| Because for a given amount of funding and engineering effort, a telescope on the ground can be a lot larger. Hubble has a 2.4 meter mirror, TMT is 30 meters. Resolution scales with the diameter of the mirror (so an order of magnitude more for TMT), while light gathering scales with the area (two orders of magnitude more for TMT). Traditionally, there were two challenges for large ground based telescopes: a) We couldn't make telescope mirrors larger than ~5 meters because the glass starts to deform under its own weight and the view gets worse rather than better; and b) we lose too much to atmospheric distortion. The solution to the first issue is to assemble large mirrors from many small mirrors in a honeycomb pattern[0]. The solution to the second one is adaptive optics[1]: Actuators and computer control can compensate for atmospheric distortion by precisely deforming the mirror in real time, several times per second, to cancel out the distortion. We still need a good location for the telescope to minimize the amount of distortion we need to handle, but with that we could for the first time achieve better resolution from the ground with the 10-meter Keck. These days the benefits of a space telescope come not so much from visible light, but from other wavelengths that are fully or partially blocked by the atmosphere[2]. Like UV, X-ray, or infrared like the upcoming James Webb space telescope. [0] https://www.wired.com/2008/10/in-the-late-197/ [1] https://en.wikipedia.org/wiki/Adaptive_optics [2] http://gsp.humboldt.edu/OLM/Courses/GSP_216_Online/lesson2-1... |