Ask Astro: How do telescopes see different parts of the electromagnetic spectrum?

By | Published: September 26, 2022 | Last updated on May 18, 2023
Scientists use telescopes capable of viewing the universe across the entire electromagnetic spectrum.
Astronomy: Roen Kelly
How do we create telescopes that see different parts of the electromagnetic spectrum?

Frank Coulter 

Pauanui Beach, New Zealand 

The range of colors or wavelengths that a telescope can observe depends on the design of both the telescope itself and the cameras attached to it.

For instance, if you’re building a telescope that will observe mostly in infrared, usually the mirror is coated in silver or gold. For the mirrors of telescopes designed to observe in both optical and infrared, aluminum, which reflects light fairly well over a wide range of wavelengths, is a better choice.

Mirrors in X-ray telescopes are usually coated with gold or iridium. But there’s a problem: If you aim a mirror directly at an X-ray source, the photons (light) won’t be reflected — they will either go through the mirror or get absorbed. To solve this, mirrors in X-ray telescopes are shaped like a funnel. This way, the incoming light grazes the mirror at a very shallow angle, like a stone skimming on a lake, allowing instruments at the bottom of the funnel to catch the light.

The detectors that register the light captured by a telescope are also tailored to different wavelengths. Many detectors are made from semiconductor materials, so that when light hits the detector, it releases electrons, which we then measure and translate into an image. Different semiconductor materials respond to light with different energies and therefore different wavelengths.

Finally, we can further restrict the wavelengths we observe by putting a filter in front of the detector. A filter is just a piece of glass that only lets through certain wavelengths. Astronomical cameras usually have several filters mounted on a wheel, and astronomers can then select which one they want to use to make their observations.
It may seem like a lot of work, but seeing the cosmos through every possible wavelength is critical for deepening our understanding of the universe we inhabit.

Juan Carlos Muñoz-Mateos 

European Southern Observatory Media Officer, Garching, Germany