Refracting telescope
A telescope that uses only the refraction of a lens is called a refracting telescope. There are two types of refracting telescopes depending on the combination of lenses.
The Galilean telescope is often referred to as a terrestrial telescope. The telescope consists of a convex objective and a concave eyepiece. Galileo was the first astronomer to observe the stars with his telescope.
Later, telescopes with convex eyepieces appeared instead of concave eyepieces. This is called the Keplerian telescope. Most astronomical telescopes in use today are Keplerian telescopes. The Keplerian telescope is unsuitable for terrestrial use because it appears upside down, but it is widely used for astronomical observations due to its wide-angle. It doesn't matter if the celestial body looks upside down.
Keplerian telescope
- If the starlight incident parallel to the objective lens comes out parallel from the eyepiece, it is focussed.
- Starlight is collected and transmitted to the eye. In other words, you can see brighter than your eyes can see (increased luminous flux density).
- If you look at the example in the picture above, the light that enters at an angle of 3˚ is changed to about 15˚. This means that you can see the image at about 5x magnification through this telescope.
- Looking at the example in the figure above, most of the light entering at an angle of 3˚ is entering the eye. As such, Kepler telescopes have a wide-angle of view.
Galilean telescope
- If the starlight incident parallel to the objective lens comes out parallel from the eyepiece, it is focussed.
- Starlight is collected and transmitted to the eye. In other words, you can see brighter than your eyes can see (increased luminous flux density).
- If you look at the example in the picture above, the light that enters at an angle of 3˚ is changed to about 15˚. This means that you can see the image at about 5x magnification through this telescope.
- If you look at the example in the picture above, the light that enters at an angle of 3˚ cannot enter the eye. As such, the Galilean telescope has a narrow-angle of view.
Three criteria for telescope evaluation
- Condensing: It gathers light so you can see brighter than you see with your eyes, and lets you see dark objects that are invisible to your eyes.
- Resolution: The blurry spread makes it easier to see what you see. In other words, it allows you to look very closely at the outline of an object that is too far away to distinguish its shape.
- Magnification: Zoom in on the image to make it look good.
Reflective telescope
The condensing of a telescope is closely related to the size of the objective lens. For example, if the objective lens diameter is doubled, the amount of light entering the objective lens is quadrupled, which is the power of two. This leads to a preference for larger telescopes.
However, refracting telescopes cannot be made as large as they are. This is because the larger the lens, the heavier and thicker it is. As you get closer to the edge of the lens, the glass acts like a prism, breaking the light down into colors. The telescope that compensates for this problem is the reflex telescope, and this reflex telescope is currently the most used for astronomical observation.
If you look at the example in the picture above, the light that enters at an angle of -1˚ is changed to about -5˚. This means that you can see the image at about 5x magnification through this telescope.