Why do we see?
We see when light interacts with matter and comes back through the lens that is our eye. We can tell i light is on or off based on its interaction with different types of matter (i.e., particles in the air).
Refraction
At sunset, the Sun appears distorted because of how light bends in the Earth’s atmosphere. This light bending is called refraction. The reason why we see the sunset as many different colors is because light, intrinsically, is all colors. Think back to images of prisms, where rainbow light is coming out. The way light is bent or refracted can cause it to display different colors. In the case of the sun, the atmosphere scatters the light, so we see things like a colorful sky. The Sun’s all color light typically is scatter by the atmosphere to show blue the most, but it changes at sunset and sunrise.
Mirrors
In a mirror, the image appears behind the mirror— a virtual image— the same distance away from the mirror, and the same angular size. A mirror doesn’t project like a convex lens, instead just showing this virtual image.
What is a telescope?
Let’s say we want to buy a telescope. What specifically are we looking for in this telescope we want to buy?
- Better level of detail— can see what you observe in more detail.
- Length of exposure, for capturing images
- Size and quality of the lens— clearness? Specific material?
Therefore, the two most important properties we’re looking for are light collection ability and angular resolution. The ability to collect more light over a greater area at high efficiency allows us to view and capture higher-resolution images of the sky. A larger angular resolution will help us capture everything in more detail, but this also requires the lens to be bigger, which can become unwieldy. For the common telescope user, we’re going to want to find a “sweet spot” where both of these are balanced, allowing us to observe with a high level of accuracy without compromising ease of use.
Light Collecting Area
The more light we can collect, the higher our ability to “see” fainter objects is. A telescope’s diameter tells us its light collection area, .
You’ll notice that this is the same formula we use for the area of a circle, , but we define light collecting area in terms of diameter because it is easier to measure a diameter then split it in half than an exact radius.
The largest optical telescopes currently in use have a diameter of about 10 meters— aka, super big. This is why optical telescopes are often not used in contexts that require exact precision or a large light collecting area, but they’re more than enough for us.
Angular Resolution
The minimum angular separation that a lens can distinguish is the angular resolution; essentially, the ‘sampling increment’ if you’re thinking of it in terms of technology. The more samples you get, the more precise it will be, because you’re putting more things together to get a result; however, you’ll get the same kind of “shape” from something with less samples, it will just be less precise. The human eye’s angular resolution is about one arc minute.
There is an ultimate limit to angular resolution, though. It comes from the interference of light waves with the telescope, and is referred to by the term Diffraction Limit. This is a fundamental limit of physics and can’t just be “engineered around,” so it leads to us needing to find a different approach if we want to get more precise.
What types of telescopes are there?
Refracting Telescopes
Refracting telescopes use lenses to focus light. These are the ‘traditional’ telescopes and the oldest type, but as mentioned before they can get unwieldy if you’re trying to use them for higher-precision tasks. They end up getting super long and large in these cases.
Reflecting Telescopes
A reflecting telescope uses a mirror to focus light. Most modern commercial and general scientific telescopes are reflecting telescopes because they are more compact than reflecting telescopes and can achieve higher resolutions for less upkeep and creation logistics and costs.
What do astronomers even do with telescopes?
Astronomers use telescopes for imaging, spectroscopy and time measurement, amongst other things. The vast majority of modern astronomers don’t spend all of their time outside with a telescope staring at the sky; instead, they capture images and data, then focus on interpreting it and making sense of it in the context of space. Astronomical detectors generally record only one color of light at a time, so several images must be captured for a final picture. This allows astronomers to also record bands of light we can’t see and assign them to visible color ranges to study.
What other kinds of telescopes exist?
Radio Telescopes
A standard satellite dish is essentially a telescope for observe radio waves. The benefits of radio telescopes are that they are too long to be meaningfully scattered by the atmosphere, and that they don’t depend on visible light, making it so that they can actually be used in the daytime. Radio telescopes can be linked together in a process called interferometry to get better angular resolution.
Observing What Our Atmosphere Blocks
Only radio waves and visible light pass easily through Earth’s strong atmosphere, but they’re just a small sample of all of the types of frequencies that are emitted into space. We need to do something else to observe other frequencies. We have to go above the atmosphere to see things like UV, infrared and X-Ray waves. Telescopes like the James Webb Telescope are high-precision out-of-atmosphere telescopes that observe different bands of light; Webb is an infrared telescope.
Beyond Light
We don’t just get light signals from outer space. We can also observe signals from neutrinos, cosmic rays and gravitational waves and use them to further observe space. Ultimately all waves and frequencies we receive are just data that can be interpreted within the same purview, albeit with different methods for each distinct type.