The CCD’s are chilled to reduce thermal noise. A computer is used to integrate the image - it gets to the point of counting individual photons hitting each pixel.
But as always, the more photons the better when it comes to imaging dim objects farther out in space than anyone has yet seen. And for that you need more surface area, which means larger mirrors. CCD’s are the modern equivalent of photographic film; they just count photons, and need something to focus them first to form a meaningful picture.