Normally it would seem silly to cut off some of the light that comes through your telescope. For many objects, we want as much light as possible to see faint detail. But for some things, like the Moon and planets we often have plenty of light. What we want to see is details that can be washed out by that light, or that are hard to see without some sort of enhancement.
Mars is near its opposition right now. That means its the biggest and brightest it will be for the next two years or so. Right now (March 2012) it's 14 arc seconds in diameter. By June it will have receded enough from us to only appear half that size.
The planet Mars has lots of fine detail that's hard to see in a telescope. This image shows haze near the polar caps and clouds. Also, there is a lot of fine detail in the reddish/yellowish face of the planet.
Mars has very faint and difficult to see detail. A lot of that detail is lost in ranges of color that our eyes aren't very good at seeing detail in--the orange-red end of the color spectrum. Fortunately there's an easy way of dealing with that--color filters.
I usually like to have four color filters, thought more are possible. Six is as many as I could see being useful. My most-used filters are red, yellow, green, and blue. For Mars specifically, I use the red, yellow, and blue filters. I don't find green all that useful on Mars, though it's very useful on Saturn and the Moon.
Here's what I use red, yellow, and blue filters for on Mars:
- Red--Best subtle detail in the plains of Mars, or for distinguishing the edges of the "seas" from the plains.
- Yellow--Best for picking out sharp small details, such as the area around Valles Marinaris, the smaller plains areas around Cimmeria, faint changes in color such as between Utopia and Elysium.
- Blue--Picks out the polar caps, sharply defining their edges. It also makes the hazes and clouds stand out, such as the polar hazes around the polar caps and the clouds in the equatorial areas, especially near the limb (edge) of the planet's disk.
Green has some minimal value at bringing out the seas and some of the detail in them. A light green filter works better here than a dark green. The dark green reduces contrast too much.
As to the other filters I could see putting in my accessory box, there's orange and violet. For Mars, orange can be very useful. It does some of the pulling out of detail that yellow does, though not as well, but it also brings out some of the subtle shadings that a red filter brings out along with sharpening the detail a bit.
Violet works well on the same things that blue does (polar caps, atmospheric haze and clouds), but it can sometimes pick out equatorial clouds near the center of Mars' disk that a blue filter won't quite show.
Check out filters for your telescope. They come in different sizes depending on whether you have 2" diameter eyepieces or 1.25" eyepieces. The larger ones are more expensive.
You don't need a full set to start with, either. Starting from scratch, I'd say the most generally useful filters are a yellow filter and a light green filter. The next most useful are the red and blue filters. Orange would be next on my list, and the violet is a specialty filter that I wouldn't rush to buy unless I already had everything else I wanted. A dark green filter is good for the Moon, but not much else, and a neutral density filter, especially a variable one, works just as well on the Moon and has other uses as well.
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