How Telescopes Detect & Measure Light

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  • 0:01 What Is a Detector?
  • 0:35 Photography & CCDs
  • 2:03 Photometry & Spectroscopy
  • 4:25 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will go over the numerous methods astronomers can use to detect and measure light, including photography, spectroscopy, photometry, and CCDs.

What Is a Detector?

What are your eyes used for? To see stuff. More technically, to detect stuff. To detect a lion about to eat you, to detect an iffy looking mushroom that might kill you, and plenty of other things.

A detector is something that measures light after it has been brought into focus - in our lesson's case, brought into focus by a telescope! While the human eye is a qualitative detector, there are plenty of other kinds of ways light can be measured.

These ways, which are more sensitive and objective than our eyes, will be outlined for you here.

Photography and CCDs

One way to record the things astronomers see in a telescope is the same way you'd record something you see with your eyes for posterity - you use photography. A telescope can be used like a camera. Its objective (the primary lens or mirror) is like a camera's lens. Thus, the eyepiece of a telescope can be removed, and a photographic plate or film can be put in the focal plane of the telescope, as it would be placed in the focal plane of the camera's lens.

Now, while you might snap a picture really quickly here and there with your camera while having fun with friends, astronomers usually use very long exposures instead. This is because long exposure times are necessary for them to get good images of celestial objects that are way too faint for your eyes to even see.

Even so, photography is not a very sensitive or efficient way to record images when compared to the use of CCDs, or charge coupled devices. You've probably heard of these guys being used in higher end video cameras. A CCD is a device that can be made up of a million or more photosensitive detectors confined to a very small area, the size of a postage stamp. Such an array of detectors is laid out in a mini chessboard-like pattern where each piece of the pattern in the CCD is known as a pixel. The sensitive nature of a CCD gives astronomers the ability to produce much better images of faint celestial objects than photography ever could.

Photometry and Spectroscopy

Photography should not be confused with photometry. Photometry uses instruments called photometers, and it refers to the measurement of the brightness of celestial objects such as planets, galaxies, stars, nebulae, and so on. While ancient astronomers used their eyes to subjectively measure the apparent brightness of these objects, a photometer does this far more precisely by way of light hitting the photometer and producing a measurable electric current as a result.

Another powerful way light can be used thanks to a telescope to measure and analyze celestial objects is known as spectroscopy. Spectroscopy is the study of patterns of spectral lines emitted by chemical substances. In spectroscopy, light entering the telescope is later dispersed according to wavelength to create a spectrum, an arrangement of electromagnetic radiation (which includes visible light) placed in order of wavelength.

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