From Tom Armstrong of the Navy Prototype Optical Interferometer:

April 4, 1995

We obtained first fringes on 28 October 1994 on Alpha Andromedae, and later the same night on Procyon, on our East-Center baseline (18.9 meters).

Since then, we have been working on bringing many of the systems into easier working order, on improving alignment procedures, and on bringing a third delay line and third siderostat on line. We have also started to prepare for the second phase of construction, in which the arms of the imaging array will be extended to 250 meters and long delay lines will be added.

DETECTORS

We have worked quite a bit on the APDs. When we got fringes in October, many of them displayed distinctly non-Poisson statistics, and the circuit generated too much heat. We have finally tamed the circuit. With modifications, the new circuit shows Poisson statistics and can count up to 400,000 photons per second. The heat load to the chiller has been cut significantly.

We are currently operating the APDs at -45C and about 25V above breakdown. At high count rates, the dead time is in the 100 - 150 ns range, but increases somewhat as the count rate is decreased. We will decrease the operating temperature and increase the voltage over the next few months.

QUAD CELLS

We have also devoted a fair amount of effort to the quad-cell angle trackers. Our quad cells consist of an array of epoxy lenslets, each 400 microns on a side, on a 5 mm substrate of BK7 glass. Where four lenslets intersect, there is a small square area of about 35 microns on a side that is not covered by lenslet. Glued to the back of the substrate are four 200 micron diameter optical fibers spaced on 400 micron centers. When the star is centered on the quad cell, about half the light should hit the 35-micron square area and thus fail to enter the fibers. (Dave Mozurkewich has demonstrated that losing the light at the center of the image actually improves the angle-tracking signal- to-noise ratio.) We have found that obtaining the best behavior of the quad cells requires considerably more care in alignment than we had originally thought, and that less light disappears into the 35-micron square than we had expected. In addition to working on alignment, we are tweaking the tracking algorithm to account for differences in sensitivity between the APDs of a given quad cell.

DELAY LINES

We currently have three vacuum delay tanks installed, and five delay carts on site, in various states of completeness. We also have an "air delay line," a pair of rails set up without a vacuum tank to be used for testing and commissioning delay line carts. We have been working on a variety of things with the current set of delay carts, many of which fall into the category of normal glitches encountered in commissioning any complex system.

Power and control signals are sent to the cart via a ribbon cable that enters the front of the tank. This cable lies between the delay-line rails, extending back to a cable-tensioning cart, and then forward to the back of the delay cart. We have found that friction between the section of cable lying between the rails and the section between the tensioner cart and the delay cart is a significant drag on the cart. We have added Teflon tape to the half of the cable that never passes over the roller of the tensioner cart, which seems to have alleviated the problem.

We have also tracked down a variety of electronic problems on various carts and have improved the performance of the delay line metrology detectors. These problems had led, by various mechanisms, to increased jitter in the delay lines. Typical jitter is 5 nm rms when the carts are working properly. We have attained that level of jitter with two of the carts currently in the delay tanks and are tracking down problems on the third cart.

The delay tanks consist of eleven 6-foot sections of 18-inch pipe, with a small gap between sections. The vacuum seals between sections are formed by a neoprene band around the joint, held in place by two hose clamps. We use a similar system to form the vacuum joints in the feed beam system leading from the siderostats to the laboratory. These seals are much less expensive than vacuum flanges, and allow some amount of motion between sections. The system works quite well for the level of vacuum we need; we have been able to hold a pressure of 100 mTorr for two weeks at a stretch without pumping.

SITE METROLOGY

One of the features of the NPOI is the capability of doing wide-angle astrometry with 2 mas precision. To make this possible, we must monitor changes in the geometry of the interferometer at the micron level. The astrometric array will eventually have three distinct metrology systems to monitor the array geometry. The first is the siderostat metrology system. Five laser interferometers, mounted on an optical breadboard in a temperature- controlled chamber behind the siderostat mount, will monitor the motion of a cat's-eye retroreflector mounted at the siderostat pivot point. Five to six additional laser interferometers, also mounted on the breadboard, will monitor the motion of the breadboard itself with respect to corner-cube retroreflectors placed in bedrock 8 meters below the siderostats. Finally, eleven interferometers will monitor the motions of three of the breadboards with respect to the fourth, master breadboard.

At present, we are installing the siderostat metrology systems on the first three astrometric siderostats. The first of these is nearly complete. The wells for the "optical anchors," the second system, are in place. Provision has been made for the third "pier-to-pier" metrology system. We plan to install the vacuum pipe for that system during the summer of 1995.

CONTROL SYSTEM

The control system of the NPOI is quite complex, consisting of Motorolla(?) 680xx processors operating the siderostats, acquisition cameras, angle trackers, delay lines, fringe detectioiin and data storage. In October, when we first detected fringes, important parts of the system were incomplete or had rudimentary interfaces to the astronomer. In order to search for fringes, we calculated the expected fringe position off-line every ten minutes. In addition, we were unable to record fringe data in October. Our detection was based on the fringe-tracking software's telling us that it had locked onto fringes. The delay at which this happened changed in the appropriate way during the evening, and two stars were detected a few hours apart, but no fringe data were recorded.

The control system is now nearing completion. The only parts missing are some of the user interface and storage of some auxilliary data. The fringe detection algorithm also needs to be refined for operations on more than one baseline.

SIDEROSTATS

Six siderostats are currently on site, four astrometric siderostats plus two for the imaging array. Three astrometric siderostats have been installed in their enclosures and connected to the control system. We have built pointing models and tested the angle trackers for these three.

NEXT CONSTRUCTION PHASE

We have started the process of getting permission from the Forest Service to proceed with the next phase of construction, which will include extending the arms of the imaging array to 250 meters and adding six long delay lines (to be used in series with the current "fast" delay lines). Construction of the next phase will start late in the construction season this year, or early next year.

PLANS FOR 1995 AND BEYOND

Plans for 1995 include installing three more siderostats--the fourth astrometric siderostat and the first two imaging (movable) siderostats-- extending the beam combiner to accept six beams, completing the site metrology systems, and beginning regular observations. We are also beginning the acquisition process for four beam compressing telescopes that will allow use of 35 cm apertures (rather than the current 12 cm, limited by the feed system optics). We expect to install the large beam compressors, and to complete installation of the long array arms and long delay lines, in 1996.

WEB HOME PAGE

Pictures of the NPOI and a bibliography of optical interferometry at the interferometry project can be found on the Web by clicking on Navy Prototype Optical Interferometer. We will try to keep that home page updated as we progress. --Tom Armstrong