Before spacecraft and very high altitude balloons, no astronomy was done outside of these windows. A 5m mirror is difficult to support and move around. As a large mirror moves, it sags out of shape. From 1949-1978, Mt. Palomar 5m was largest telescope on Earth. A telescope is only as good as the instruments that record the information.
Photometers record the flux of light in a given range of wavelengths from a given part of the sky.
Filters in the light path allow selection of the wavelengths studied. Photometes are especially useful for studying objects that vary in brightness, especially in short periods of time. Spectrometers break light into component wavelengths. Usually use a diffraction grating, which bounces light off of a finely ruled mirror. Different wavelengths diffracting off the lines are bent by different amouns, producing a spectrum.