Farther and Fainter
Astronomers always want to see farther and fainter, to gather a lot of light from dim, faraway objects. The 200-inch Hale Telescope at Palomar Observatory is a big “light bucket” that allows observers to study very distant stars and galaxies.
This 40-inch-wide, 1,250-pound glass plug is a part of California astronomy history. It served as a center support for the 200-inch, 14-ton Palomar mirror as it was being ground and polished. The process took 11 years. After the mirror was complete, construction workers removed the plug from the center of the mirror. It was later sent to Griffith Observatory for display.
Using Technology to Build Bigger Telescopes
Astronomers want large, sensitive mirrors to gather more light from astronomical objects. The larger the mirror, the farther, fainter, and smaller they can see.
Big mirrors present big challenges. The mirrors in the large telescopes at Mount Wilson and Palomar Observatory are incredibly heavy. Designers created perfect glass surfaces that would hold their shapes despite the enormous weight.
Polishing the Perfect Mirror
Casting glass is only the first step in creating a very large and precise mirror. The mirrors for the Hooker 100-inch telescope at Mount Wilson and Hale 200-inch instrument at Palomar Observatory had to be polished to the perfect shape. Optical experts did this slow, careful work, often by hand. Nearly 10,000 pounds of glass were ground off the Hale mirror over the course of 13 years to make the final surface.
New Mirrors, New Astronomy
Computers make it possible to design, build, and operate very large telescope mirrors made of lightweight reflecting segments. The Keck Observatory mirror is a mosaic of 36 thin, hexagonal mirrors. The result is a 400-inch reflecting surface. The Keck telescopes, along with Gemini, Subaru, and other observatories, observe the cosmos from atop Mauna Kea on the Big Island of Hawaii.
A set of 18 segmented mirrors forms the heart of the James Webb Space Telescope, the infrared-sensitive successor to the famous Hubble Space Telescope. Its reflector segments are made of lightweight beryllium, which can detect very faint infrared emissions.