Agena AstroProducts Observing Guide: The Planet Mars

By: Brian Ventrudo
June 28, 2018

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Mars Observing Guide
Figure 1 - Mars imaged by Damian Peach in June 2018. Image credit: Damian Peach.

1. Overview

As one of the five bright planets visible to the unaided eye, Mars has perplexed and tantalized stargazers since antiquity. It's red-orange color, a consequence of iron-oxide in its surface sands, prompted classical astronomers to name the planet after the Roman god of war. Telescopic astronomers found Mars was remarkably Earth like. It had polar caps, seasons, an atmosphere, and surface features that seemed to change over days and weeks. Until the mid-20th century, a few astronomers even speculated there might be intelligent life and an advanced civilization on Mars.

Thanks to dozens of orbiting space probes and landers, we now understand that Mars appears to have no life, no liquid surface water, and a barren and inhospitable landscape. But for amateur astronomers, Mars remains a fascinating sight because it's the only planet to reveal an appreciable amount of surface detail in a small telescope, and it also features occasional surprises such as dust storms and local fogs and cloud banks.

Seeing Mars takes a little practice, however, as well as the right tools for the job. This guide will help you understand what you can see on the surface of Mars, especially during the time before and after the opposition of July 27, 2018, when Mars makes its closest approach to Earth since 2003. And it will help you select telescopes, filters, eyepieces, and other accessories to help you get the best view of this remarkable world.

2. When to See Mars

Except when it's in conjunction with the Sun and lost in the solar glare, Mars is almost always visible in the sky. When the planet is at its farthest point from Earth at an average distance of about 225 million kilometers (140 million miles), Mars shines at a modest magnitude of +1.8 and appears just 3.5" (arc-seconds) across. Even the largest telescopes reveal little detail on Mars when it's so small.

Mars Observing Guide
Figure 2 - Mars reaches opposition and closest approach to Earth when it is opposite the Sun in the sky.

Like all superior planets, Mars is best observed when it's near opposition, that is, opposite the Sun in the sky and at its closest point to Earth. The period in the weeks before, during, and after the opposition of Mars is often called an apparition. Oppositions happen, on average, every 780 days (about two years and two months) as the faster moving Earth catches up to the slightly slower-moving Mars as each planet orbits the Sun.

Mars Observing Guide
Figure 3 - Variation in relative apparent size of Mars during oppositions from 1995-2005 (credit: NASA)

Not all Martian oppositions are created equal. Because the orbit of Mars is substantially elliptical, there's a big variation in the Earth-Mars distance at opposition. If opposition occurs when Mars is closest to the Sun, the two planets can get as close as 35 million miles (56 million kilometers), and the planet grows to a respectable 25" in apparent diameter and a magnitude of -2.8. When opposition occurs at aphelion, Mars gets to within just 60 million miles (100 million kilometers), subtends an angle of just 15", and reaches magnitude -1.8 at opposition. Jupiter, by comparison, grows to nearly 50" in apparent diameter at opposition.

Every 15 or 17 years, Mars reaches opposition within a few weeks of its closest approach to the Sun and the planet grows especially large and bright. The closest approach of Mars to Earth in the past 60,000 years occurred in August 2003 when Mars came within 55,758,006 km. Subsequent oppositions were less impressive. The opposition of July 27, 2018 will be the best since 2003. During this opposition, the planet will reach a very bright magnitude -2.8 and about 24.1" in apparent diameter.

After 2018, Mars will appear smaller at each opposition through 2025. In the next opposition on October 13, 2020, Mars will reach a maximum apparent size of 22.3", and in December 2022, the planet will span just 16.9".

Mars Observing Guide
Figure 4 - The apparent size of Mars during it's 2018 apparition. Image credit: ALPO

3. What You Can See on Mars with a Telescope

Backyard telescopic observers will see three types of regions on Mars: one or both polar caps, the lighter red-orange regions covered with rusty dust, and the darker regions which are large areas of exposed volcanic rock. The rotation of the planet and the occasional unexpected dust storm on Mars bring these features in and out of view each night, which is partly what makes Mars so interesting to observe.

