Observing Mars

Observing Mars


As an outer planet, Mars is best observed around its Opposition. Mars’ opposition occurs about every 26 months. Mars appears as a red/orange disk with the proper filters you might detect the polar ice cap or even atmospheric conditions, like clouds, on Mars.


Where To Look




If you were to trace the path of the Sun across the sky, the Sun’s path is a line called the Ecliptic.  The Ecliptic rises and falls during the year:  The highest point is the Summer Solstice and at the lowest point, 6 months later, occurs on the Winter Solstice.  Once you get a feeling of where the Ecliptic lies, you might discover that the moon and all the planets, with the exception of the former planet Pluto, lies within a few degrees of the Ecliptic.  The Ecliptic represents the edge view of the Solar System.


Scanning the Ecliptic will help you locate the moon and planets. To pinpoint a specific planet at a specific time, you may want to: consult magazines like Sky and Telescope or Astronomy, or use software (see below), or one of the new handheld computerized realtime gadgets (see below), or consult a website like Astro Planet.[ 🙂 Ed.]


Optimal Times


There are certain times in the orbit of a planet when a planet is “optimal for viewing.”  For the outer planets: Mars, Saturn, Jupiter, Neptune, Uranus and Pluto the point of best viewing is at the Opposition.



For planets outside the Earth’s orbit (Mars, Jupiter, Saturn, Uranus, Neptune and Pluto), the months around Oppositions are the best time to view these. An Opposition occurs when the planet is opposite from the Sun, relative to the Earth. At Opposition the planet will rise as the Sun sets and will set as the Sun rises providing an entire night of observation. Also at Opposition the planet comes physically closest to the Earth in it’s orbit so it appears as large as possible.




Due to its brightness, Mars is visible about 30 minutes before sunrise or after sunset.

Equipment Needed


Because planets are bright, though tiny in size, a large telescope isn’t necessarily required for viewing planets like Mercury, Venus, Mars, Saturn and Jupiter.  Large aperture telescopes are very beneficial to make dim things bright, like nebulae, galaxies, star clusters and the far outer planets: Uranus, Neptune and Pluto.


Almost any telescope capable of magnifying 100 to 200 times is great for viewing planets and our moon.




Eyepieces control magnification, field-of-view and eye relief.  You can consider the eyepiece half of your optical system.  Typically you will want high magnification eyepieces (100x-200x) for the moon and planets, while low power, wide field eyepieces are used for deep sky objects.


Calculating eyepieces


Each telescope is designed with a focal length.  Eyepieces also have a focal length. This value is usually printed on the side or top of the eyepiece. If you divide the focal length of the telescope by the focal length of an eyepiece that will give you the power or magnification that eyepiece will give with that telescope.


For example – An 8-inch Schmidt-Cassegrain telescope has the focal length of around 2000mm.  If you use a 10mm eyepiece with that telescope you will have 200 magnification (2000/10). A 30mm eyepiece in the same telescope will produce 67 power (2000/30).


So the lower the focal length of an eyepiece, the higher the power.


Sometimes eyepieces are also specified with Apparent Field-of-View measured in degrees.  If you were to divide the Apparent Field-of-View by the power you will calculate the Actual Field-of-View that that eyepiece would have with the telescope.


Planetary Filters

Because they reflect light directly from the Sun, most planets are very bright.  If you think of a telescope as a light amplifier, then most telescopes will produce an image of a planet that is too bright to pick out subtle details.  There are two ways to adjust the light: planetary Filters and Off-axis masks.


Wratten System


The Kodak company developed a numbering system to specify color filters for use with black and white film.  This is known as the Wratten System.  Astronomy uses the same numbering system to specify planetary filters.


Because observational astronomy lacks color in the views of astronomical object until one gets into very large aperture telescopes (greater than 10 inches), using a planetary filter is like using a color filter with black and white film.  They will reduce the brightness and enhance various features seen on the planetary disk.


For more information on Planetary Filters, click here.


Suggested Filters for Mars

Wratten Number Color Feature
12 Yellow to brighten the plains and darkens blue/brown features
15 Yellow to brighten the plains and darkens blue/brown features
21 Orange to increase contrast and detect dust clouds
25 Red to maximize contrast and enhance fine surface details
29 Red to maximize contrast and enhance fine surface details
30 Magenta to enhance red/blue features and darken green features
32 Magenta to enhance red/blue features and darken green features
38 Blue to detect clouds and enhance the polar caps
46 Dark Blue to detect clouds and enhance the polar caps
47 Dark Blue to detect clouds and enhance the polar caps
57 Green to darken red/blue features enhances polar regions
64 Blue-Green to detect Òice fogsÓ and Òpolar hazesÓ
80 Blue to detect clouds and enhance the polar caps
23A Orange to increase contrast and detect dust clouds
38A Blue to detect clouds and enhance the polar caps


Off-axis Masks


Using an off-axis mask on the front of a telescope is another way to reduce the light gathered by a telescope for observing planets.  An off-axis mask is a plate that fits in the front opening of the telescope with a smaller hole located between the center and the edge of the opening (off-axis).  Frequently off-axis masks are built into the dust cover of some Newtonian reflector telescopes.  The hole is placed off-axis to avoid being blocked by the secondary mirror, usually located in the center of the aperture.


Using and off-axis mask has two advantages of filters.  They do not introduce false color and by reducing the usable aperture makes the telescope less sensitive to poor seeing conditions caused by the turbulent atmosphere.

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