Brands - JMI

JMI

Menu

Tips and Planning Tools for the 2018 Mars Opposition


Observing Mars

We need to state at the outset that Mars is a rather difficult object to observe. One member of our staff would go so far as to call it "a difficult little dirt ball." Mars is likely to be disappointing the first time you see it. That does not mean it will stay that way, given the right combination of equipment, atmospheric conditions, and a little bit of patience, but you should not expect to see the kind of detail visible in a Hubble Space telescope such as the one above. A wealth of detail can be seen, provided that both your optics and the seeing conditions allow for magnifications of 200x or more.

They say a picture is worth a thousand words, but when it comes to visual astronomy, a sketch is often better. Anyone who has ever read The Year Round Messier Marathon and spent any time at the eyepiece of a telescope can tell you that the sketches contained in that book are a good representation of what you can expect to see in the eyepiece. The sketch below was drawn by our own Janice Romer in 1984. The telescope was a Celestron C14, but Mars had an angular size of only 15 arc-seconds at the time. Mars will have approximately the same angular size as of June 1, and will go on to reach a maximum angular size of 24.3 arc-seconds when closest to Earth on July 31. Given that Mars will be more than 50% larger in the eyepiece than when this sketch was originally made, you can expect to see this level of detail in a far more modest telescope this summer. One again, this assumes that you have the right combination of equipment, atmospheric conditions, and a little bit of patience.

Planning Your Observations of Mars

The first and most obvious thing to know is where to find Mars this summer. Throughout the summer Mars will be to the east of Sagittarius and slightly west of the bikini asterism in Capricorn. The path of Mars over the next few months is shown below; click here for the larger source document. Experienced observers will immediately recognize that this means Mars will be a late night object this summer. Transit times for the Philadelphia area, meaning when Mars is positioned due south and at its highest elevation in the sky, will be after midnight for the ten days prior to and after the closest approach on July 31. Challenge number one this year is that Mars will be a late night object.

The US Naval Observatory provides an online calculator to find the rise, transit, and set time for Mars from any location and date you might find interesting. It also reveals the maximum altitude Mars will reach at transit. For the Philadelphia area that maximum altitude ranges from a high of only 27 degrees on July 1, to a low of 23 degrees in the middle off August, before rising again to 27 degrees on September 30. Challenge number two this year is that Mars will be low in the sky and subject to a great deal of atmospheric disturbance.

Another tool we like comes courtesy of NASA. The Mars Interactive map allows you to view the planet from any angle. This tool is useful for familiarizing yourself with the location of various surface features before observing, or confirming what you may have seen afterwards.

What Can You See?

The easiest thing to see on Mars are the polar caps, which are small white regions made up of carbon dioxide and water ice. Note that both the Hubble image and sketch above show the northern polar cap; what appears to be the southern polar cap is actually Hellas Planitia. This year the southern pole is pointed towards Earth, and Spring has just begun in the southern hemisphere of Mars. This means that the southern polar hood will fade from view and that the southern polar cap will begin to shine in brilliant white. As Spring continues to warm the southern hemisphere the polar cap will begin to recede. This is one reason why it is important to observe Mars often because the features you see will change over time.

The next thing to look for is Syrtis Major, a large dark albedo feature depicted in both images above. The contrast of this feature changes over time. it regularly darkens as the season moves from Spring into Summer, which lead early astronomers to speculate on the existence of vegetation on Mars. Today we know that this has more to do with the different illumination of the planet and to a lesser extent, the redistribution of dust.

Directly beneath Syrtis Major is Hellas Plantina, a large impact basin that could easily be confused for the southern polar cap if it is not pointed towards earth at the time. As the Mars Interactive map shows, Hellas is actually much larger than the southern polar cap. The southern polar cap will be pointed towards Earth this summer, so both features should be visible.

As we reach the second half of July and the closest approach nears, it may be possible to see sections of the south polar cap to detach from the main body as the ice recedes. As that ice recedes, a dark band begins to separate the brilliant white ice and the rusty red of the surface. This was easily seen with modest instruments back in 2003 when Mars was slightly closer to Earth.

One regularly recurring remnant of the receding polar cap is know as the "Mountains of Mitchel" and may be visible as two distinct white dots to observers using 8-inch or larger telescopes. This feature will be located south of Hellas Planitia, but will be very challenging to see. Consider this one a challenge!

Equipment for Observing Mars

This may seem a little silly, but the best telescope for observing Mars will be the one that you can both afford and easily take out and use night after night. The optics should ideally be capable or reaching 200x magnification or more. Using the 50x magnification per inch of aperture rule of thumb, that tends to set the floor at four inches of aperture. Some optical designs, such as apochromatic refractors, can easily exceed that rule of thumb. Others, particularly very inexpensive beginner telescopes, have difficulty achieving that level of performance. Owners of quality 80 mm and larger apochromatic refractors shouldn't rush out to buy a larger instrument, although we would be glad to help you if you want to. Our first recommendation for observing Mars is to verify that your telescope can achieve 200x or more magnification without serious image degradation.

