Refractor Vs. Reflector Telescopes
The Two Types
The two types of telescope that you mainly want to compare: refractor vs reflector telescopes. The difference is easy to keep track of: refractor telescopes use glass lenses similar to eye glasses. Reflector telescopes use mirrors - you see your reflection in a mirror... That's how I keep it straight.
Pretty simple, right? I always think so until I look a little more into it, then decide things aren't as they seemed.
You can always tell the difference between the two types just by looking at them. Refractor telescopes are long and skinny like a tube from a paper towel roll. Reflector telescopes are usually short and wide like a can of pie filling. Another way to tell is that the eyepiece is always on the back of a refractor telescope and always in the middle-front of a relfector telescope.
What the Difference Is
Why are there two types? One company said theirs was better? No. What the difference is often depends on the purpose of the telescope. You see, advances were made with glass lenses first so a lot of telescopes were made with glass lenses. It wasn't until Newton that they were really practical for anything but looking. I'm not sure if it was Newton that discovered this coming property or not, but it gave rise to reflector imaging.
Refractor lenses don't focus all colors on the same point. Mirrors do.
I think of light like most scientists do: a collection of wavelengths blended together to make the colors we see. There are many types of light you know by name but don't associate with light. Microwaves, radio, infra-red, visible light, ultraviolet, X-ray, cosmic, and gamma rays. The visible light you see with your eyes actually spans a very narrow window of the light that is out there. The light that comes from the sun and lands on the earth's surface is mostly visible light (with a little IR and UV mixed in). Thus, we took longer to discover that there are more types of light out there.
Most people think of radio waves in terms of frequency. I tend to think of all light in terms of wavelength - the two are very related, but I opt for wavelength. The shorter the wavelength, the higher the frequency and energy. Blue light has not quite twice the energy of red light.
What does this have to do with the lenses? Well, when you split an image into the colors and then focus the images, people were finding that when red was in focus, blue would be slightly out of focus. They would focus the blue and suddenly the red would come out of focus. This problem only occurred in refractor telescopes.
It is a Big Deal!
For small-scale operations it's all a matter of preference and is not a big deal. When you go snap a picture with your friends, the red and blue are so close together in focus that you can't tell - so it doesn't matter. But when you got a telescope as big as the Hubble or any that has an observatory built around it, then it will most probably be a reflector telescope.
When I said that visible light is a narrow window in the spectrum, that means that the red and blue won't be far out of focus from each other. How about when you look at X-Ray Vs. Microwave? It is a big deal! If you were trying to take a picture of an event with both wavelengths, one would be so far out of focus that you wouldn't be able to identify what you were looking at. But with a reflector telescope, the microwave will be just as in-focus as the X-Ray. That is why it is a much sharper image when using a reflector to look at a wide range of colors.
When I first started looking into telescopes and saw a diagram of a reflector telescope, I almost blew it off as bullshit. Why would anyone stick a mirror in the middle of oncoming light like that, especially in the center of attention? It would be like waving a hand in front of a camera - it would block the image you're trying to take a picture of.
Then I started wondering why your iris contracting in your eye doesn't create a dark circle in the edge of your vision. Or the aperture in a camera?
Then I realized that if you wave a hand ten feet in front of the camera while focused at a hundred feet, the image can still be seen with a very blurred hand in the middle. The image can still be seen in focus. The smaller the object in front of the camera and the closer it is to the camera, the more it will dim the image as opposed to blurring it. When waving your hand in front of a large-aperture telescope, the entire image can still get through. Tricky logic, eh? You will not have an image of a hand stuck in the middle of an image of the moon - the hand will be so out of focus and dim that you might not be able to tell the hand was there at all. That's the same with the mirror - it might block ten percent of the light, but it will not create a void in the center of your image like I'd previously thought. Since the mirror in the telescope is small it will only dim the image as opposed to blurring it or creating a void in it.