Saturday, April 25, 2009

How powerful is it?

I hear this question all the time.

Just how powerful is this/a/your/ telescope? It's a misconception of just how a telescope works although the simple answer from today's amateur is somewhat inaccurate. That answer is, "power is meaningless". This is partly true, but does not explain the function of power. By power we mean, "magnification", the actual image scale of what we are looking at. Magnification is required to view just about anything, so saying it is meaningless is essentially wrong. What is meaningless is "advertised power" on today's junk trash scopes found in today's department stores. Advertised magnification claims on such telescopes are trying to dupe you into believing you are buying something with a "tremendous spec". Try using that 500X dinky scope with it's shaky tripod. You'll see what I am talking about.

Astronomers will say that "aperture" is the most important". This is mostly correct. The most correct answer is "the largest aperture you can afford and transport easily". Aperture or diameter of a telescope is the function of light gathering, which in turn, is a function of resolution, or the ability to see detail. After you get the light gathering, you can apply the a point.

Not all objects in the sky need tremendous power (surprise!). In fact, very few do. Most of the "power hungry objects" are the moon or the planets. Deep sky objects benefit from low or high power depending on their size. Try looking at Andromeda Galaxy at high power and you'll see what I mean. Try looking at the Blue Snowball at low power and see what I mean.

There are rules to magnification, but they are rules-of-thumb or guidelines. You can obey them, bend them and break them. The generally accepted rule is 50X per inch or 2X per millimetre of aperture. Simply put, a three inch telescope is generally able to "power up" to 150X, more or less.

The guideline assumes that all things are equal. But they seldom are. The general rule with power assumes that the atmosphere is stable (unlikely on most clear nights), the optics have reached ambient temperature and quality of the optics. This is where it gets a bit muddled. We have pushed high quality, little scope well past the 50X per inch guideline either because it was stable or the optics reached ambient. Well past the breaking point of the guideline. Or we have been restricted on larger instruments to well below the guideline because the reverse is true.

So just how powerful is it? I don't know. Depends on the night, depends on the scope, depends on the object. One thing is certain, it's something that needs no advertising.


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