During early days of telescopic observation, the light regions on Mars were thought to be continents, so they were given names that described land masses. Major regions of this type include Elysium Planitia (the Elysium Plain), Arabia Terra (Land of Arabia), the circular Hellas Planitia (Plains of Greece), and Amazonis Planitia (Amazon Plains).

The darker regions were named after seas, lakes or other watery features. There's the very large Mare Erythraeum (Arabian Sea) in the south, Mare Acidalium (named after a legendary fountain) in the north, and most strikingly, the large wedge-shaped feature Syrtis Major named after the Gulf of Sidra off the coast of Libya. Many of these dark regions appeared to early telescopic observers to change size during the Martian year. They suspected this was caused by changes in vegetation or rainfall. It turns out the dark regions do not change their dimensions, but are instead occasionally obscured by atmospheric dust.

The Martian polar caps are also striking features that wax and wane with the Martian seasons. Most of each cap consists of a layer of carbon dioxide frozen out of the atmosphere. The northern polar cap has a layer of frozen carbon dioxide just 1 meter thick, while the southern polar cap has a layer about 8 meters thick. Underneath these layers is a base of water ice about 2 km to 3 km thick.

In a small telescope, only the largest features of the Martian surface are visible. The trick, especially for beginners, is sorting out which feature is which. The map below from Sky and Telescope magazine shows the most prominent non-polar regions of the planet projected onto a disk for a range of longitudes. In this image south is up, so if you are using optics that do not invert the image, you must make mental accommodation for this map.

Mars Observing Guide
Figure 5 - The major features of the Martian surface (excluding the polar caps). Each disk shows the major features visible through a telescope for the full range of Martian longitude. In this image, south is up. Credit: Damian Peach/Sky and Telescope. Click to enlarge and open in a new window.

Because of the rotation period of Mars, the view of the planet changes slightly from night to night. Mars rotates in the same direction as Earth every 24 hours and 37 minutes, just a little longer than the rotation period of Earth. If you observe Mars with a correct view with north up and east to the left, the planet will appear to move from left to right such that features on the surface will appear to move out of view 37 minutes earlier each night. The overall effect makes it appear Mars makes a full backward revolution in 40 days.

The Mars profiler from Sky and Telescope is a very useful tool to help you find out which surface features are visible each night. You can access the tool at the link below.

4. Tips for Observing Mars

Observing Mars through a telescope is like playing a musical instrument. It's a learned skill that takes continual practice. Even experienced observers need to practice their observing skills and learn to see the planet anew at each apparition. This ochre world is notoriously small and hard to observe visually, but it rewards the patient stargazer with sights unseen elsewhere in the solar system. It's worth the effort.

Some key tips for observing the Red Planet:
  • Observe near opposition. As mentioned earlier, Mars is only large enough to reveal detail about three or four weeks before and after opposition when it's closest to Earth.
  • Pick a Night with Steady Air. Seeing is critical when observing Mars. Pick a night when the air is steady and the stars aren't twinkling too much. Sometimes, nights with a little haze have steadier air than crystal-clear nights. On a night of bad seeing, Mars is a boiling orange blob. On a night of excellent seeing, it's a little world.
  • Acclimatize Your Telescope. Bring your scope out at least 30-60 minutes before you plan to observe. By cooling it down to the ambient temperature, you'll prevent air currents inside your scope from degrading the image of the planet. Scopes with large mirrors or lenses, and those with closed tubes, take longer to acclimate than smaller telescopes and those with open tubes.
  • Observe Frequently. Because Mars rotates every 24 hours and 37 minutes, you'll see almost the same face of the planet at the same time each night for several nights in a row. So extend your observing session over several hours on one night, or at the same time over four to six weeks to see both sides of the planet.

5. Best Telescopes for Observing Mars

On a night of good seeing near opposition, even a small telescope with 60mm to 80mm aperture will show the largest and most prominent features on the Martian surface, although the image will be small and the resolution too low to reveal much detail. If you want a larger and clearer image of Mars for visual viewing or imaging, consider a telescope with as long a focal length and as large an aperture as you can afford and physically transport and handle. Longer focal lengths give larger images, while larger aperture gives better resolution of fine detail.

