Infrared Venus Imagin

Although Venusian upper cloud features are mainly ultraviolet features, some duskier markings are also visible in the near infrared. A filter with a bandpass of 700-1000 nanometers will reveal these markings. The U.K. imager Damian Peach has experimented with imaging in both UV and IR with Venus and then synthesizing a green component by adding the two results together. This can be used to make a false color RGB image consisting of infrared, green (IR+UV), and ultraviolet. The resulting image gives a picture that shows UV detail as deep blue and IR detail as red, a sort of highly exaggerated colour picture compressing Venus' subtle colors into the visual range. Professional astronomers used a similar technique when imaging Saturn's Moon Titan from the Cassini probe.

But what may interest amateur astronomers far more is the possibility of imaging the Venusian Ashen light in the near infrared. The reader of this book may well pose the question: "Surely someone must have tried this by now?" Indeed, they have. Both professionals and amateurs, but the attempts have been few and the visual Ashen light sightings are still hard to explain or discount. Probably the first observer to report the Ashen light was Johannes Riccioli in 1643. In the mid-1980s, Dr. David Allen and his colleagues at the Anglo-Australian Telescope showed that the Venusian night side did indeed glow strongly in the infrared. Unfortunately, it was far into the infrared, at wavelengths of 1.7 and 2.3 microns. However, do not give up hope just yet! The Galileo Near-InfraRed Mapping Spectrometer also imaged the Venusian night side (in 1991) at a wavelength of only 1.05 microns, far closer to the near infrared. The features discernible indicated that the detector was able to see the heat coming off features below the clouds, i.e., surface features. With the surface of Venus able to reach temperatures up to 460° Celsius in daytime, perhaps this is not surprising?

The amateur astronomer Christophe Pellier did achieve successful imaging of the Venusian night side in May 2004 (Figure 11.6). Using a Celestron 14 (355 mm aperture) at f/11 and an ATiK 1HS monochrome CCD he recorded an Ashen light type image with stacked 10-second exposures using a narrow-band 1000 nanometer filter. The night side was clearly visible, although there was only the vaguest suggestion of it using a 780-1100 nanometer filter on the same night. Maybe, you might think, this is the first evidence that the ashen light is real? However, the extreme red limit of the dark-adapted human eye is usually reckoned to be around 700 nanometers, so the idea that even superhuman observers could see to 1,000 nanometers still seems absurd: more research is needed.

In concluding this chapter, I would like to repeat, with some caution, that some amateur astronomers have imaged Venus in broad daylight. This is not difficult as, if you know exactly where to look, Venus can be spotted, in daylight, with the naked eye. Seeing in the daytime is usually much worse than at night, but the planet is at a much greater altitude, offsetting the poor daytime turbulence. However, for the beginner I would not recommend daytime imaging of Venus (or Mercury) at all. With the Sun in the sky and both planets quite close to it, this is a dangerous pursuit unless you have quite a few years of experience behind you.

CHAPTER TWELVE

Imaging Mars

Talk about the planet Mars and one immediately conjures up thoughts of science fiction: specifically, H.G. Wells' War of the Worlds and Orson Welles' radio dramatization of it (which scared a nation on October 30, 1938!). One also conjures up visions of the alleged canals on Mars, primarily "hyped" by Percival Lowell and the belief, even up to the mid-20th century, that intelligent life might exist on the red planet. The visual telescopic observers of the late 19th and early 20th centuries strained at the eyepiece to see any evidence of the Martian canals, but the damage done by the Earth's atmosphere meant that even with a lifetime of experience and good equipment most were still not sure if the canals were real or illusory. Only those few observers who had observed the planet under near-perfect conditions realized that canals simply did not exist. However, it took the fly-by of the spacecraft Mariner 4, in July 1965, to prove that Mars was a highly cratered world and not one able to support any intelligent life. The extent to which people believed that Martians might exist is hard to appreciate in the 21st century. However, the following anecdote, often quoted by Sir Patrick Moore, may make things clear. On December 17, 1900, a prize of 100,000 Francs (the Guzman prize) was offered to anyone who could establish contact with a being from another world. However, Mars was excluded from the award, because contacting a Martian was considered to be too easy!

Mars has an equatorial diameter of 6,794 kilometers (compared to 12,756 for the Earth). It orbits the Sun in just under 23 months (687 Earth days) and Earth overtakes, or laps Mars, on the inside track, in just under 26 months (780 Earth days). So, roughly three Earth months after each Martian year, we come closest to Mars. Mars orbits the Sun at an average distance of 228 million kilometers, roughly 50 percent further away than the Earth. However, the distance actually varies between 207 and 249 million kilometers. If Earth passes Mars when the red planet is closest to the Sun, it can come within 56 million kilometers. At such times (late

August/early September encounters) Mars can appear a whopping 25 arc-seconds across. Unfortunately for North American and European observers this takes place when the planet is low down in the southern sky. When Earth passes Mars at the red planet's furthest point from the Sun, it only spans 15 arc-seconds, quite a challenging object on which to resolve much detail.

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