Ground Based Optical Telescopes

Small amateur telescopes are great for exploring the night sky from our gardens, but what tools do professional astronomers use to explore the universe? The truth is that they use a wide variety of ground based telescopes, space based telescopes as well as other experiments and detectors such as gravitational wave detectors. This post will be about ground based optical telescopes.

The purpose of any telescope is to capture light. Most of the light in the universe can’t be detected by our eyes. The electromagnetic spectrum describes the full range of light, a chart below describes this in better detail. The light from the wavelengths we can see is called the visible part of the spectrum. All parts of the spectrum are useful to astronomy in different ways. For example, cool regions of gas are seen in the radio part of the spectrum where as extremely hot areas of gas are seen best in gamma rays.

Take the following image of the Whirlpool Galaxy, it combines data from multiple wavelengths of light to create the four images. Each picture is different depending on what wavelength is used. Different parts of the galaxy are more prominent in certain wavelengths. It takes many different types of telescope to gather these images. Some of these telescopes need to be in space as the atmosphere blocks certain wavelengths of light so ground based telescopes can’t see them.

The telescopes that can see in visible light are called optical telescopes and they resemble the structure of an amateur telescope albeit a lot bigger. When building large telescopes it is important to ensure it is located in the best possible place. The location needs to be free from light pollution, in an area that has a lot of clear nights, high in altitude to get above as much of the atmosphere and weather as possible and far away from cities where pollution could affect it. Sites like this are not all that common and this means that telescopes are generally grouped into a few sites. The main three sites are Mauna Key in Hawaii, The Paranal Observatory in the Atacama Desert in Chile and the Roque de los Muchachos Observatory on the island of La Palma in the Canary Islands.

The biggest single aperture telescope in the world today is called the Gran Telescopio Canaria. It measures 10.4m in diameter and is located in La Palma. Not all optical telescopes are constructed from one mirror. The larger the mirror is, the harder and more expensive it is to make. For this reason, large telescopes often are made up of many smaller segments. The larger the diameter of the mirror, the better the light gathering power is and the higher the resolution of the telescope. Some telescopes also use a technique called interferometry to increase the resolution of the images it can produce.

Interferometry is when two telescopes look at the same object, at the same time and combine the data captured into a final image. One example of this is the Keck Interferometer. This uses both of the 10 meter Keck telescopes in Hawaii to achieve the same resolution as an 85 meter telescope. One of the drawback to this technique is that for optical light it is very difficult and requires the light be combined using optical fibers. The funding for the Interferometer on Keck was cut in 2012 so although the telescopes are still active the interferometer is not.

The turbulence in the atmosphere of the Earth can cause the light from celestial objects to get bounced around. As mentioned already, the telescope is positioned on a mountain top to get above as much of the atmosphere as possible. Adaptive optics is also used to mitigate the effects of atmospheric turbulence. This is where an artificial “star” is created using a laser. The telescope then uses this star to calculate the turbulence in the atmosphere and little pistons move the mirrors slightly to counteract the turbulence.

The European Southern Observatory (ESO) is currently constructing a huge telescope 20 km from the Paranal Observatory. This telescope is called the Extremely Large Telescope (ELT) and the mirror will have 5 segments in it. The total diameter will be 39.3m! Construction started in 2017 and the first observations are expected to take place in 2024 (although this could be pushed back as COVID-19 has delayed construction). This telescope will also feature adaptive optics.

Most optical telescopes are similar in design. They usually have one big mirror, called the primary, that collects all of the light and focuses it onto a smaller secondary mirror. The light is then reflected back through a hole in the main mirror to the detector. All of this is usually mounted on a moving mount and housed in moving dome. Where the telescopes differ is the detectors and instruments that are used. These vary depending on the application and it is common for instruments to change over the lifetime of a telescope and some instruments are designed for a specific study and placed on the telescope only for that study.

There are a number of uses for optical telescopes such as finding asteroids and comets, imaging celestial objects and conducing spectroscopy which is where light is split into a spectrograph to see what elements are present. This is only the tip of the iceberg when it comes to the astronomy that can be done with other telescopes such as radio and space based telescopes. Check out future posts to find out more.