Tuesday, January 24, 2012

Observing Mars

Mars is getting close to its "opposition". That's the best time to observe Mars, generally (depending on conditions otherwise.) That's because that's when Mars is closest to Earth, and it is its brightest.

Image by amateur astronomer Arnomane of Wikimedia
Image by Arnomane

That also means that Mars is at its largest, as viewed from Earth. Oppositions occur roughly once every two years. This time, Mars will be about 13.9 arcseconds across, at a distance about 1.66 times the distance to the Sun. For comparison, the Moon is about half a degree across, or just about 30 arcminutes. Each arc minute is sixty arc seconds. That means that the Moon is about 1800 arcseconds across.

The Moon is large enough to see a fair amount of detail by eye. But to our eyes, Mars appears only as a bright dot, even at its largest. The minimum angular resolution of the human eye is about 1 arc minute, or 60 arc seconds. Mars would have to be over four times larger for us to see it as anything other than a dot by eye. It certainly never gets as large as the Moon! This is known as the Mars Hoax, an internet hoax that's been going around, and around, and around on the internet for about nine years now. It just can't look that big!

Make It Bigger

Since we can't see a lot of detail by eye, we use magnification to see the detail of Mars's surface. At the low magnifications of ordinary binoculars (7x to 20x), Mars shows as a small disk, but little or no detail is visible. A stable mount and higher levels of magnification are needed, which means that a telescope is the best instrument for observing Mars.

Mars will begin to show details at about 35-50 powers of magnification. This is low magnification for your telescope. At this level large things like the polar ice caps and Syrtis Major (pronounced like "Sir Tiss Major"), a large dark area on Mar's surface, become visible. To see more details requires increased magnification.

How much magnification you can use will depend on the observing conditions, the quality of your scope's mount and tracking, and your experience in astronomical observation. Mars is a good object to learn with. It's bright, it has both strong and subtle details. If you want to get good at using higher powers (over about 150-200x) on your telescope, this is a great opportunity.

For myself, I do my detailed observation at about 350-400x. At this level I find I can get the most detail with my 8" Newtonian telescope without it being a strain to track Mars with my scope, or having to break off my observation too often. Larger telescopes, and very well made refractors with good mounts will be able to use higher powers, but for most observers with little high power experience, anything over about 200x will be challenging.

Now in Living Color!

To tease out the finest details of Mars, it can be very beneficial to use colored filters. I have two sets of colored filters that I use for general observation. Dim objects don't work well with filters because the filter cuts off too much light to leave a bright enough image to see. Fortunately, Mars is quite bright.

I observe Mars with all the different colors of filter, but I find the red and yellow color filters to bring out the most unique detail. A blue filter is often worth observing with, it brings out polar caps and clouds on Mars. Yellow and orange filters bring out details, usually better than a red filter.

There are no hard and fast rules I've found in my own observation of Mars with colored filters. Different viewing conditions and different seasons on Mars each have an effect on what will be seen. So I usually cycle through all of them in an evening, then go back to those that brought out the most unique detail.

Take a Picture, It'll Last Longer

To really teach yourself to see detail, try doing sketches of Mars at the eyepiece. I draw several circles on a sheet of paper ahead of time for the outline of the planet (or just print out a sheet with circles about 2" in diameter on it), then sketch in basic light and dark areas with a normal pencil.

I then take notes of the time, date, magnification, and any filters that I am using. I make sketches with each of the different filters that I find work well on Mars during that observing session.

When I'm observing without a filter, I make notes on the colors of the details and try to fix them in my mind. Later, when I'm inside with light, I color in areas of my sketch with colored pencils to try to capture the colors, or color differences, that I saw on Mars.

Drawing Mars really helps you to learn to see the details on Mars. It also gives you a personal record of what you saw. You can go to the internet with the time that you saw Mars, or to your favorite astronomy program, and look up Mars's orientation at the time you drew it. Compare what you find with what you drew. It will never look the same, but you can see a match between different parts of your drawing and the image of Mars as it was when you drew it. Going back later, you may see other details that you didn't capture in your drawing.

At which point it's time to start another drawing. :)

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