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Buying a Telescope - Part 3

Day 27

Some parting thoughts.


There’s a common misconception that telescopes are all about magnification. One sure sign of a questionable telescope is one that puts its magnification up front. Worse is one that advertises 200x, 300x, and more.

It works like this. The main mirror or lens, called the “objective,” collects light, information, and produces a real image somewhere in the back of the telescope. It’s usually inside the eyepiece holder. It’s called a real image because it acts like a little picture floating there in space. If you point a telescope at something and look at the back of it, with no eyepiece, you can see the real image.

The quality of the image, including it’s resolution, depends on the amount of information collected by the objective, generally the larger the aperture, the more detail is in the image. The eyepiece is a little magnifying lens you use to look at the image, just like you use a magnifier to look at a small object like jewelry or a photograph.

If you imagine taking something with a picture like a newspaper, magazine or a book (if you can even find one of those), and look at it with say a 10x or 20x magnifier, you can probably see more details in the picture. You might be able to make out something that you missed without the magnifier.

However, if you then find a 100x or 200x magnifier and look at the picture, what do you think you’ll see? You’ll probably see the dots the picture is made of, or just a fuzzy blur but you won’t see any more details in the original picture. There’s not enough detail, resolution, information in the picture for 200x to work.

That’s the way a telescope real image works. Even if the manufacturer includes an eyepiece that gives 200x magnification, you’ll just see an unimpressive blur, if you could ever find the object at that magnification.

Worse, you’re also magnifying the speed at which an object in the telescope appears to move (as the earth turns). At 30x to 50x this isn’t that much a problem. At 100x it’s moving but you can keep up. At higher magnifications, unless you have a mounting with a clock drive, the object goes screaming by.

The basic rule of thumb is 20x to 30x per inch of aperture for normal observing, and you might double it to get a closer look. For a 2.4-inch telescope that means 40x to 80x. Note in the previous post that the Orion 2.4-inch has eyepieces for 28x and 70x. That’s perfect! 200x or 400x for a 2.4 is absurd.

For the 4.5-inch telescope, 90x to 180x in theory are good. It’s eyepieces at 36x and 91x are okay, but you could add an accessory to take the 91x up to 150x to 200x but I’d expect you to use those magnifications rarely.

Back in the old observatory where we had a 14-inch Celestron Schmidt-Cassegrain on a permanent pier mounting. It was an f/11 telescope meaning the focal length was 154-inches (14 x 11) which equals 3912 mm. Our typical eyepieces when public observing were a 32-mm and 10-mm. The 32-mm gave 122x and 24-mm at 162x. (Magnification is just the telescope focal length divided by the eyepiece focal length). We might use a 10-mm or so for 391x, but usually not for the general public. At some point, the atmosphere itself limits the detail in the image, regardless of how large the telescope is. Astronomers call that effect the “seeing.”

Clock Drives and Tracking

As discussed above, we’re standing on a moving platform, the earth. For that reason, the whole sky is constantly turning as if we’re on a merry-go-round. The sun, moon, stars, everything in the sky rises in the east and sets in the west for this reason. Some telescopes have an equatorial mounting and a clock drive or computer driven tracking system to follow the stars across the sky. If you’re just doing visual observing, and you have a good mounting, you can easily keep the object you’re observing in the field of view. There’s great joy in visual observing this way.

If you do become interested in some types of visual imaging or more serious observing of certain types, then you may want to look at a telescope with a tracking system like this, but I wouldn’t start with one.

Try Before You Buy

It’s a great idea to see what it’s like to use a telescope. Visit your local planetarium or observatory on an observing night or find a local astronomy club. Maybe go to a meeting or two, one where they offer some presentation or have a speaker on some topic and then, either at the same meeting or a separate one, where members bring their telescopes which they’ll happily show you along with views of a variety of objects through them.

Use the Internet

I’ve skipped explaining a lot of terms and concepts that surround amateur astronomy and telescopes. Excellent explanations are readily available on the Internet. Orion has nice tutorials and beginner guides on their site. Of course Wikipedia has articles on many topics surrounding amateur telescopes. And you’ll find other sources.

You should also download the free software for your computer, Stellarium. This is a beautiful “planetarium program” that shows you the current night sky or the sky from any location, time and date on earth. You can use it to learn constellations, what planets are visible, and every other object in the sky an amateur would be interested in. You can also print star maps to take outside.