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Observing Mars

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Observing Mars

By Richard S. Bell


Appearing as a brilliant rusty red beacon on a clear night, the planet Mars has attracted the attention of skywatchers for centuries - and rightly so. Of the eight planets to circle our star, Mars is the only one whose surface we can study through a telescope. Mercury huddles too close to the Sun, while Venus’ hellish surface is enshrouded in a perpetual layer of clouds. The remaining worlds are all gas giants, with no surfaces to study at all. Mars is also the most earth-like of all the planets. Polar caps wax and wane with the seasons, puffy white clouds drift across the surface, and a roughly 24-hour day all remind us of home.


Orbits & Oppositions

Mars observers must be patient. While Jupiter and Saturn are always pleasing sights through a telescope, Mars is often too small to enjoy. The Red Planet may be our neighboring world, but it can have an angular diameter as small as 4″ when on the far end of its orbit.  The best time to observe Mars is around opposition, when a planet is opposite the Sun in the sky as seen from Earth. Oppositions of Mars occur every 26 months, the interval needed for Mars to catch up to Earth (or more accurately, the time for Earth to lap Mars in its quicker and shorter orbit about the Sun). However, due to the elliptical nature of Mars’ orbit, not all oppositions are created equal.


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Next to Mercury, Mars has the most elliptical orbit of all the major planets. It has an eccentricity of 0.0934, meaning it deviates from a perfect circle by about 9%. This deviation makes its solar distance vary between 128 and 155 million miles. Thanks to this eccentricity, the separation between Earth and Mars during opposition can also vary dramatically over time. The most favorable Martian oppositions recur every 15 to 17 years. Known as perihelic oppositions, they occur, as one might assume, when Mars is at both opposition with Earth and at perihelion with the Sun at nearly the same time. At its peak, the angular diameter of Mars can “swell” to 25.1″ at a distance of about 35 million miles (or 0.38 Astronomical Units).  During an aphelic opposition, the gap between Earth and Mars can be as much as 65 million miles. The Red Planet’s angular size in this instance is a paltry 13.8”.


The last perihelic opposition occurred in 2018 and the next will not take place until 2035. So, am I two years late (or 15 years too early) with an article about tips and techniques to view the Red Planet? Not at all. In fact, the coming observing season for Mars will be better – for us – than it was in 2018.


Observing Gear

Mars will be a pleasing sight with the unaided eye and binoculars this Fall. However, if you want to explore its rusty red surface in any detail, you’re going to need a telescope. Just about any aperture or style telescope will do, but these are my recommended minimum apertures by telescope type:

·         4-inch Refractor

·         6-inch Reflector

·         5-inch to 8-inch Catadioptric


The refractor should preferably have an apochromatic lens to provide razor sharp views free of chromatic aberration. Achromatic refractors aren’t quite as sharp and will show a secondary spectrum around the Red Planet, but your pocket book won’t take as large a hit. Any refractor should have a minimum focal ratio of f/6 or f/7 to provide sufficient magnification to reveal Mars’ surface features.


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Most Newtonian reflectors available for purchase are designed for deep sky viewing. They will have focal ratios between f/4 and f/5. Their secondary mirrors are fairly large so contrast will be sacrificed.  A 6-inch f/8 Newtonian is a good compromise between planetary performance and portability. Larger apertures with high focal ratios are available, but their tube lengths make transporting them to suitable sites prohibitive.


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With catadioptric telescopes, focal ratios are not a problem and their folded optical designs makes them highly portable.  Even 5-inch Schmidt-Cassegrains have focal ratios of f/10. That secondary mirror will result in loss of contrast, but lower costs compared to apochromatic refractors and high portability make it a worthwhile compromise.  One model of catadioptric telescopes, the Maksutov-Cassegrain, rival even apochromatic refractors. Maks (as they’re affectionately known) with apertures of 6 or 7-inches have focal ratios of f/12 or f/15. Loss of contrast is minimized thanks to small secondary mirrors and the color contrast is fantastic. Maks aren’t as widely available at Schmidt-Cassegrains, but they’re not difficult to track down. Manufacturers like Celestron, Orion, and Sky-Watcher have models 7-inches in aperture available. When no planets are visible, they’re great for double star observing!


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Any telescope you use should have a sturdy mount.  The best optics in the world would be wasted if the mount can’t minimize vibrations in less than a second or so. Non-tracking mounts like Dobsonians will do, but a clock-driven mount will allow you to focus on viewing Mars’ features. If you don’t have a telescope in the suggested aperture range then no need to worry. I had great success viewing Mars during the historic opposition of 2003 with a Tele Vue 70mm Pronto (a long discontinued semi-apochromat).


