Telescopes

Telescopes come in two basic types: refractors and reflectors. Refractors, or refracting telescopes, use lenses to focus electromagnetic radiation to a focal point to be detected by some sort of detector: eye, CCD computer, camera. Reflectors use mirrors to reflect light to a focal point to be captured by the detector.

Refracting telescopes are basically used to observe one particular type of electromagnetic radiation, visible light. The telescope has two basic pieces; the aperature or objective lens, and the eyepiece or detector. The eyepiece was used by up until the invention of photgraphic astronomy, and now can be replaced by a camera or a computer to take images. Lenses have a property called focal length which is the distance parallel light coming into a lens must travel after passing through the lens to come to a point. The focal length depends on the curvature of the lens.

Knowing the focal length, we can define a couple of terms.

• telescopic magnification power is the ratio between the focal lenght of the objective lens and the focal length of the detector. The larger this ratio, the stronger the telescopic power
• light-gathering area is only dependent on the area of the objective lens. With more area, more light can be gathered and therefore a distant object will be brighter than with a smaller lens. Compare images of Jupiter with a telescope to just your eye. The aperature of a telescope is much bigger than that of the human eye.
• Resolution is how small a detail a telescope can make out. Depends on both the diameter of the objective and the wavelength of light. The longer the wavelength, the bigger the diameter of the telescope's objective to have good resolution. This is why visible light telescopes have smaller diameters than radio telescopes.

Refracting telescopes can only get so big before the lenses cannot support their own weight and will sag in the center. To resolve this problem, reflecting telescopes were invented. There are typically two types: a Newtonian reflector and a Cassegrain reflector. Newtonian reflecting reflect light to a secondary mirror which then reflects the light off to the side of the telescope. Cassegrains reflect light to a secondary mirror which then reflects the light back down through a hole in the primary mirror (or objective mirror).

The Three Types of Telescopes mention in This Blog

Image Credit:

Me

  

Arecibo Observatory in Puerto Rico. This is a radio telescope which is basically a giant reflecting telescope. The detector is at the top, at the focal point of the mirror.

Image Credit

NAIC - Arecibo Observatory, a facility of the NSF