Schmidt-Cassegrain and Maksutov-Cassegrain telescopes are a great choice for planetary observers and imagers. These scopes pack long focal lengths into a small optical tube, and larger apertures of 6", 8", 9.25" or more also offer higher resolution for revealing fine detail in good seeing conditions. Larger apertures also give brighter images at high magnification than smaller-aperture scopes.

For visual planetary observers, Celestron's NexStar and CPC telescope systems are a good choice. They offer good optics and long focal lengths in compact packages as well as computerized alt-azimuth mounts. Meade's LX-90 systems and the new ETX125 also give good visual views of the planets, as do the more advanced Meade LX200 systems.

Mars Observing Guide
Figure 6 - A Schmidt-Cassegrain telescope has a long focal length and large aperture, ideal for observing Mars

For even longer focal ratios and focal lengths, and therefore larger images, the razor-sharp optics of a good Maksutov-Cassegrain telescope are a good choice for serious planetary and lunar observers. Unlike Schmidt-Cassegrains, these scopes rarely require alignment and have smaller secondary mirrors for higher image contrast. The 127mm, 150mm, and 180mm Mak-Cass scopes from Sky-Watcher are excellent planetary performers.

Newtonian reflectors, especially affordable Dobsonians, are also a good choice for planetary observing. These telescopes have shorter focal ratios Schmidt-Cassegrain or Maksutov-Cassegrain telescopes, and they are usually longer and heavier, but they offer a lower cost for a given aperture. A Dobsonian telescope with 6" to 10" aperture offers good resolution and focal length while still being portable enough for one person to transport and set up.

With a lack of a central obstruction, refracting telescopes generally offer the best contrast for visual planetary observation. Semi-apochromatic apochromatic refractors with doublet objective lenses are a particularly good value for visual observation of Mars and other planets. These scopes are sometimes called "ED" or extra-low dispersion refractors. The f/9 100mm ED refractor and f/7.5 120mm ED refractor from Sky-Watcher are good choices and come at an affordable price, but their smaller aperture and shorter focal lengths are not ideal for observing Mars because of the planet's relatively small disk. Larger refractors with 127mm or even 152mm objectives are a better choice, but these telescopes are long, heavy, and pricey, and they require a very stable mount.

6. Eyepieces for Observing Mars

Which eyepieces are best for observing Mars? Nearly any good eyepieces will do, especially those that will give you a medium and high magnification. The highest usable magnification depends on the seeing conditions and the aperture of your telescope. Generally, a magnification of 30-50x the aperture (in inches) works well on nights of average-to-good seeing. For example, if you have a 4-inch telescope, try 120x to 200x. If you have an 8" scope, try 240x to 400x. Again, experiment to get the best view each night.

Mars Observing Guide
Figure 7 - An Agena Starguider eyepiece, an affordable eyepiece for planetary and general-purpose observation

Serious planetary observers favor eyepieces that have a minimal number of lens elements so that contrast and light transmission are maximized. Such eyepieces have an apparent field of view (AFOV) of less than 60° compared to wide-angle eyepieces. Many manufacturers make a series of eyepieces for planetary observing. Some good choices for planetary observing eyepieces include:

Plossls. Plossls are a reliable performer for planetary and lunar observing. They are also a good value for observers on a budget. Many manufacturers make a Plossl design including GSO and Tele Vue Optics. Agena's Starguider eyepieces, which are similar to the Plossl design, are also an economical and well-regarded choice.

Brandons. A long-established design that dates back to the 1940s, Brandon eyepieces are favored by planetary observers for their high-contrast and bright views of the planets. They work best with telescope of focal ratio f/7 or greater.

Vixen HR (High Resolution) Eyepieces. These eyepieces offer excellent contrast and eye relief. With focal lengths from just 1.6mm to 3.4mm, however, they can result in a magnification that is TOO HIGH for many telescopes.

Orthoscopics. Also a long-time favorite of planetary observers, Orthoscopic eyepieces from Kokusai Kohki, Baader Planetarium, and other manufacturers give excellent contrast in a small form factor.

Tele Vue Delites. A premium eyepiece from Tele Vue Optics, the relatively new Tele Vue Delites have replaced the company's Radian eyepiece line which was popular among planetary observers. With a 62-degree AFOV, Delites are also a good general purpose eyepiece.

