Development of the NPOI

Please click on the small images below to display full-sized images.

Jul 1989 Work commences on the project.

Funding for the Navy Prototype Optical Interferometer (NPOI) was initiated by the Oceanographer of the Navy and the Office of Naval Research. Design work began on all phases of the project. Fabrication of major components, including the siderostats, the elevator and array feed cans and the fast delay line optics carts began at the USNO instrument shop.

Sep 1991 USNO, NRL and Lowell sign contract.

The US Naval Observatory, Naval Research Laboratory and Lowell Observatory sign a contract to build the NPOI on Lowell's dark sky site at Anderson Mesa, southeast of Flagstaff, Arizona. Lowell Observatory provided the developed site and site infrastructure. During the 12 months following the contract signing, Lowell worked with the Forest Service, obtaining the necessary permits to clear the site for construction. Also, sub-contracts were let to local construction firms during this period. The photo at the side shows a view of the Mesa, near the present array center and looking north, before construction began.

Sep 1992 Construction begins on Anderson Mesa.

Jan 1994 Control building, lab building and astrometric huts completed.

After approximately a year and a half, the initial phase of construction was completed. This included the concrete piers to hold the siderostats, beam compressors, elevator cans and center feed cans for the imaging and astrometric arrays, the control and lab buildings and the astrometric huts. The photo at right is an aerial view of the site after this phase of the construction. The lab buildings and astrometric huts are labeled in the picture but the control building is not shown. It lies out of the field of view to above and to the left of the lab building.

Aerial view of early phase of construction

Jun 1994 First astrometric telescopes and visible light beam-combiner optics installed.

The photo at top left shows the installation of a siderostat assembly in one of the astrometric huts. This operation is normally performed with our crane truck, the back end of which may be seen in the photo. The assembly is carefully lowered until it rests on the set of three kinematic locating points on the siderostat pier inside the hut.

The critical laser metrology systems necessary for astrometric observations were installed in the astrometric huts. These consist of a laser interferometer that measures the position of the siderostat relative to bedrock in each of the huts. In the bottom left photo we see a picture of the five laser beam launch tubes (copper pipes kept under vacuum) originating from the metrology interferometer and pointing toward a retroreflector on the back side of the siderostat mirror.

Oct 1994 First visible light stellar fringes obtained.

Mar 1996 First visible light closure phase data obtained.

After the initial closure phase demonstration at the NPOI, we began a series of observations of the spectroscopic binary star Mizar A. In the graphic to the left, we show a gray-scale representation of one of the series. The graphic illustrates that closure phase data may be used to produce real images. The separation between the two stars is about 6.5 milli-arcseconds, about 8 times the resolution of the Hubble Space Telescope. These data were important to establish an accurate brightness difference between the two stars.

May 1996 Concrete work begun for outer part of array.

Jun 1998 First two imaging telescopes placed on array.

The photo on the left shows a view looking along the east arm of the interferometer, with the siderostat covers at stations 2 and 4 rolled back to expose the mirrors. These covers were the first versions a cover for the siderostats in the imaging part of the array. The white building in the background to the right is the east astrometric hut.

Jul 1998 Infrared wavelength beam-combiner optics installed; first infrared fringes recorded.

The photo on the upper right shows the beam combiner table for infrared light with relay mirrors and beam splitters installed. The alignment laser and white light sources are to the right in the foreground, separated from the optics by a black baffle. Detector locations are at the right edge of the photo behind the alignment sources, on a separate breadboard table. Only one pair of telescopes can be used at this time to give a single baseline. The photo on the lower right shows a filtered fringe signal for a star, one of the first obtained with the infrared system at a wavelength of 1.25 µm.

Nov 1998 Assembly work begins on optical system for the outer part of the array.

In November, we began installing vacuum feed pipe for the east arm of the interferometer. After locating the station at the end of the arm to be used as an alignment fiducial, we placed in the 8-inch aluminum feed pipe. Much of the installation work was done in the snow, as may be seen in the upper photo on the left. Later, after the weather cleared, we began to install the elevator cans at the outer stations. In the photo at lower left we are installing the cover to the elevator optics skeleton at station 9.

The east arm feed pipe installation was completed by December 1998. We then turned our attention to the north arm. Pipe installation was started in February and finished in June 1999. The full run of feed pipe to station 8 on the north arm is shown in the top photo on the right. The west arm was the last to have feed pipe installed, beginning in April and finishing in July 1999. The photo in the middle on the right shows the first line of pipe. Note the crossover ladder bridging the feed pipe. This allows foot traffic access to the astrometric huts from the control and lab buildings without having to walk around the entire perimenter or crawl under the pipes.
In July 1999 a second generation siderostat cover was designed and constructed. The new design is a fabric-based retractable cover, much smaller and more portable than the first generation covers. The prototype for this second version is shown in the bottom photo on the right.