Telescope aperture determines the resolving power of a telescope. All else being equal, the larger telescope will be capable of resolving finer detail, up to the limitations of the atmospheric seeing conditions. Of course "all else being equal" is generally never the case; you cannot really compare an unobstructed instrument such as a refractor to an obstructed instrument such as a Newtonian. As a general rule of thumb, instruments with small or no central obstruction produce images with greater contrast than those having larger central obstructions. Ignoring the resolution advantages of larger instruments, it is the opinion of this author, based upon observations of Mars oppostions in 2003 and 2005, that the following telescopes produce the most visually enjoyable images:

  • Apochromatic refractors
  • Maksutov-Cassegrains
  • Schmidt-Cassegrains
  • Slow Newtonians, with focal ratios of f/8 or higher
  • Faster Newtonians

Note that most Newtonians have for-vane spiders to hold their secondary mirrors. These produce bright diffraction spikes, or a white cross emanating from bright objects like Mars and Jupiter. Some of us find this particularly annoying when studying the planets.

One last thing to keep in mind is the telescope mount. In order to truly appreciate Mars you will need to spend a lot of time at the eyepiece, focusing on the planet itself and not re-positioning the telescope. If you have a tracking mount in your arsenal of observing equipment, that is the one you will want to use.

Moving on from telescopes, eyepieces are the next most important item on the equipment list. We've already stated that you will want to achieve magnifications of 200x or greater in order to get a good look at Mars, and this implies shorter focal length eyepieces with most telescopes. Other points to consider are eye relief and contrast. Getting the most out of Mars requires a lot of time and patience at the eyepiece. When pushing the limits of magnification up against the limits of atmospheric seeing conditions, there will be moments of razor sharp focus separated by periods of softer focus due to turbulence in the atmosphere. Many of us find it easier to spend time at the eyepiece when the eye relief is more generous. Holding your eye in just the right position when eye relief is less than 8 mm can be very tedious.

Contrast is the other major factor to seek in an eyepiece. High contrast is necessary to discern subtle difference in planetary features. Simpler eyepieces tend to provide better contrast than ultra-wide angle eyepieces having many more glass elements. Mars is a rather small object to begin with, so there is no need for ultra-wide field eyepieces to observe it anyway. High quality Plossls can be a good choice for planetary observing. Unfortunately the eye relief of a Plossl eyepiece is directly proportionate to its focal length, and many of us find Plossls of 12 mm or lower focal length somewhat tedious to use.

One of our favorite premium eyepieces for planetary observing is the TeleVue Delite. These eyepieces combine a more than adequate 62 degree apparent field, a generous 20 mm of eye relief across the entire line, and excellent contrast. The TeleVue Delos eyepiece line has similar characteristics, albeit with a wider 72 degree apparent field and a higher price tag. For those whose scopes require an eyepiece in the 3 mm focal length range to achieve high magnification, the unique 3 mm - 6 mm Nagler Zoom by Tele Vue offers the greatest versatility when it comes to dealing with a turbulent atmosphere. You can simply dial in the highest magnification usable for the current seeing condition. Its more limited 50° apparent field of view lends itself better to scopes on tracking mounts.

Let us now turn our attention to filters. Mars will be as bright or brighter than Jupiter throughout much of July and August. Jupiter is bright object, even in a telescope with only four inches of aperture. Reducing that brightness and glare is a good first step towards be able to comfortably observe Mars for long periods of time. A simple and inexpensive variable polarizing filter is really something that every lunar and planetary observer should have in his or her eyepiece bag. The Orion Variable Polarizing filter, available in both 1.25 and 2-inch formats, removes the glare and allows you to adjust the light throughput from 40% down to 1%, all while remaining color neutral. Observers using larger apertures should give this filter some serious consideration.

When it comes to colored filters, the Wratten #23A red filter is the classic recommendation for Mars. This filter increases the contrast between lighter and darker regions of the planet's surface. Specialized filters such as the Orion Mars Filter take it a step farther by transmitting both the violet and blue as well as the red orange portion of the spectrum while blocking most of the yellow and green light. The Baader Moon and Skyglow filter may be the ultimate in terms of Red/Green/Blue enhancement, by isolating these three primary colors while blocking the intermediate colors separating them.

Tips and Techniques for Observing Mars

We've already mentioned two challenges this season. One of these was the late night nature of this year's event, which there is nothing you can do about. The other was the relatively low position of Mars above the horizon. While you can't do anything about it, there are a few things you can and should do to minimize the impact. The most obvious of these is to pick an observing site with a good southern horizon. When setting up to observe in a field or clearing, position yourself to the northern end of the field in order to extend your southern horizon. If travel is an option, travel south. For every degree of latitude you go south, Mars will appear one degree higher above the horizon. Travelling south from Philadelphia to Charlotte, NC or similar latitude will place Mars five degrees higher above the horizon. Travelling to Jacksonville, FL will place Mars ten degrees higher than in Philadelphia.

The second thing to think about is how to minimize the negative impact of atmospheric disturbances. Because Mars is going to be low in the sky, you will be observing through a good deal more of the Earth's atmosphere than if it were higher in the sky. While you cannot change the atmosphere per se, you can certainly minimize localized disturbances by carefully choosing your observing site. Rising heat causes air currents that soften the image and limit the useful magnification. Avoid observing across a blacktop parking lot or rooftops in the close foreground. Remote park sites without a lot of man made heat source to the south are definitely helpful. Large bodies of water can have a calming effect on the atmosphere; try setting up to the north side of a lake or other large boy of water if the opportunity arises.

Further Reading

Observing the Planet Mars by Jeffrey D. Beish -The definitive work on the topic of Mars observation, although the equipment section id rather obviously dated.

Observing the Planets with Color Filters by Jeffrey D. Beish - This covers all of the planets but has extra emphasis on Mars.

Coming Up Next

In our next edition we will share our own observing experiences with a variety of instruments to give you a better idea of how this opposition is shaping up, and what you should challenge yourself to see.

Printable Version