Artem has a doctor of veterinary medicine degree.
Eyes, Telescopes, and Images
From the outside, a person's eyes are just a couple of golf-ball sized things stuck inside their head. But if you've ever taken an eye apart in a classroom, you clearly would've seen the very complex nature of this amazing natural apparatus. It's got a lens, iris, retina, fluidy stuff inside, and way more than that. And under the microscope, it gets even more complex!
A telescope is pretty similar. A telescope you can get at the local store is just a tube to me, but like our eyes, it's made up of many parts.
While we've got an anatomy and physiology course that covers how our eyes detect light, this lesson's big mystery and focus lies in figuring out how a tube somehow takes light coming from a distant object and then produces an image your eyes can then appreciate. We'll end this lesson with a cool comparison of the light-gathering abilities of your eyes compared to that of telescopes.
The Objective and Focal Plane
A reflecting telescope, aka reflector, or a refracting telescope, aka refractor, both use something called an objective, a lens or mirror that focuses light. Since telescopes are all about great visuals, let's walk through this lesson with lots of images, otherwise what I'm about to say won't make too much sense.
For simplicity's sake, let's start off this lesson by saying our telescope is using a large diameter lens, the objective lens. The larger the diameter of the objective, the greater the light-gathering power of the telescope.
Once light coming from our celestial object enters the telescope, it will hit the objective inside the telescope. The objective will then form an image in something known as the focal plane, the plane where the objective of a telescope forms (or focuses) an image. The distance from a lens or mirror to its focal point is called the focal length of that lens or mirror.
The Eyepiece Lens
But we're not done yet! While our large diameter, long focal lens at the front of our telescope forms the image, we need something else to help us to actually be able to see the image!
That's where a smaller lens, the eyepiece lens, located at the back of the telescope comes in. You can see how this lens has a much shorter focal length than that of the objective lens:
This is important! That's because the magnification of the image made by the objective is equal to the focal length of the objective lens divided by the focal length of the eyepiece lens. The smaller the latter, the smaller the denominator, and thus the larger the magnification.
To help you remember that you want a bigger focal length of the objective and a smaller one for the eyepiece to magnify your image, just look at the telescope itself and how it's designed! The eyepiece is tiny compared to the diameter of the telescope itself where the bigger objective sits.
The eyepiece is important for the same reason in a reflecting telescope as in a refracting one. But the way the images are formed are just a bit different. Again, it helps to look at an image.
In a reflector, the objective (a mirror as opposed to a lens) is located at the back of the telescope. Light hits the mirror and focuses at an imaginary point beyond a second mirror. This secondary mirror brings light to a focus from point 1 to point 2 on your screen. Just as before, the eyepiece takes over thereafter to help magnify the image for your eyes to appreciate.
Comparing Everything to the Human Eye
Your eyes can only appreciate so much of whatever they look at without the multiple powers of a telescope. During this lesson, I purposefully squeezed in a couple of important notes. Here was one of them:
- The magnification of the image made by the objective is equal to the focal length of the objective lens divided by the focal length of the eyepiece lens.
When astronomers talk about magnification of a telescope, they are actually talking about the ratio of the size of an object as seen through a telescope compared to its size when seen with the naked eye.
Hence, if a telescope has an objective with a focal length of 90 cm and an eyepiece with a focal length of 3 cm, the magnification is 30X (90 divided by 3). This obviously means that the image formed by the telescope allows us to see a larger image of whatever it is that we're looking at with far more detail than with our eyes alone.
The other point I squeezed in this lesson was this:
- The larger the diameter of the objective, the greater the light gathering power of the telescope.
More specifically, the light gathering power of a telescope is directly proportional to the square of the diameter of its primary (or objective) lens or mirror.
Let's compare our eyes' ability for this with that of a telescope. Your eyes have a pupillary diameter of about 5 mm when they are in the dark (and thus the pupil is at its maximum size). The pupil is the black hole in the middle of your eye (technically, it's the middle of the iris).
Anyways, a small telescope may have an objective lens that has a diameter of about 5 cm. It doesn't seem like it's that much of a difference. Five cm is only 10 times greater than 5 mm.
But remember, we're dealing with squares here. Thus, because the diameter is 10 times greater, the light-gathering power of that small telescope is actually 100 times greater (10^2 = 100)!
This means that even such a small telescope will allow you to see objects that are 100 times fainter than you could without it. Some of the largest telescopes in the world allow you to see objects millions of times fainter than your eyes can ever appreciate.
Be it the inner workings of the eye or a telescope, it's all pretty fascinating stuff. But since this is an astronomy course, let's set aside the comparisons for our summary and focus on solely the telescopes.
A reflecting telescope, aka reflector, or a refracting telescope, aka refractor, both use something called an objective, a lens or mirror that focuses light. The plane where the objective of a telescope forms (or focuses) an image is known as the focal plane, while the distance from a lens or mirror to its focal point is called the focal length of that lens or mirror.
Light entering a refractor will pass through the objective lens at the front of the telescope, form an image in the focal plane, and then will be magnified by the eyepiece for your viewing pleasure. Light entering a reflector will bounce off of an objective mirror at the back of the telescope, will have the light redirected by a secondary mirror to a different focal point, and will then have the image magnified by an eyepiece for you to see.
After you've reviewed this video lesson, you should be able to:
- Define objective, focal plane, and focal length
- Identify diagrams of reflecting and refracting telescopes and recall their differences
- Explain how both types of telescopes magnify images
- Compare the magnification of telescopes to what is seen by the human eye
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