The following is the project description given by the Navy Precision Optical Interferometer and client Jim Clark:
The central goal of all astronomical observation is to better understand our universe and how it works. Earth and our solar system are just one possible astrophysical arrangement, so what we can learn by looking around our own neighborhood is limited. By observing the billions of other systems out there, we can develop a better understanding of what other arrangements are possible, how they might have formed, and what the implications are for potential future off-earth explorations.
Just as when you view the world with just one eye, there are limitations to viewing celestial phenomena from a single point, i.e., a traditional telescope. Observation of many 2D features benefit from combining multiple views of the target from points that are spread out spatially, a technique called interferometry. For example, the centroid of binary systems changes as one star orbits its partner, and it is the centroid of the binary system that a single telescope measures to generate a catalog of stellar positions; other applications include star-spots (which have yet to be seen!), accretion discs (as the stars spin and toss out matter of different temperature and mass), star rotations (what is the spin rate and orientation of polar axis?) and other interesting science that single telescopes simply cannot measure; we may eventually be able to use nulling interferometry for exoplanet detection.
The Navy Precision Optical Interferometer (NPOI), an astronomical long-baseline optical interferometer, has been in operation on Anderson Mesa, just outside Flagstaff, Arizona, since 1994. This facility was funded by the Naval Research Lab and the U.S. Naval Observatory and built on Forest Service land supplied using a Special Use Permit (SUP), held by Lowell Observatory. An aerial view of the site, shown in Figure 1, illustrates the general shape and layout of the 2.2-meter to 437-meter baseline array. The NPOI has a unique capacity for detecting and determining motions and orbits of binary systems. Many regional partners collaborate with NPOI to take advantage of its unique capabilities, including Northern Arizona University, New Mexico Tech, Seabrook Engineering, Tennessee State University, and Lowell Observatory.
The NPOI collects and combines light from up to six apertures simultaneously to form a high spatial resolution synthetic aperture. The wavelength range of operation is currently in the visible spectrum, 400 nanometers to 800 nanometers, and will soon include infrared wavelengths. Reconfigurability of the array generates baselines from 2.2-meters to 437-meters, and the light collected at each station is transported as a 12.7 centimeters beam through evacuated pipes to a beam combiner. Reconfiguration of the array is analogous to a zoom lens on a Digital Single-Lens Reflex (DSLR) camera.
Many components require maintenance or repair, especially the heart of NPOI, the Fast Delay Lines (FDL). The Fast Delay Lines (Figure 2), housed by vacuum tanks are used to zero the path difference, within 10’s of nanometers, from each siderostat to allow for the beams to be combined in phase. At the front of these tanks, snoots (Figure 4) are used to equalize air distances through the entire optical path. Snoots allow stellar light into the FDL’s and back out after respective reflections. The Snoots contain custom vacuum windows allowing stellar light to enter and exit of the FDL tanks. The FDL tank Seal Plates also allow the metrology beam into and out of the tank which allows for highly accurate measurements of cart positions, on the order of a nanometer. These inner tank components from time to time must be repaired. This entails the front and/or rear seal plates to be moved/stowed and FDL tank inners worked on in an appropriate clean environment. Then all components must be reinstalled easily. Presently, an overhead gantry crane and multiple technicians are required for disassembly of the FDL seal plates and snoots.
Over the course of Fast Delay Line (FDL) use, there is need for periodic maintenance within the vacuum tanks confining the FDL Carts. On the front of the FDL’s there are seal plates which capture snoots to relay the light in and out of the inner room. Figure 3 shows the snoots exiting the delay line tanks. Figure 4 shows the snoots as seen from the inner room. These seal plates and snoots use many features which take long lengths of time to disassemble and assemble. This issue is apparent on the front and rear of the tanks, although the rear seal plates do not have optical pass throughs. Reducing the use of large tools (gantry crane) and quantity of technicians needed to complete this task would be most beneficial to the Opto-Mechanical Group (OMG) at NPOI.