Hexbyte – Tech News – Ars Technica |
If Lowell Observatory’s Gerard van Belle gets his way, you’ll soon be watching an exoplanet cross the face of its star, hundreds of light-years from the Earth. He can’t show you that right now, but he should be able to when the new mirrors are installed at the Navy Precision Optical Interferometer in northern Arizona. They’re arriving now and should soon start collecting starlight—and making it the highest-resolution optical telescope in the world.
Van Belle recently showed Ars around the gigantic instrument, which bears almost no resemblance to what a non-astronomer pictures when they hear the word “telescope.” There are a couple of more traditional telescopes in dome-topped silos on site, including one built in 1920s in Ohio, where it spent the first few decades of its life.
Hexbyte – Tech News – Ars Technica | Going big
The best way to improve imagery on these traditional scopes is to increase the diameter of the mirror catching light. But this has its limits—perfect mirrors can only be built so large.
The Keck Telescopes in Hawaii pushed the boundary by constructing a 10-meter mirror made of many smaller hexagonal mirrors arranged together. The Keck works because the only requirement for a telescope, van Belle explained, is that every ray of celestial light must travel exactly the same distance, even as it strikes the telescope’s primary mirror, bounces off the smaller secondary mirror at the telescope’s snout, and arrives at the camera. No matter where light hits the hardware, every possible bouncing path must be the same length to within 50 nanometers—just one-thousandth the width of a human hair.
A bigger mirror provides two advantages: it catches more light (making fainter objects visible) and it produces a higher-resolution image. If you give up on the first advantage, you can go all in on the second by laying out a handful of small mirrors over a considerable distance. The total mirror area (and therefore light collection) won’t be that great, but the tremendous diameter of the array cranks the resolution up to 11.
That’s the principle behind the Navy Precision Optical Interferometer, a Y-shaped installation with a functional diameter of up to 430 meters. Three vacuum-evacuated tubes run along each arm of the “Y,” capable of carrying light from a telescope mirror to the center. At various points, a 5-inch telescope mirror bounces star light to a cylinder affectionately called a “lizard head” that redirects it down one of those tubes. Currently, the instrument works with up to six of these mirrors at a time, spanning a total diameter of 100 meters in “zoom” configuration, or hugging the center when in a “wide angle” mode.
This is where it starts to get tricky. It still has to be true that the light hitting all these mirrors travels the same distance (to within 50 nanometers) before hitting the camera. That’s made a bit tricky by the hardware—if there are two mirrors on an arm of the Y, one is obviously farther from the center. But the sky itself poses other challenges. As stars appear over the horizon, their light has to travel across the array to hit the mirrors on the far side. Something has to be done to very precisely sync up all these rays of light.
Hexbyte – Tech News – Ars Technica | A series of tubes
That’s why there are a series of extra tubes extending off the building that houses the camera. Mirrors are placed at different positions in each tube to make light that had a shorter route run an extra lap, so to speak. Inside the building, there is a second set of fine-tuning tubes to accomplish the same task with increasing precision. Inside each of these tubes, the mirrors sit on movable carts. And on the cart, there are three more levels of adjustment: a lever arm on the cart, a voice coil (like a speaker driver) on the lever arm, and a tiny, expanding piezoelectric crystal on the voice coil. The end result is that all the light paths can be dialed in to even less than the 50nm requirement.
Once properly adjusted, the light is ready to meet a camera. But if you’re expecting to see a simple little electronic box stuck on the end of these tubes (as I was), you’re way off. Instead, you enter a climate-controlled room that looks like a college physics classroom just after the students left. Tubes enter through the wall and… end. The light shoots out into the open room. An incredible series of hand-sized mirrors arrayed on tables (referred to as “the switchyard”) redirect the light toward an enclosed table on the side of the room that itself holds a Tinkertoy arrangement of mirrors, light-splitting panes, and prisms. Here, light coming from the various mirrors is combined and fiddled with before finally traveling down a fiber-optic cable to reach the image-capturing sensors in the next room.
To be fair, this particular setup is about to be replaced by a modernized version that is slightly less complicated. There are still a couple mirrors in each line, but they will feed into delicate fiber-optic lines connected to something resembling an integrated circuit chip that performs all the functions of the current camera table, only in miniature. Behind that lies new off-the-shelf cameras built for medicine. “It’s all this technology that came out of astronomy, went into medical imaging for mammograms, for doing fluorescence microscopy,” van Belle explained. “It’s all been commercialized and so it’s easy to use now. It comes with software!”