Most eyepieces in our collections feature ultra-wide fields-of-view for deep sky viewing. While these are suitable for viewing Mars, the generous fields they offer are overkill.  There are specialized eyepieces available for planetary viewing. Tele Vue’s DeLite eyepiece series still have a generous 62° apparent field-of-view, but offer fine planetary performance at a more affordable price than their high-end Nagler and Ethos eyepieces. Baader Planetarium offers the Hyperion eyepiece line. Focal lengths range from 5 to 24mm and have apparent fields of 68°. These eyepieces can even be reconfigured for afocal (eyepiece projection) imaging with DSLR cameras.  Even more affordable are Orion’s Edge-on series of planetary eyepieces. These range in size from 3 to 14.5mm and have 55° apparent fields. Both the Hyperion and Edge-on eyepieces come in sets that will save you even more money.


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Old school eyepiece designs like Orthoscopics and Plössls will also do well for planetary viewing. Orthoscopics are a little hard to come by these days, but Plössls are widely available and very affordable. Exact focal lengths are difficult to recommend. It depends on the telescope you use and the seeing conditions on any given night. It’s best to have several focal lengths in your in collection.


A great way to increase your range of magnifications for viewing Mars is to add a Barlow lens to your collection. Barlows artificially increase (usually double) the focal length of an eyepiece with the use of a negative lens element. Most are more affordable than eyepieces, but there are higher-quality versions to match high quality eyepieces. Tele Vue’s Powermates perform the same task as Barlow lenses, but use a positive lens element. This preserves the exit pupil of the eyepiece being used and is free of other aberrations some Barlows have. They are more expensive, of course, but still cheaper than high-end eyepieces.


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Colored planetary filters are essential for the observation of surface detail that is often virtually invisible without filtration. Most are inexpensive and can be purchased in sets to save even more. While every colored filter does something for Mars, here’s a breakdown of the best ones and the features they’ll enhance:

·         #12 Yellow: Increases contrast of maria and helps to spot clouds in the Martian atmosphere.

·         #21 Orange: Sharpens boundaries between yellow-orange areas and blue-green regions on Mars, resulting in a darkening of edge-detail in the maria.

·         #23A Light Red: Useful for the outlining of the Martian polar ice caps, maria, and dust storms. Probably the most commonly used filter for Mars.

·         #56 Light Green: Excellent for the observation of polar ice caps as well as yellow-tinted dust storms.


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A minus violet filter is recommended for observer’s that want to view Mars in its natural light with an achromatic refractor. The minus violet filter will mask the secondary spectrum inherit in the optical design. Baader Planetarium’s version is called the “Contrast Booster Filter” and costs about $90. There are even specialized filters just for Mars now available. Tele Vue used to offer the Bandmate Mars Filters Type A & B, but they have been since discontinued. You could probably find them on the used market. Orion has the 1.25-inch Mars Observation Eyepiece Filter for about $40. Their Mars filter transmits violet/blue and orange/red wavelengths where detail is rich, while blocking overbearing yellow/green wavelengths. Polar ice caps appear more sharply defined, while differences in landscape shading are nicely resolved. Polar hazes and cloud cover also become visible.


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Finally, when viewing Mars through the eyepiece, it helps to know what you’re looking at! In fact, it makes the experience much more enjoyable. Desktop planetarium programs like TheSkyX and Starry Night will show you which features are currently facing Earth, but their fancy graphics don’t match what Mars looks like in the eyepiece. Sky & Telescope has a handy app on their website dubbed “Mars Profiler.” Starting the app shows Mars’ current face, but you can enter various dates and times (using Universal time, just add 4 hours during daylight time and 5 for standard time). Look for the central-meridian longitude and then you can use the handy-dandy Mars Map I assembled for the 2018 opposition while out in the field (using a map from the Association of Lunar & Planetary Observers). My favorite program for Mars viewing is called Meridian. It hasn’t been updated for 8 years, but runs fine on Windows 10. After downloading and installing the program, you must enter your language preference, date format, latitude, and time zone every time (which is kind of irritating).  Select Mars from the planet menu. The graphics may not be fancy, but that’s the point.  It’s a realistic view of Mars through a telescope.


