0 W

Figure 12.2. Almost a whole rotation of Mars is captured in this sequence of webcam images from top left to bottom right shot by the author over a four-week period using a 30-cm LX200 at f/22 in July and August 2003. South is at the top and roughly 60 degrees more of the planet emerges from the left-hand evening terminator in each frame, giving a night to night illusion of the planet rotating backwards! The major features in the middle of each frame are the Sinus Meridiani; the Syrtis Major; Mare Cimmerium; the Amazonis Desert; Solis Lacus to the left of center; Solis Lacus to the right of center.

I think the best way to describe the Martian features to a beginner is to imagine they are drifting into view escaping from the evening terminator with south at the top, in other words, in the opposite direction to the planet's rotation and as they would appear over a five-week period, to the observer. It is worth noting that a beginner will need some time to identify the major features visually as atmospheric stability (in the Earth's atmosphere!) is essential for resolving fine details. Mars is often a very small body and, perhaps surprisingly, can vary considerably in appearance. Remember, Mars has an axial tilt of almost 24° (like the Earth), so sometimes we get a better view of the southern hemisphere, and sometimes the northern hemisphere is favorably presented; sometimes both hemispheres are equally favored. Also, depending on whether it is Martian spring, summer, autumn, or winter in the hemisphere we are looking at, the relevant polar cap may be large, shrinking rapidly, or small. The axial tilt is crucial to how the major features at high latitudes appear; especially the polar caps. Add to this a disc that may only be a few arc-seconds in diameter, or may span 25-arc-seconds, and the extra factor of potentially considerable dust storm activity and you can see how Mars looks very different every year you observe it. On top of this is the problem I have already mentioned, i.e., that you may not see features on the opposite side of the planet for over a month, simply because Mars rotates in 24 hours and 37 minutes. While an observer in Florida is studying, say, the Syrtis Major, his colleague in Hong Kong may well, 12 hours later, be studying the Solis Lacus, but neither will be able to see the other's subject well for several weeks.

If we start first with the Syrtis Major, well, I have to admit a psychiatric problem here. To me, the Syrtis Major and adjoining features always look like a small bat clinging to an orange! Above (to the south) of this huge dark V is a noticeably lighter oval feature called Hellas. Many dust storms seem to originate first in this area. As the days tick by, observing at the same time of night, you will see more features emerging from the left-hand evening terminator. After a week or less you will notice a rarity of dark albedo features emerging, except for a dark stripe moving upward away from the Srytis Major region. This is the Mare Tyrrhenum/Mare Cimmerium region and marks the start of what, as a Mars novice in the 1980s, I used to call "the boring side of Mars"! By that I meant the hemisphere with a rarity of dark albedo features, except in the high southern latitudes. After a week or more of tolerating this "boring side," the beginner will start to see a fascinating feature emerge from the evening terminator. This is the Solis Lacus or "Lake of the Sun." At first it just looks like a dark smudge on the limb, but when on the meridian it resembles a dark eye or the hub of a wheel, with a hint of spokes radiating out from its center. Beyond and bordering the Solis Lacus' lighter surrounds is Aurorae Sinus and then, way down in the north a rare dark northern feature, the Mare Acidalium swings into view. Only a few days later you will see another distinctive marking emerge. It is called the Sinus Meridiani, and to its left, a long dark line is seen, the Sinus Sabaeus. To me (and remember my problem with a bat clinging to an orange!) the Sabaeus-Meridiani feature looks like the arm of a bear with a claw (Meridiani) at the end. Meridiani is so-called because it is at the point designated as zero degrees longitude on the Martian globe. Finally, a few days after the whole of Sinus Sibaeus emerges we are back to the giant dark V of the Syrtis Major, emerging from the evening terminator; and we have now been observing Mars for over a month.

Mars' expanding and shrinking polar caps are unique features for the Earth-based observer to study, but they can also be a real problem for the webcam imager as they are simply so much brighter than anything else on the planet (see Figures 12.3 and 12.4). The skilled visual observer still has a slight advantage here as the eye-brain combination can tolerate a much larger brightness range than an 8-bit webcam frame. Sometimes the only solution to the polar cap brightness is to image the planet at two different exposures simply to reveal any detail within the caps themselves. Features within the caps are rare, but they do occur, at the cap boundaries, when local spring causes them to shrink. Typically, with the southern cap, two rifts have appeared (Rima Australis and Rima Angusta) and then a detached part of the cap is left. (Schiaparelli called this Novissima Thyle). At a later stage this too breaks up into the so-called "Mountains of Mitchel," first seen by O.M. Mitchel at the Cincinnati Observatory in 1845. Similar changes can be recorded in the northern cap, too. With the onset of autumn, an overlying polar haze can confuse the situation. But, regardless of all this, the polar caps are daz-zlingly bright and a combination of short exposures, filters and careful use of brightness, contrast and gamma controls are needed to produce images that show the Martian features and the polar cap edges well.

