This aerial view shows some of the main parts of the NPOI, located on Lowell Observatory's
dark sky site at Anderson Mesa approximately 15 miles southeast of Flagstaff, Arizona:
the Control Building,
the Lab Building,
the Astrometric Array
which includes 4 stations,
the Imaging Array
with another 27 stations,
and the Long Delay Lines.
Inside the lab building, there is also
the Fast Delay Lines, and
the Inner Room
where the beam combining is done.
Each arm of the array is 250 meters (820 feet) long. The longest baselines will be
(1433 feet) in length, while the shortest baseline is
(56 feet). The current ~1 milliarcsecond resolution will be increased to 200 microarcseconds when completed.
This tour of the NPOI shows how various parts of the instrument work and how all the parts work together to produce stellar
interference patterns. The tour follows the starlight path through the telescope, beginning at a station on the array...
There are two types of stations on the NPOI array -
used for observing programs that image stars, and
used for accurate measurements of the positions of stars.
There will be a total of six
which can be moved to different locations on the array.
There are four
at fixed positions.
Inside an astrometric station,
the main components are the
with a 50 cm mirror (20 inch), the
(Wide Angle Star Acquisition) CCD camera, and the
(Narrow Angle Tracking) tip-tilt mirror. The movable
under one cover, and the
When starlight reflects off the
the star is imaged by the
which has a
view of the siderostat mirror.
The image is analyzed and information fed back to adjust the
siderostat pointing so that the light is reflected to the
and into the feed system...
THE FEED SYSTEM
The Feed System is a series of
vacuum pipes and "cans"
containing mirrors which route the light from the array stations
back to the Lab Building where the light is recombined.
Each arm of the array has
three levels of pipe
allowing up to three stations on any one arm, or any combination of
up to six stations total from the three arms to be used at once.
At each station on the array there is an
reflected into the elevator can,
from the NAT mirror, through a window in the top of the can.
Inside the elevator can,
there is a moveable platform to direct the light onto any of the three
pipe levels, while letting light pass straight through from
stations farther out the arm. At pipe intersections,
at the branches to the astrometric stations
and at the array center, there are
which direct the light beams around or through the intersections.
Inside the feed cans,
there are mirrors on slides on each level that will intercept a beam
and reflect the light in a different direction or slide out of the way
to let the light pass through.
The 255 cubic meters (9000 cubic feet) of
in the feed system, spanning the three 250 meter (820 feet) arms of the array,
is all under vacuum so the light will travel at the same speed
across the array. The distance the light has to travel to the beam combiner
is different depending on which stations are being used. This
fixed optical path difference
has to be compensated for before the light can be recombined.
The projection of the starlight onto the array, creates an additional
variable optical path difference,
that also has to be compensated for. This is done by the delay lines...
THE DELAY LINES
The NPOI has two types of "Delay" lines. The Long Delay Lines, or
compensate for optical path differences in incremental amounts. The LDLs
are 110 meter (361 feet) long vacuum tanks with cans, that each contain two
"pop-up" mirrors, spaced at 6 intervals. Inside the lab building the beams
are brought to the same level through
periscope cans and then will be sent, twice, out and back a delay line,
reflecting back off the pop-up mirrors. The mirrors may be popped up in
different cans creating a wide variety of incremental delay distances
up to 440 meters (1444 feet).
The Fast Delay Lines, or
are 18 meter (59 feet) long vacuum tanks in the Lab building.
Inside each tank is an
riding on parallel steel rails. Each starlight beam travels
to the optical cart where it is reflected back by a
The optical cart, with it's position monitored by a
laser metrology system,
can travel at speeds of several centimeters (~ 1 inch) a second with a precision
of a few nanometers (~ 0.0000001 inches).
continuous delays lengths
up to 36 meters (118 feet), covering the intervals between LDL pop-ups,
and compensating, while tracking a star, the changing optical path differences
due to Earth's rotation.
At the front of the cart is a
that strokes in and out, at 500 Hz. Setting the piezo mirror strokes to different
amplitudes, between 500-8000nm, on different FDL lines allows the signal from each
baseline (a pair of stations) to be determined. This is necessary because each
output beam contains up to six different baselines after the light is combined...
The Beam Combining Table, located in the Lab Building, holds the
Narrow Angle Tracker (NAT) quad cells,
which feed back error signals to the NAT mirrors keeping the beams
centered through the feed system and aligned into the beam combiner, the
Spectrometer prisms and lenslet arrays,
which disperse the light from the beam combiner and then feeds
the spectral components, covering 550-850nm, through 16 optical fibers to Avalanche Photo
Diodes (APDs) where the light photons are converted to electrical
signals, as well as the heart of the system - the
Beam Combiner optics. The
consists of two 3-beam wide beam splitters
(BSA and BSB),
two 3-beam wide flat mirrors
(M3A and M3B),
a 2-beam wide flat mirror
and one single-beam flat mirror
light beams enter the Beam Combiner,
from up to six different array stations, impinging on either BSB or M3A first,
and then propagate through so that each of the three exit beams
are the combination of light from four stations, or six "baselines"
(pairs of stations). Then each of
the three exit beams
passes through a Spectrometer prism and is focused
onto one of the three Spectrometer
where the spectral components are sent through optical fibers to
banks of APDs
where the photons are converted to electrical signals
- and that is the end of the light path.
From the NPOI control room, using the primarily GUI based
software, the observer can control the siderostats
tracking and flux into the instrument, the
weather and "seeing" conditions
(estimated from NAT tracking errors). The observer can control the
delay lines, initiate
fringe searching, adjust tracking parameters and monitor fringe SNRs
(Signal to Noise Ratios), as well as
monitor data recording and log observations.
In addition to numerous computers in the control building, the
behind the software for the NATs, FDLs and Fringe engine are
located in the Lab Building. The electronics
for each siderostat
are located on the array, at or near each station, and are in
the process of being upgraded.
While the observer can
manually control all the telescope systems,
most of the observing processes are also automated so that an entire observation
sequence can run by simply selecting the star and
pressing one button.
This allows one observer to operate the entire NPOI and, in
as little as three minutes, complete each stellar scan...
HOW IT WORKS
When the light from a star shines down on the NPOI
array, unless the star is directly overhead,
the light travels farther to reach different stations
and this delay has to be compensated for before
combining the light from the different stations.
To observe the star, first the optical carts in the fast
delay lines slew to starting positions and
the siderostats point to the star so that
the light propagates through the telescope.
Then the fringe engine takes over control of the optical
carts in the Fast Delay Lines, moving them, until
the optical path lengths match the delay distances
and the fringes are found.