However, all of the light collected by the Keck Observatory primary mirrors (75.76m2) is sent to the secondary mirror and the instruments, compared to GTC's primary mirror, which has an effective light-collection area of 73.4m2, or 25.4 square feet less than each of the Keck Observatory primary mirrors. Because of this fundamental difference in design, Keck Observatory's telescopes arguably remain the largest steerable, optical/infrared telescopes on Earth.