Annual Question:
One of the scientific heroes of our time is Pieter van Dokkum, professor in the Yale astronomy department and author of a recent book, Dragonflies. The book is about insects, illustrated with marvelous photographs of real dragonflies taken by van Dokkum in their natural habitats. As an astronomer he works with another kind of dragonfly. The Dragonfly Observatory is an array of ten sixteen-inch refractor telescopes arranged like the compound eye of an insect dragonfly. The refracting lenses are coated with optical surface layers designed to give them superb sensitivity to faint extended objects in the sky. For faint extended objects, the Dragonfly Observatory is about ten times more sensitive than the best large telescopes. The Dragonfly is also about a thousand times cheaper. The ten refractors cost together about a hundred thousand dollars, compared with a hundred million for a big telescope.
The Dragonfly Observatory recently finished a search for faint dwarf galaxies orbiting within the gravitational field of our own galaxy. About fifty dwarf companions to our galaxy were discovered, more than were expected from computer models of galactic evolution. Each dwarf galaxy is embedded in a halo of dark matter whose mass can be determined from the observed velocities of the visible stars. The dwarf galaxies have about a hundred times more dark mass than visible mass, compared with the ratio of ten to one between dark and visible mass in our own galaxy. The Dragonfly observations reveal a universe with an intense fine-structure of dark-matter clumps, much clumpier than the standard theory of big-bang cosmology had predicted.
So it happens that a cheap small observatory can make a big new discovery about the structure of the universe. If the cost-effectiveness of an observatory is measured by the ratio of scientific output to financial input, the Dragonfly wins by a large factor. This story has a moral. The moral is not that we should put all our money into small observatories. We still need big telescopes and big organizations to do world-class astronomy. The moral is that a modest fraction of the astronomy budget, perhaps as much as a third, should be reserved for small and cheap projects. From time to time a winner like Dragonfly will emerge.