Zoom Eyepieces. A high-quality zoom eyepiece is ideal for getting a wide range of magnifications without the bother of changing the ocular. The Baader Hyperion Mark IV eyepiece is excellent for planets and for all around use. The 3-6mm Nager zoom eyepiece is intended for use in small f/6 or f/7 refractors to give high-contrast planetary views and relatively high magnifications.

A good 2x or 3x Barlow lens also comes in handy for extending the focal length of your telescope and increasing image size with your set of eyepieces.

7. Filters for Observing Mars

With so many features on the surface of Mars and in the planet's atmosphere, a serious observer should consider color filters to help extract maximum detail.

Celestron offers a dedicated Mars observing filter that passes light at specific wavelengths to significantly enhance the visual appearance of the planet. If you're going to get just one filter to observe Mars, this is probably your best choice.

Mars Observing Guide
Figure 8 - The Celestron 1.25" Mars Observing Filter

Many standard color filters also help with observing the features on the Martian surface and in the atmosphere. Here are some suggested color filters for Mars observing sessions:

  • Orange (#21 or #23A) increases contrast between light and dark features and penetrates hazes and most clouds.
  • Yellow (#12, #15) can brighten desert regions and darkens bluish and brownish features.
  • Red (#25, #29) gives maximum contrast of surface features, enhances fine surface details, dust clouds boundaries, and polar cap boundaries.
  • Light Green (#56) darkens red and blue features, enhances frost patches, surface fogs, and polar caps
  • Blue (#80A,#38A) and deep blue (#47) shows atmospheric clouds, discrete white clouds, and limb hazes, equatorial cloud bands, polar cloud hoods, and darkens reddish features

Celestron also offers two Mars Observing Kits. One kit includes a 2x power-doubling barlow lens to increase the magnification of a 1.25" eyepiece as well as #80A Blue, #56 Light Green, and #25 Red planetary filters. The deluxe kit includes all above accessories plus the Mars Observing Filter.

8. Cameras and Accessories for Imaging Mars

With the right tools, good sky, and some patience, it's possible to get very good visual views of the planet Mars. But many amateur astronomers want to try to image the Red Planet. Getting an excellent image of Mars through a telescope is not an easy task, and it's not a project for beginners.

For those who want to try their hand at imaging Mars, the first step is to acquire a good planetary astronomy camera. Such a camera needs a relatively small sensor with small pixels for good resolution, and it needs to be matched with a long focal length telescope on a solid mount. ZWO makes excellent planetary astronomy cameras suitable for imaging Mars. The ZWO ASI290-series cameras are a good choice, as are the ZWO ASI224-series cameras. The Linked TEXT are good for imagers on a budget. A color camera works well, but some experienced imagers use a monochrome camera with a set of color filters and a filter wheel to produce a color image. A TEC-cooled camera is generally not required for planetary imaging, although if you acquire a cooled camera you can use it for other imaging applications as well.

Experienced imagers can also use, along with a camera, the ZWO Atmospheric Dispersion Compensator (ADC) to counteract the effect of our atmosphere in refracting light of various colors. Proper use of the ADC results in significantly sharper planetary images.

You can learn more about choosing an astronomy camera at this link. You can learn more about the ZWO ADC at this link.

9. Summary

The planet Mars is one of the most interesting and rewarding object in the solar system to observe. When it's near opposition and its closest approach to Earth, the planet can reveal subtle surface features and atmospheric phenomena. In fact, Mars is the only planet on which you can see many surface features at all. This guide has helped you understand what to look for on the face of the planet Mars through visual observation with a small telescope, and it suggested some good stargazing tools and accessories to help you get the best view.

Brian Ventrudo
About the Author

Brian Ventrudo is a writer, scientist, and astronomy educator. He received his first telescope at the age of 5 and completed his first university course in astronomy at the age of 12, eventually receiving a master's degree in the subject. He also holds a Ph.D. in engineering physics from McMaster University. During a twenty-year scientific career, he developed laser systems to detect molecules found in interstellar space and planetary atmospheres, and leveraged his expertise to create laser technology for optical communications networks. Since 2008, Brian has taught astronomy to tens of thousands of stargazers through his websites and