The Martian Moons

A real challenge for the amateur observer, even at a favorable opposition, is spotting the Martian moons Phobos and Deimos. In a large amateur telescope these 11th and 12th magnitude moons would not normally be a challenge, but they never stray far from the brilliant Martian disc and their rapid orbits mean they move considerably even in one night. Phobos is the larger Moon at over 20 kilometers across but never strays more than a Martian diameter from the planet, making it the hardest to see. Deimos, at around 12 kilometers across can stray three Mars diameters from the planet, making the fainter Moon easier to see. The best plan for the visual observer is to use an eyepiece with a thin "occulting bar" at the eyepiece focus. Placing Mars behind the bar reduces most of the dazzling light enabling both moons to be seen. However, the exact position of the moons needs to be checked each night with a planetarium software package such as Guide 8.0. Phobos and Deimos orbit the red planet in 8 and 29 hours, respectively.

Of course, with a webcam, using an occulting bar is not essential, because a webcam chip, unlike the eye, cannot be dazzled (although Mars will certainly end up being saturated and burnt out). Because Phobos and Deimos orbit so close to Mars, fitting the planet and moons in the same image is not hard, and, to make a nice composite, you can easily cut and paste a short exposure Mars image on top of the overexposed blur in the main image. It might be thought that a webcam could not reach objects as faint as 11th or 12th magnitude with short exposures, but in fact they can. (An alternative solution with ATiK or modified webcams is to take a long exposure beyond the normal 1/5th second limit of a commercial webcam.) Even a humble ToUcam Pro can just record stars of magnitude 11 or so in 0.1 second exposures at f/10 with a 25-cm aperture. With a monochrome webcam and 0.2 second exposures stars as faint as magnitude 12 or 13 can be recorded on a stack of hundreds of frames.

CHAPTER THIRTEEN

ImagingJupiter v 3

Of all the planets in the solar system, Jupiter is the most rewarding to study because changes can be glimpsed from night to night as the huge weather systems move with respect to one another. I never tire of seeing the Great Red Spot appear over the limb or the satellites and their shadows crossing the giant planet. Jupiter is the fastest rotating planet in the solar system and even a novice observer will spot the features moving across the disk in a half-hour session. Jupiter's huge invisible shadow, trailing behind the planet, creates a fascination of its own as the four Galilean moons drift in and out of the shadow and sometimes even eclipse and occult each other. On nights of perfect stability, small markings can just be glimpsed on these moons.

Jupiter is truly massive, with an equatorial diameter of 142,880 kilometers, that is, just over 11 times the Earth's diameter. Even though the planet is mainly gas and liquid, with (probably) a relatively small rocky core, it still has a mass of some 318 Earths! Jupiter's equatorial regions (System I) rotate in 9 hours, 50 minutes, and 30 seconds, with the rest of the visible regions (System II) taking 5 minutes, 11 seconds longer. A third rotation system (System III), based on radio signals from the planet, gives the rotation period as 9 hours, 55 minutes, and 29 seconds. With there being no visible solid surface, the system longitudes transiting the meridian at any given time need to be calculated without reference to solid features. Thus, the longitudes of certain features (such as the Great Red Spot) tend to drift slowly with respect to the system they are in. Planetarium packages like Project Pluto's Guide 8 or handbooks like that of the BAA can be used to check on the system longitudes at a given time. Jupiter orbits the Sun at a mean distance of 778 million miles. At its closest it can appear as large as 50.1 arc-seconds in diameter, although an opposition size of nearer 45 arc-seconds is more typical. The giant planet orbits the Sun every 11.86 years and so spends half that time above the ecliptic (good for northern hemisphere observers) and half below (good for southern hemisphere observers). As always, observers on the equator have no reason to ever complain! Jupiter reaches opposition 32 days later each year. The belts, zones, and jet stream currents on the giant planet are highly complex, but hopefully Figure 13.1 will make things slightly clearer than mud! Figures 13.2 through to 13.6 show some high resolution webcam images of Jupiter.

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