What are the advantages of large telescopes?*
Technology has dramatically changed since Galileo first made improvements to the telescope and then began making contributions to observational astronomy more than 400 years ago. Telescopes have grown bigger, better, and more costly. But, what is the advantage of bigger telescopes, why do astronomers want costly telescopes in space, and what can we observe?
Larger telescopes allow for more light to be collected, and more light collected means clearer pictures and images of more distant objects, giving gained sight of fainter objects with higher resolution. But problems come with larger size. Refractor telescopes, like the one used by Galileo, require longer focal tubes as they are scaled up to larger sizes. The tubes of large scale telescopes begin to bend, or bow, under their own weight and that reduces image quality.
The invention of reflecting telescopes solved the problems caused by instability in tubes but it did not solve other large problems; namely, human made light pollution affects the quality of observations we make from Earth, ground based telescopes are limited by the effects of blurring caused by Earth's atmosphere, and Earth's opaque atmosphere blocks most electromagnetic radiation so imaging into the spectrums of ultraviolet, x-ray, and gamma rays , as well as most infrared, can not be accomplished from the ground. Earth was a problem - space was an answer. Placing telescopes in space was the way to eliminate those issues and gain that broader view into the electromagnetic wavelengths which could not be observed from Earth.
Since the 1970's many telescopes have been placed into orbit making observations in the gamma ray, x-ray, ultraviolet, visible, infrared, and microwave wavelengths. But it was the Hubble Space Telescope (HST), placed in low Earth orbit in 1990, that became the most popular of the space telescopes that were launched into space in the 20th century. The HST was no simple accomplishment, and problems plagued the development and deployment of the device. An advantage of the low-Earth orbit which Hubble was placed in was that it could be serviced in space, and it was serviced five times. The successful end results of those missions were thousands of beautiful images that were appreciated by many. Those images made the Hubble dear to the hearts of viewers, but the cost of this instrument became, itself, astronomical and cost has been the biggest drawback of space based observation.
In about 400 years we have progressed from a very simple telescope that allowed remarkable observations of visible-light objects, to space based platforms that allow us to see beyond the limited range of our own eyes and peer into the universe as no other group of living beings on Earth has been able to do before. Complex instruments imaging in wavelengths that were not needed for us to succeed on this Earth, but help us in ways that let us understand our past and adapt to our future, are now working above our heads among the stars to help us prepare for epochs yet to come.
*for partial completion of the course: Astronomy: Exploring Time and Space by Chris Impey