Occasionally some concern has been voiced that the Martian colors, obtained by using a color webcam like the ToUcam, are not exactly true to life and well below the purity that could be obtained using a true RGB filter set. The filters built into

Figure 12.3. Mars imaged on August 22, 2003, by Damian Peach, at high altitude, from La Palma in the Canary Islands. A 25-cm Schmidt-Cassegrain working at f/40 was used, as well as a ToUcam Pro webcam. The major dark "eye" above and to the right is the Solis Lacus.

Figure 12.4. Mars imaged one day later than in Figure 12.3, and 30 minutes earlier. The Solis Lacus is on the right hand of the disc, Aurorae Sinus near the center, and Sinus Meridiani on the left hand of the disc.

the ToUcam pixels (one blue, two green, one red in each 2 x 2 pixel cluster) cover a wider bandwidth than narrowband scientific filters, so they can grab more signal. Damian Peach and Singapore imager Tan Wei Leong studied this issue some time ago and concluded that the use of a UV-IR rejection filter combined, crucially with a magenta filter (effectively a green subtraction filter) would lead to better filtering and the final ToUcam image could then be cleanly separated into red and blue channels with green being created from red + blue to synthesize an aesthetically pleasing color picture, too. However, with the advent of low-cost monochrome webcams and filter sets, this technique has rarely been used. When one takes into account atmospheric dispersion, individual telescope characteristics and individual observer's picture tweaking methods, one can conclude that the only truly scientific images are monochrome ones taken through precision filters before they are combined to make a pleasing color image. However, another approach in imaging Mars, pioneered by Antonio Cidadao in Portugal, is to image the planet in IR and UV and synthesize the green channel completely, by combining the IR and UV images. See Figure 12.5 for the result that captures the surface details and high-altitude limb haze. A more standard approach, by Don Parker, is shown in Figure 12.6.

Mars is often neglected by amateur astronomers when the disc shrinks to less than 10 arc-seconds in diameter. However, good views can still be obtained when the planet is high up and seeing is good. Take a look at the stunning image by Damian Peach, in Figure 12.7, for proof.

Figure 12.5. Antonio Cidadao from Portugal pioneered the technique of simulating the green channel in planetary imaging. Here the technique is used on Mars. The infra-red image captures the Martian surface markings well and the ultraviolet image captures the limb haze. By combining the IR and UV images to give a synthetic green, a clean color image of Mars showing everything of interest is captured, and only two filters are needed. A 250-mm SCT with Stellar Products adaptive optics device was used, alongside a Finger-Lakes Instruments CM7-1E CCD. Image: A. Cidadao.

Figure 12.5. Antonio Cidadao from Portugal pioneered the technique of simulating the green channel in planetary imaging. Here the technique is used on Mars. The infra-red image captures the Martian surface markings well and the ultraviolet image captures the limb haze. By combining the IR and UV images to give a synthetic green, a clean color image of Mars showing everything of interest is captured, and only two filters are needed. A 250-mm SCT with Stellar Products adaptive optics device was used, alongside a Finger-Lakes Instruments CM7-1E CCD. Image: A. Cidadao.

f^sif

RGB Syrtis Blue Cloud

Figure 12.6. Mars imaged by Donald Parker of Coral Gables, Florida, on April 24th 1999, with his 40-cm f/6 Newtonian and a Lynxx PC CCD camera through red, green, and blue filters. Note the different appearance of the planet in each color. The images show a "cold front" coming off the North Polar Cap, a brilliant orographic cloud over Olympus Mons on the evening (left) limb, and clouds over the Elysium volcanic shield near the center of the disc. Also note the famous Blue Syrtis Cloud on the morning limb, in a slightly later enhanced color shot in the top right image.

RED GREEN BLUE

Figure 12.6. Mars imaged by Donald Parker of Coral Gables, Florida, on April 24th 1999, with his 40-cm f/6 Newtonian and a Lynxx PC CCD camera through red, green, and blue filters. Note the different appearance of the planet in each color. The images show a "cold front" coming off the North Polar Cap, a brilliant orographic cloud over Olympus Mons on the evening (left) limb, and clouds over the Elysium volcanic shield near the center of the disc. Also note the famous Blue Syrtis Cloud on the morning limb, in a slightly later enhanced color shot in the top right image.

Figure 12.7. This amazingly hi-res image of Mars was obtained from the U.K. by Damian Peach, on December 18, 2003, in a twilight sky. A Celestron 11 telescope and ATiK 1 HS webcam were used. The resolution is slightly better than one would expect for an instrument of this aperture: note the arc-second scale down the side and on the planet! Image: Damian Peach.

Figure 12.7. This amazingly hi-res image of Mars was obtained from the U.K. by Damian Peach, on December 18, 2003, in a twilight sky. A Celestron 11 telescope and ATiK 1 HS webcam were used. The resolution is slightly better than one would expect for an instrument of this aperture: note the arc-second scale down the side and on the planet! Image: Damian Peach.

Was this article helpful?

0 0
Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment