move further and further apart in the weeks to come. But, for now, we can see the moon Titan and can measure its angular distance from Saturn. We now have four good angular measurements, five if we count the suspect measurement of Dec 16, and they are shown plotted below. Hmm, ok, it seems to be some sort of motion around Saturn and it will be interesting to get more data points and plot the motion. The Titan info page for Titan shows it has a bit more than 15 day orbital period, so we won't have to wait too long to get measurements over one whole orbital period.
|Tracking the angular position of Titan from Saturn (Source: Palmia Observatory)
So, here we are on December 22, waiting for our chance to go outside and take some more photos. The combined tripod, camera, red dot finder and 600mm telephoto lens is easily moved about. I usually make two trips so as not to overstress the little 1/4 inch screw connection that mounts the lens to the tripod.
|Ready to go out with tripod mounted DSLR with 600 mm telephoto lens (Source: Palmia Observatory)
The weather forecast is for Dec 22 evening clouds. Darn! Jupiter is just barely visible at about 5:15pm, but by about 5:30, the clouds have moved in and swamped out any chance of making an observation. Well, that is just the luck of the astronomical observer.
See you next time,