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Observing Tips

Aside from overcast skies, the greatest menace for Mars observers is atmospheric seeing. However, even heat escaping your telescope can distort your view of the Red Planet. So, make sure you set up your telescope early enough for it to reach thermal equilibrium. To guarantee peak performance, be sure to also collimate your telescope as accurately as possible. The advantage of refractors is that they require infrequent collimation – in some cases never. The only exception is if your refractor takes a good bump.  Some models can be collimated by the user, but others must be returned to the manufacturer. Newtonians can be collimated while you wait for the sky to darken thanks to Cheshire eyepieces and laser collimators. Collimating systems are available for Schmidt-Cassegrains, but most people align their optics on a star. Maksutov-Cassegrains share the same advantage as refractors, in that they require infrequent collimation.


Many amateur astronomers go to great lengths to seek out dark skies for observing faint fuzzies. The advantage of planetary observing is that it’s largely unaffected by light pollution. In fact, when observing Mars (or any other planet) don’t worry about dark adaption at all! Now, don’t go thinking you can observe in a supermarket parking lot (the seeing would be terrible), but any city park with an unobstructed view would do nicely. Your eyes become less sensitive to color as they adapt to dark conditions.  Planets like Mars have a plethora of colors! Feel free to even use a white flashlight when checking out your KAS Mars Map. I know, it feels weird to say that!


Observe Mars at its zenith, when it’s due south and at its highest altitude for the night. This is easy to determine with any desktop planetarium software. On the night of opposition, October 13th at 1:36 am EDT, Mars will be 53° above the southern horizon in the constellation Pisces and 22.3″ in angular diameter. Compare that to the perihelic opposition in 2018, when Mars reached a maximum altitude of only 22° on July 29th. This is why this year’s opposition is better than 2018. Mars may have been 2″ bigger, but that extra 31° in altitude makes a world of difference for the stability of Mars in the eyepiece.  If you’re planning a trip to more southernly latitudes in October, then pack a telescope. The further south you go, the higher Mars gets.


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Michigan weather can be unreliable (to say the least), so don’t fret if it’s cloudy on the night of opposition. In fact, Mars will be 20″ or larger between September 9th and November 2nd. Comfort is also very important when viewing through the eyepiece. There are many observing chairs available for purchase and many clever designs for ones you can build yourself.  The more comfortable you are, the more relaxed you are. This will help you focus, because when you’re at the eyepiece you need to be patient and stare at Mars intently. The seeing will distort Mars much of the time, but there are those brief moments of stability when features pop into view. You’ll miss them if you’re standing and swaying around. So, sit down, relax, be patient, and enjoy the view.  The longer you look, the more you will see.


Finally, sketch Mars at the eyepiece. Nothing focuses your attention better than taking the time to draw what you see.  Use a graphite pencil and smudge stick to emulate the mottling on the Martian surface. And remember, you can use a white flashlight to see what you’re doing!


Top Sights to See

South Polar Cap. Mars’ South Polar Cap is at a higher elevation than the North Polar Cap, thus it is cold enough for the water ice to be permanently covered by a layer of frozen carbon dioxide (dry ice). It’s less than half the diameter of the northern cap and will be tilted by about 10° in our direction. The northern cap will be tilted out of view, but you should be able to spot the north polar hood. That’s a cloud zone that develops over the polar caps during late summer and persist through the winter, and sometimes into early spring. It is composed of water ice clouds.


Syrtis Major. This prominent V-shaped feature was discovered by the famous Dutch scientist Christiaan Huygens on November 29, 1659. He used it to determine that Mars had a roughly 24-hour day like Earth. First known as the Hourglass Sea, it was renamed Syrtis Major Planum by noted Mars observer Giovanni Schiaparelli in 1877. The orbiting Mars Global Surveyor spacecraft discovered that it was a low-relief shield volcano. Its dark color comes from basaltic volcanic rock and a relative lack of dust.


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Hellas. Mars is too distant to observe impact craters...with one notable exception. Hellas Planitia formed during the Late Heavy Bombardment period of the solar system about 4.1 to 3.8 billion years ago. It is the 3rd or 4th largest impact crater in the solar system and the largest impact crater visible from Earth. It is 1,400 miles in diameter and 23,465 feet deep. That makes it a little hard to miss, even from Earth!


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Solis Lacus. The Latin to English translation for this prominent Martian feature is “Lake of the Sun.” It is also known as the “Eye of Mars.” Several features help form the entire eye, but Solis Lacus itself is the dark pupil in the center.



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There you have it! Your guide to observing Mars during the Fall of 2020. A global dust storm spoiled the 2018 opposition, so let’s hope the Martian skies remain clear this Fall. Take advantage now. Do not wait until 2035! May your skies be steady and clear.


Richard Bell has been an amateur astronomer for over 40 years and resides in southwest Michigan. He is the current president and a lifetime member of the Kalamazoo Astronomical Society.

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