Monday, March 12, 2012

Mars and Magnification

Normally I tell people not to worry much about high magnification. Most observing, especially for beginners, is done best at low levels of magnification--below 200x, and more commonly at something like 50-100x (power of magnification).

Mars, however, needs a bit more magnification if you're going to see it at its best. Even when it is at its brightest and closest, as it is right now (March 2012) and will be every 26 months.

Mars, with a range of small, delicate details that require magnification to see.

The largest details on Mars--the polar caps and Syrtis Major--can be seen at magnifications as low as 35-50x depending on your viewing conditions. But Mars will show far more if you can get to 300x or something close to it.

But doing this takes more than a high power eyepiece.

First, we have to have a sturdy mount, which I talk about in many posts, including the linked post. Next, we need to have some way of tracking the sky. To work at high powers, your scope needs to have some sort of automatic drive mechanism, whether it's a computerized clock drive or a kitchen timer with a friction wheel pushing your telescope along. It should work smoothly, and for periods of time at least 15-20 minutes long without needing manual interference.

Then you're ready for a higher power eyepiece or a Barlow (which multiplies the magnification of your current eyepieces.)

But what if you have a Dobsonian or some other manually moved telescope?

In that case, you'll be limited to medium level magnifications, and you won't be able to pick out as much detail. That's because you won't be able to just sit and relax at the eyepiece for as long. I regularly observe through my all-manual Dobsonian at magnifications up to about 250x. It's not as pleasant as deep-sky views at 56x, but it can be done.

You could modify your scope, but to be honest observing detail on Mars is a pretty specialized pursuit compared to working your way through a variety of deep sky objects at low to medium powers. It's not worth taking away from a scope that does that well just to look at Mars.

Instead, consider getting another scope. A smaller one will do, Mars is plenty bright. That keeps telescopes from taking over the household with their size. Plus, a smaller scope can be driven by a smaller drive mechanism. Just make sure it's got a high quality mount that keeps it steady.

Also, the sort of scopes that have good optical qualities for looking at the deep sky are not as good for objects like Mars. They usually have "fast" focal ratios. That is, their f/-number is a small number--six or lower. Scopes with higher f/-numbers, f/7 or higher, will generally give better contrast and detail in views at high powers than a light bucket with an f/4.5 focal ratio.

On top of that, an unobstructed optical train, that is, one without a secondary mirror in the light path, gives the instrument an advantage. An unobstructed 3" to 4" scope, on something like Mars at high power, will perform as well as a 6" or larger telescope with a secondary mirror in the way. Like a Newtonian or Schmidt-Cassegrain.

This is why we can't expect to buy one telescope to do everything. Different scopes do different things well. If we try to make a scope do everything, it becomes compromised for everything. It's better to have a scope that does a lot of things very well, though it's not optimal in other areas. It's best to accept that, enjoying what it does well.

In fact, the best scope isn't necessarily the one that gives the best views. The best scope is one that fits your lifestyle and needs well enough that you take it out and use it a lot.

Tuesday, March 6, 2012

Seeing Mars with Color

An inexpensive addition to a new scope is color filters. They're usually sold in packs of three or four at an affordable price. They're threaded to screw into the end of your eyepiece barrel.

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.

Planet Mars
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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