Introduction
The Keck Interferometer will use Michelson beam combination between the two Keck telescopes, providing a baseline of 85 m. The interferometer will utilize the phased pupils provided by adaptive optics on the Kecks.
Cophasing of the array will be accomplished by fringe tracking on an isoplanatic reference to enable high-sensitivity science observations. Key components of the cophasing system include active delay lines in the beam-combining lab and dual-star modules at each telescope. Several back-end beam combiners will be provided, including two-way beam combiners at 1.5-2.4 µm for fringe tracking, astrometry, and imaging; a multi-way combiner at 1.5-5 µm for imaging; and a nulling combiner for high dynamic range observations at 10 µm.
Click on the links above for more details on each of the subsystems.
Interferometer Subsystems
The Keck Interferometer integrates a number of different subsystems to enable its scientific measurements. The block diagram (below) traces the light path from the telescope to the back-end instruments.
Keck Interferometer block diagram (Click to enlarge)
Telescopes
The interferometer uses the two existing 10-m Keck telescopes.
First fringes with the interferometer used siderostats, similar to those used at
Palomar Testbed Interferometer. They are installed at the site adjacent to Keck 2; the siderostat shelters can be seen in the foreground in the image below.
The siderostats are the tiny white objects visible just below the Keck Telescope to the right. They simulate telescopes for the testing/debugging of the interferometer.
Cut-away view of the Keck Telescopes, showing the 100 foot domes as well as the workshops and basement laboratory.
Wavefront Correction
The Keck Observatory developed an adaptive optics system for the Keck-2 telescope. As part of the interferometer project, a second adaptive optics system was developed for Keck-1, providing phased pupils at near- and mid-IR wavelengths.
Test images taken by CARA's Keck-2 Adaptive Optics System.
The image on the left is a corrected image (adaptive optics on), the image on the right is uncorrected (adaptive optics off).
Dual-Star Module
A dual-star module (DSM) is located at the Nasmyth focus of each telescope to enable cophasing by producing two collimated beams from two separate stars. The figure below illustrates the beam train from the Keck AO system through the Keck DSM for the bright star when the DSM is configured for imaging.
Optical path from a Keck Telescope through the adaptive optics system and Dual-Star Module.
Coude Train and Beam Transport
To propagate the light from the dual-star module on the Nasmyth platform of each Keck telescope, the Keck coude train has been completed with the addition of coude mirrors M4-M7 (M4 is the output mirror of the dual-star module, while M7 is the fixed mirror at the base of the telescope). Since two beams are propagated from the dual-star feed, each coude mirror is actually a mirror pair. For the primary star, these mirrors are located along the coude centerline; for the secondary star, these mirrors are offset from the centerline and are actuated to compensate for azimuth rotation. After mirror M7, the light is directed into the beam-combining lab located in the basement of the Keck facility.
Coude path through a Keck Telescope mirrors M4-M7
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Schematic of the interferometer beam combining lab in the observatory basement.
Isometric view of the beam combining lab at the observatory.
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The beam transport system employs oversize flats which direct both the primary and secondary beams. The geometric compressed beam size is 10 cm, while the transport optics allow for a 15 cm unvignetted aperture for each beam to accommodate diffraction and alignment tolerances. Beam transport for the Kecks is through the coude tunnel which connects the two domes.
The beam transport system directs light to the beam combining lab, which is at the basement level adjacent to Keck 2. Attention has been given to controlling the thermal environment in the lab. As shown in the schematics and photograph, modular clean rooms partition the basement into separate areas for the long delay lines, the fast delay lines, and the beam-combining optics, providing a second layer of attenuation to environment disturbances, in addition to helping to maintain cleanliness of the optical system.
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Photograph of the beam combining lab.
Delay Lines & Metrology
Starlight bounces off mirrors on left of screen, goes to long delay lines in the middle, and to fast delay lines on the right.
