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I traveled about 50 feet for my field trip. But I looked over 300 million miles.
 The planet Jupiter But there is one place in the solar system that puts on a show over a time span of days or even hours—Jupiter and its satellites. First discovered by Galileo in 1610, the four innermost moons of Jupiter seem to shuttle back and forth, across and behind the giant planet. At certain times a change is visible over an hour or less. It's like watching from the outside a small solar system running on a fast clock, and since it runs fast I could measure it in a relatively short period of time. My goal was, if possible, to observe and record the positions of the moons so I could determine their relative distances from Jupiter and their patterns moving around it, using the data to construct an accurate model of the inner Jovian system. When to Look, How to Look Jupiter was in a favorable position for viewing during the spring of 2004, a bright dot slowly arcing across the southern sky. Even at its closest, Jupiter is too far away to show a disk to the unaided eye, though a small pair of binoculars will show Jupiter and the inner moons very well. But just looking is one thing, quantifying is quite another. To make measurements I needed a bigger image and a way to record those images: a telescope and a camera. The telescope was a 10" Schmidt-Cassegrain. For a neophyte it had the wonderful capability of automatically finding a known object in the sky and then tracking it through the evening with minimal adjustments. For a camera I used the family digital camera. Unlike film cameras, digital cameras have the advantage that images can be examined immediately after exposure and additional shots taken if the first ones didn't come out well. The images can also be manipulated with standard computer photo software, which made the subsequent measurements much easier. Since we have no permanent mounting for the telescope, this project meant setting it up each evening in the driveway in front of the house (the 50 feet mentioned earlier). After turning it on and getting it tracking, I took pictures of Jupiter and its moons each available night from when it first got dark enough (about 7:30 p.m.) until as late as midnight. Observing sessions were usually ended by either the ground fog common in our area in the spring, or by falling temperatures, which caused dew on the optical surface of the telescope. Actually, most sessions never happened at all due to the famous Oregon cool-season cloud cover. Taking astrophotos is not easy. Just centering the image in the camera field can be a hair-tearing experience. To get decent-sized images, I used an eyepiece that gave a magnification of about 250 times (and operated the camera at a 3x zoom setting); at such magnifications the telescope could be fairly described as one of the better vibration detectors commonly available. Even a moderate breeze could move the scope enough to make the image wander out of view, and by just walking too hard near the tripod I could turn it into a blur. And focusing the image using the electronic camera screen was a project in itself. (Take a picture, enlarge it, tweak the focus, take another, and so on until the image was sharp.) Even very elementary astrophotography can be painful. But it's also rewarding, as the sequence in the illustration shows. |
 Jupiter and the Three Inner Moons (Click to enlarge) Seeing Is Believing—When You Can See It All While the moons of Jupiter may be predictable, the sky in between us isn't. Look at the problem. First, no viewing in the daytime. Second, even if the sky is dark, Jupiter has to be well above the horizon (lights and hills). Third, clouds or thick haze mean no images. And even if the sky appears clear, there is the question of seeing, the term for the stability of the atmosphere. The air can act like a distorting lens as one looks up through layers of different winds and temperatures. On poor nights, looking at Jupiter was like looking through a heat mirage on a summer highway, and getting good pictures was impossible. At higher magnifications the image of the planet would pulse and stretch like a water drop on a hot plate. Favorable viewing combinations are not the norm in western Oregon in the spring. In practice I was able to collect pictures on nine nights over a two-week period in the first half of April 2004. Then the clouds rolled back in. Recording the Observations—Endless Pickiness Even with clear skies, recording the necessary data was no picnic. I had to use exactly the same setup for each observing session. Since I was going to be making quantitative measurements of the photographs for the distances of the moons from Jupiter, I needed to settle on a particular combination of telescope eyepiece and camera settings and use that every time. I couldn't vary the magnification or the size of the images would vary, making my measurements incompatible. I also had to attach the camera to the telescope so that I would have the same view each time, and have extreme steadiness, but also have the option to move the camera around a bit if necessary. This was complicated by the fact that the camera wasn't really designed for this kind of photography in the first place. The problem was solved by gluing a camera filter adapter into some PVC plumbing pipe, which was then cut and sanded to slip snugly over a smaller piece of pipe on the telescope eyepiece holder. When the adapter was screwed onto the camera, the camera/pipe assembly could be snugged over the scope eyepiece. The camera was focused to infinity, and the image was focused in the camera using the telescope. I collected data on the three innermost Galilean satellites: Io, Europa, and Ganymede. This focus was due to the final eyepiece/camera configuration, which gave just enough of a view to see Ganymede and Jupiter at their greatest separation in the same frame. |
        
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