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Observing the Moons of Mars (from 2005)

Other articles about mars:

Observing Mars in 2010

My Dream of Asaph Hall (an observing essay)


Note: mars will not be large enough for serious moon hunting until at least the 2016 opposition.  Until then I am leaving this page from 2005 up for general reference.


November 2005:

So by now you've probably heard that the August 2003 opposition of Mars was the closest in recorded human history.  Now, two years later, mars is back at opposition again and although it won't come quite as close as in 2003, it is still a good opportunity to observe mars and its moons.  As October opened the planet was 18" in diameter and the first two weeks of November finds it at the maximum of 20".  Not only will Mars offer terrific views during this time span but we also have the chance to observe its elusive moons, Phobos and Deimos.

Now is the time to try it for yourself!  This guide is deigned to help you find success.  All you need is dedication to the task, a good observing site, at least a 4-inch telescope, and a bit of luck.  But most of all you simply need to believe that it can be done and that you can do it.

Deimos, the outer moon, is the fainter of the two but paradoxically considered by many the most easily observed.  The great difficulty in observing these moons is that they are overwhelmed by the glare of the much brighter planet.  One trick for successfully observing them is to try when they are at their most distant.  These most distant points are called elongations and occur twice every orbit, once to the east and once to the west.  Elongations occur every 3 hours 50 minutes for Phobos, and every 15 hours 09 minutes for Deimos.  Your best chance to see Phobos is within 30 minutes of an elongation.  For Deimos there is more leeway, perhaps as much as two hours before/after elongation.

Deimos will appear as an 12.0 magnitude star that can be separated from Mars by as much as 70" (down from 87" in 2003).  Phobos will appear as a 10.9 magnitude speck that may be separated from Mars by as much as 28" (down from 35" in 2003).  For comparison, at their greatest distances from the planet during the 2003 opposition Phobos was 35" away while Deimos reached 87".  Although that is a reduction to 80% of the apparent distance from the planet, future oppositions are only going to get worse; they next time the moons will be this separated from the planet will be in 2018!

Your greatest enemy will be the glare from the planet.  A telescope that scatters as little light as possible will be a great asset.  Use high medium-magnification and try putting mars just outside of the field of view.  Be persistent.  Reliable reports came in during the 2003 opposition of observations of Deimos in 6-inch and even 4-inch telescopes!  The overriding factor in success is the atmospheric conditions at the time.  Look for a dark night with little scattered light (halos) around the bright stars.  The preceding day should have a deep blue sky.

General Tips for Success

I've assembled these tidbits gleamed from recent personal experience that I hope will help others find success.

  • Wait until Mars is at least 30 degrees above the horizon; the higher the better.

  • Look for a dark night from a reasonably dark location.  Moonlight and light pollution will hurt your chances.

  • Spend a great deal of time--be prepared to spend many hours at the eyepiece.  In scopes smaller than 16-inches this is a real feat of observing skill.  Patience is the greatest observing skill of all.  Have a comfortable chair to sit in (if you can sit) and be sure to be as comfortable at the eyepiece as possible.

  • If at first you don't succeed... Yep, keep trying.  Atmospheric conditions are highly variable.  An hour from now things may be better, or even tomorrow night, or perhaps the next.  Just keep at it.  I spent a great deal of time trying to spot these moons during the 2000 opposition and never did.  Yet, on a fateful night two years later with the same scope at the same spot I was able to see them both clearly.

  • Try every moderate-high power eyepiece you own or can borrow.  Forget about what you may have heard about using "less glass", etc.  Just try them all earnestly.  I found 250x worked best for me, but I was able to see both moons at 150x and 430x.  For me, an 8mm Radian worked best.

  • Use software to predict the locations of the moons.  It is essential to observe them while they are well away from the planet.  It helps to know where to look.  Estimate the distance to each moon in terms of mars diameters.  I knew, for instance, to look for Phobos above the equator of the planet at a distance of one mars radius from the edge of mars.  The polar cap tells you where south is to get you oriented in the eyepiece.

