Why do we need very large stationary telescopes in addition to the orbital telescopes?

Question

Most of the existing telescopes are located on Earth since it is easier and cheaper to construct, build and operate on Earth. Space launches are very expensive, and, moreover, if there is some problem with the telescope, orbiting in space, it is quite complicated to fix, since a team of astronauts has to be sent and work in open space is much more complicated than on Earth.

However, telescopes that operate in space offer a lot of benefits, comparatively to those on Earth, since the precision of the Earth's telescopes is limited by the atmosphere.

Hubble was a real breakthrough at the time of launch and discovered a lot of things and extended our knowledge about the space and Universe. As far as I understand, this progress would be impossible with any of the existing telescopes, located on Earth.

The upcoming launch of the Webb telescope is supposed to reveal a lot of facts about the early universe. In comparison to Hubble, this telescope has larger reflector: $~7$ meters for Webb vs $2.4$ meters for Hubble. In addition, it will be orbiting on a distant orbit far from Earth, with a radius of 1.5M kilometers. Therefore, there would be even less noise and hindrances in the observations.

At the same time, there are actively developing projects for construction of very large stationary telescopes: The extremely large telescope and Thirty meter telescope.

Why do we actually need these, if they are located on Earth and will be limited in precision, comparing to the Hubble and Webb telescope? Is it the case, that they are solving somehow different tasks, and the huge diameter of the reflector will allow them to observe something that cannot be seen by either Hubble or Webb?

Answer 1

Disclaimer, I'm no expert on the details, but I know the general idea.

Large ground telescopes are great, and often superior to orbital telescopes. The reason is as you said - it's cheaper to build ground telescopes, which means for the same budget you can build a bigger, more powerful ground telescope. It's true that space-based telescopes don't have to deal with the Earth's atmosphere obscuring things, but then there are also so-called adaptive optics that mitigate this advantage.

Three of the biggest advantages of ground telescopes are:

• You can make them very large. Because of the way optical resolution works ($\theta = 1.22 \lambda/D$), big telescopes have a fundamental advantage over small telescopes, and you can make bigger ground telescopes for the same price. You can actually see this in your numbers. The James Webb telescope has a 7m mirror, while the Thirty Meter Telescope is four times as big.
• In the same way, because they are larger, they can collect more light in a given amount of time. To get the same amount of light with a smaller telescope, you need to observe for longer, which is bad (telescope time is at a premium in astronomy; most of the time astronomers need to apply for time).
• They are easy to repair. If something in the Hubble Space Telescope breaks, you might need to send up astronauts and conduct a space walk, which is obviously very expensive. Comparatively even the most inaccessible ground locations (like the South Pole) can be reached for a fraction of the price of going to space.

The discrepancy is actually such that many space probes aren't worth funding because one should just build a bigger, more powerful ground telescope.

Given the above one could flip the question around and ask, why bother with space-based telescopes then? There are reasons, some of the most important being:

• The atmosphere obscures some wavelengths of light. If you want to observe in those wavelengths you must go to space.
• Ground telescopes are susceptible to local weather conditions. If it's raining or cloudy, you can't observe.
• Space telescopes can observe in all directions all the time. On the ground, you can only observe at night, and even then you can only observe half the celestial sphere at best (because the Earth is in the way of the other half).

See e.g. this source for more details.

Answer 2

Why might a family of four own both an expensive 2020 car and a cheap 2005 car?

A telescope can only look at one thing at a time. There a lot more than one astronomer on Earth who want to look at space. Thus, many telescopes are needed. The value of being able to build multiple medium quality telescopes on Earth for the same cost as one excellent space telescope should not be underestimated.

Also, different telescopes look at different wavelengths and distances. Some are compact cars, others are pickup trucks.

Answer 3

Another point for ground based telescopes, is the extreme flexibility in the actual instruments used as detectors.

Once built the mirror, dome etc are unlikely to needed replacing (though the mirror can be re-coated when needed) but things like the computers and detectors can be upgraded as new technology is developed or simply swapped out to perform different science.

Yes you can have multiple detectors on a space based telescope but the mass and volume is limited, and has to be fixed early in the design process of the satellite. Thus you must know what detectors you want before launch. On Earth you can can have as many instruments as you want stored near the mirror to be swapped out when necessary. Or replaced when damaged, or replaced by a new detector that wasn't even planned when the mirror was originally built.

Also on Earth the mass and volume are almost irrelevant for the detectors, they can be big and bulky making them easier to build and repair when damaged. If you instrument goes even slightly over the mass/volume budget on a satellite then that is a big problem, on Earth you may not even have a mass/volume budget but perhaps only guidelines based on what can fit on a truck (even then you can build the detector in parts and ship separately).

You can also mostly ignore having to worry too much about things like power, cooling, bandwidth for data. Its a lot easier to cool a detector when you just need to attach some fans to air cool it (or hook it up to an air conditioner). On a telescope you will have to build a full model of how the heat is generated and design in a complicated thermal management system to try and cool the telescope in space (and the total cooling is limited on a satellite). Some ground based telescopes simply ship hard drives full of data which is cheap, but slow, way to get a lot of data from a remote site back to base. A satellite has a limited data bandwidth for returning the actual science.

Answer 4

The resolution of ground-based telescopes is not strongly limited by atmospheric turbulence. Large telescopes use adaptive optics to compensate in real time for the blurring effects of turbulence. Usually, this provides only partial compensation, and post-processing algorithms like blind deconvolution can bring ground-based images close to diffraction-limited quality.