Mercury Maps

We have already seen that Mercury, at its greatest elongation from the Sun, is a mere 7 or 8 arc-seconds in diameter. So what features, if any, are visible on the disc?. The British Astronomical Association, long associated with quality observations of all the planets, recommends using the albedo map produced at the IAU Planetary Data Center at Meudon, prepared by J.B. Murray, under the direction of A. Dollfus in 1971. Initially, Murray prepared a map based largely on high-resolution New Mexico State University Observatory photographs from 1965 to 1970. Visual observations from Lyot and Dollfus at Pic du Midi (1942-1966) were then added to several areas. The IAU coordinate system, recommended in 1970, was then added. Although much higher resolution Mariner 10 images exist from 1974, these are really too detailed (confusingly so) to show what Earth-based observers can record. The map in question, refined by David Graham of the British Astronomical Association (BAA), is reproduced in Figure 11.1. This was originally

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Figure 11.1. David Graham's map of Mercury: produced primarily for BAA observers, it is a refined version of the map produced by the IAU. Map: courtesy David Graham.

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Figure 11.1. David Graham's map of Mercury: produced primarily for BAA observers, it is a refined version of the map produced by the IAU. Map: courtesy David Graham.

published in the February 1995 Journal of the British Astronomical Association. Of course, with webcams, the details glimpsed by the most eagle-eyed observers at Pic du Midi in the 1940s to 1960s can now be recorded with modest apertures, seeing permitting. More recently, the amateur astronomer Mario Frassati of Crescentino, Italy, produced an excellent map (Figure 11.2) for amateur observers based on his own visual observations of the planet, with a 203-mm Schmidt-Cassegrain. This too was reproduced in the Journal of the British Astronomical Association in June 2002. The drawings made to compile Mario's map were secured between January 1997 and May 2001. From a total of 78 high-quality drawings during that period, 54 were used to compile his map. Mario, his telescope, and his son are shown in Figure 11.3 and some of his excellent eyepiece sketches are shown in Figure 11.4.

Mercury is, of course, very close to the Sun and this produces its one single advantage when taking webcam images. The surface brightness is high. Mercury's albedo is only 6%, that is, it reflects 6% of the light landing on its surface. This is a similar albedo to that of our Moon. However, because Mercury orbits the Sun 2.6 times closer (on average) it has a surface brightness nearly 7 times greater than our Moon. This can be vital for clinching short exposures that freeze the atmospheric seeing. However, it should be remembered from our previous discussions about webcams that the actual exposure time indicated by the software can be false. In manual mode, an exposure at 5 or 10 frames per second will be 1/5th or 1/10th of a second, regardless of the indicated 1/25th exposure. Also, increasing the frame rate with a USB 1.1 webcam will lead to substantial image compression, which corrupts the data. We can use the brightness of Mercury (and the unfiltered Venus) to our advantage by reducing the exposure to, say 1/100th (while keeping the frame rate at 10 frames per second) and increasing the f-ratio to reduce interpixel noise to below 0.1 arc-seconds. Also, even with a narrow-band IR filter, the planet will still be bright.

Figure 11.2. Mario Frassati's amazing map of Mercury, based purely on his own visual observations with a 203-mm Schmidt-Cassegrain.

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Figure 11.4. Sketches of Mercury made visually, at the eyepiece, by Mario Frassati with a 203-mm Schmidt-Cassegrain.

Figure 11.5. The best earth-based image of Mercury, obtained by the Boston University team of Jeffrey Baumgardner, Michael Mendillo, and Jody K. Wilson. An RS 170 surveillance camera was used at the f/16 focus of the 60-inch (152-cm) Mt, Wilson reflector. This produced an image scale of 0.09 arc-seconds per pixel. The best 60 images from 300,000 (1/60th second) exposures taken on August 29, 1999 were used to form this hi-res result. The planet was 7.7 arc-seconds in diameter at the time. Image: courtesy Jody Wilson.

Figure 11.5. The best earth-based image of Mercury, obtained by the Boston University team of Jeffrey Baumgardner, Michael Mendillo, and Jody K. Wilson. An RS 170 surveillance camera was used at the f/16 focus of the 60-inch (152-cm) Mt, Wilson reflector. This produced an image scale of 0.09 arc-seconds per pixel. The best 60 images from 300,000 (1/60th second) exposures taken on August 29, 1999 were used to form this hi-res result. The planet was 7.7 arc-seconds in diameter at the time. Image: courtesy Jody Wilson.

The highest resolution image of Mercury obtained from Earth is shown in Figure 11.5. Although a 1.5-meter telescope was used, it is quite possible that amateur webcam users could rival this resolution given suitable seeing conditions.

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