  • If you use a Newtonian and can rotate the tube, do so until the flare from the spider vanes forms a triangle about the area in which you hope to spot a moon.  At the very least, try to get the moon out of the glare of one of the vanes.

  • Use software to predict the locations of nearby 10-12th magnitude stars.  I found that spotting an 11th magnitude star in the same field as Mars gave me a lot of confidence when it came to spotting a similar speck of light closer to the planet.

  • If you own a reflector make sure to collimate it carefully.  A smudge of light is much more difficult to spot than a tiny, tightly focused speck.

  • Focus on a nearby faint star rather than on Mars.  Try to relax your eye and imagine that you are looking into the distance while focusing.  Most people don't focus neutrally--the eye isn't completely relaxed and focused at infinity.  While looking for a moon you should relax your eye, yet this can cause a small shift in focus unless you are careful.  Good focus is critical!

  • Use averted vision; experiment with your eye placement rather than just stare at the expected location.  I had my first glimpse of Phobos while looking at the planet.  Try looking in a widening circle about the position.

  • Look out for internal reflections masquerading as moons.  With such a bright target in the scope all sorts of tiny ghost images and other aberrations can appear.  If your scope doesn't track the sky this is pretty easy.  Did the speck you saw move with the planet?  If id did this is probably your moon.  Reflections typically move very differently, often in the opposite direction of the objects in the eyepiece.

  • Make an occulting bar (see below).  This is less critical in larger instruments (16-inch or greater), but even then may be the critical difference.

Making an Occulting Bar

Many observers have successfully made an occulting bar to block the glare of Mars.  To do this, tape a tiny strip of aluminum foil spanning the point of focus of a medium- high magnification eyepiece.  Some eyepiece designs will be more appropriate for this than others.  My 8mm Radian, for instance, comes to focus inside the eyepiece and I wasn't about to take it apart.  Plossls often work well though.  Carefully look through your eyepiece holding a pencil tip near the other end. Be careful not to scratch or damage your eyepiece!   Can you see the pencil tip?  Can you find a spot where it is close to being in focus?  Eyepieces usually come to focus at the position of the field stop -- a sort of baffle in the eyepiece.  This can also be handy place to attach the strip.  Tie each end of your foil strip down with a tiny bit of clear tape.  I use a mechanical pencil to push the foil into place and smooth the tape out.  Once in place you should be able to place Mars behind the silhouette of your makeshift bar, blocking most of its glare while leaving the moons visible to either side.

How Large Does my Telescope Have to Be?

Spotting both moons in an 18-inch or larger telescope is a bit of a challenge, but once you do they are easily seen on a good night.  I found Phobos to be the easiest, despite it's proximity to Mars.  I estimate that a very dedicated experienced observer from a good location should be able to spot at least one moon in a 10-inch.  Even an 8-inch should not be out of the question.  I probably spotted Deimos in my 6-inch f/8 Newt. during the excellent opposition of 2003.  I say probably because I was not able to be absolutely certain.  Any observation in smaller apertures is going to be marginal.  At best you will glimpse a faint speck at the proper position then it will disappear.   If this happens be patient.  It may reappear again.  The more times you spot it the more certain you will be of your observation.  And you may just get lucky enough to see it for several seconds--long enough to be pretty darned sure.  But there are a lot of stars out there.  Be sure that you have eliminated the possibility of a field star near Mars.  Could it be done in a 4-inch?  I really think it might be possible with the right observer under just the right conditions.  Doing so would be a real feat!

Maximum Elongation Times

Deimos will be at greatest eastern elongation at 2005 October 1 15:52 UT.  Phobos will be at greatest eastern elongation at 2005 October 1 17:56 UT.  To find subsequent times of greatest elongation add 3 hours 50 minutes for Phobos and 15 hours 9 minutes for Deimos.  

All simulations via our SkyTools software.