Jul 1999 Installation of the first long-delay line vacuum pipes.

In order to use the full extent of the interferometer, we must use sections of fixed delay -- called the long delay line -- to augment our tracking delay lines. The long delay line consists of discrete stations of pop-up mirrors located along a 100 meter run of 20-inch diameter steel pipe. The photo at the right shows nearly complete runs of the first two of six such pipe lines.

Oct 1999 Work started on north astrometric hut & interpretation of metrology results.

Construction was started on the laser metrology system for the north astrometric hut. Vacuum laser launch pipes were fabricated and a visitor viewing window was installed. Work was begun to understand and correct for the effects of atmospheric dispersion in the metrology measurements. We expect this will permit 10 mas accuracy astrometry in the near future.

Apr 2000 Upgrades to the visible light system completed.

Our visible light detectors are avalanche photodiodes (APDs) that must be cooled well below ambient temperature to reduce the detector noise contribution. We have installed a new set of detector cooling units (called dewars) that now allow us to achieve an operating temperature of -70 °C. Four dewars are required to house our 120 APD detectors. One dewar contains the quad cell detectors used for guiding the six tip-tilt mirrors; the other three dewars house the APDs used to sense the output from the three spectrometer channels. In the photo on the upper right we may see two of the four dewars mounted on the outside wall of the inner room. The dewar on the left has the cover for its electronics open, exposing the APD amplifier electronics. The photo on the lower right shows a detailed view of the interior of one of the dewars. The coolant lines may be seen running horizontally across the dewar with the 8 individual cold heads attached. Each cold head holds 4 APD detectors, mounted facing down in the photo, with their electrical leads running to the top of the dewar. Light is brought into the dewar by fiber optics couplings at the bottom.

We are also preparing the system for the first experiments with six-telescope beam combination. This will increase the sampling of the image five-fold and give us the capability of constructing much more complex images. The photo on the upper left shows the rebuilt switchyard table in the inner room. This table redirects the light toward the beam combiner table after it leaves the fast delay lines. Three pairs of mirrors are shown in the photo. Each pair transfers the light from one delay line to the beam combiner table.

The beam combiner table, shown in the photo on the lower left, is the final destination for the light. Here the individual beams are combined through a variety of mirrors, lenses and beamsplitters and sent to the APD detectors. A portion of this complex optical arrangement may be seen in the photo.

Dec 2000 Six siderostat stations installed.

Nat White and Don Hutter are operating the crane truck. In the picture to the right, they are picking up the siderostat from the crane truck in preparation for installation at station 7 along the southwest arm of the array. The siderostat is lifted by means of a three-armed jig that is secured to the siderostat bottom plate and provides for a balanced load.

Here, the siderostat is being transferred to the station pier. The siderostat cover, retracted for this operation, is stowed in the foreground of the siderostat station support pipe. The NAT/WASA cover may be seen to the left in the picture, directly above the vacuum feed pipes.

In the picture on the left, the siderostat has been lowered onto the kinematic mounts. Nat is about to remove the lifting jig from the siderostat plate. In this picture, one may clearly see the stowed siderostat cover on the near side of the support pipe. The center and east astrometric station huts may be seen in the background of the picture.

This is a view from station 8 on the west arm of the interferometer, looking toward the newly-installed station 7. In the background, one may see the astrometric telescope huts. The white building in the left foreground is a vacuum pump and electronics control station house.

Lifting the north astrometric siderostat

In this picture, the north astrometric station siderostat is being transferred to its kinematic mount. The astrometric hut has its roof rolled back for access.

Here is a view of the siderostat in place on its kinematic mounts. One may see the mirror beneath a plastic dust cover. Note that the lifting jig has not yet been removed from the siderostat.

With this operation, we have completed the installation of six siderostats in the field, in preparation for 6-way beam combination. We already have modifications in place to the beam-combining table to accept light from six telescopes simultaneously. The layout geometry for the first 6-way experiments is shown in the cartoon figure to the left. Note that the telescopes are distributed with 3 along the southeast arm, two along the southwest arm and one along the north arm. The maximum baseline available will be about 64 meters (between station 7 on the west arm and the east astrometric station). The minimum baseline (station 2 on the east arm to the center astrometric station) will be about 7 meters.

May 2001 All LDL vacuum pipes and stations installed.

In the picture on the right, the last section of LDL vacuum pipe is being transported by long-reach forklift to its place in the system. As of May 2001 all six sections of pipe have been fully installed. They have now been coated with a reflective white paint to reduce pipe heating and band deterioriation. All are now under vacuum.

In this picture we see the end covers for the LDL pipes on the inside of the lab building. Connection of the LDL pipes into the optical system awaits completion of the new periscope assemblies which will be installed in the foreground of the picture.