Rich Kendrick, of CARA technical staff, working on aligning a long delay line at the summit
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The light from the beam transport system is directed to the beam injection and switching optics, which feed the light into the long delay line system. The long delay line system employs flat mirrors mounted on sleds which move along the coude tunnel in the basement. Each sled delays both the primary and secondary beams from a telescope by up to 170 m; the longer delays use a double pass through the delay line system. These are "move and clamp" delay lines, which are stationary during an observation and are only repositioned between targets.
After coarse delay by the long delay lines, the beam injection and switching optics direct the light into the fast delay lines. These move along tracks in the beam combining lab as shown in panorama above. Each delay line has a physical travel of 7.5 m, so that for a single baseline, a delay range of +/-15 m is available without moving the long delay lines. The delay lines are similar to the laser-monitored units used at PTI. They employ a four-stage servo design with a PZT, two voice coils, and a microstepped tractor motor, and provide full position and rate commanding.
Andrew Booth, of JPL, is the software engineering lead for the Keck Interferometer.
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The delay lines employ local laser metrology for real-time control of the servo systems. This is similar to that used at PTI, except that the laser sources are fiber remoted to the control room to minimize heat dissipated in the beam combining lab. In addition, separate end-to-end laser metrology which monitors the entire optical system is implemented as required for narrow-angle astrometry and cophasing. This "constant term" metrology terminates at end points in the DSMs at each telescope. Optical path changes prior to the DSM are common to both the primary and secondary star, and so do not affect cophasing. If warranted, accelerometers could be used on those unmonitored optics to provide vibration attenuation through feedforward to the fast delay lines.
After exit from the fast delay lines, the 10-cm geometric pupils are compressed to 2.5 cm and directed to a switchyard table which directs light to the various starlight sensors.
Schematic of the beam-combining lab and coude tunnel with some of the star light beams coming from both Keck 1 or Keck 2 telescopes.
Fringe Tracker
Several 2-way H- and K-band Michelson beam combiners support the various observational modes of the interferometer. The foreoptics for each combiner are implemented on an optical breadboard. The output beams from two breadboards are fed via single-mode fluoride optical fibers to a 4-input near-IR camera. The camera uses one quadrant of a HAWAII infrared array with subarray readout to provide fast frame times. A schematic of the dewar, and a photograph of the first dewar during assembly, are shown below. The array signal chain uses commercial video and clock buffer electronics with a custom second-generation clock generator and interface card to provide fast access for real-time use. Good array performance has been obtained with interferometer frame rates to 500 Hz. The use of post-combination single-mode fibers improves visibility calibration, yet allows the end-to-end laser metrology to use the same beamsplitter as is used by starlight.
Rob Ligon and Dr. Gautam Vasisht assemble the
FATCAT
"tree", which hold optics inside the FATCAT dewar.
Fringe detector dewar schematic and photo during assembly.
Angle Tracker
The angle tracker also uses a HAWAII infrared array in fast readout mode; the images from the telescopes are arranged on a single quadrant. Angle tracking is provided at J and H. For the Kecks, where the AO system already provides high-speed angle tracking, the offsets from this angle sensor are used to correct for slow drifts. For application to cophased observation, including astrometry, a second angle tracker system will be implemented to provide slow guiding on the faint science stars.
Keck angle tracker during laboratory integration and test at JPL prior to shipment to Hawaii.
Sam Crawford, JPL technical staff, assembling the angle tracker.
Nulling Combiner
The nulling combiner will be the primary instrument for the detection of exozodiacal dust disks. It uses an achromatic nulling interferometer to null the light from the central star on two parallel 85-m baselines, which then feed a fringe-scanning beam combiner. The output of this combiner is spatially filtered and directed to a low-resolution 10 µm array camera. The nuller design draws heavily on work at JPL on the deep nulling of visible light.
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| Visible-light
laboratory nulling experiment at JPL. |
Results from visible-light
laboratory nulling experiment:.a stabilized null with an average null depth of better than 10,000:1 was
achieved for a white-light source with an 18% bandwidth and a single polarization. |
Control System
The control of an instrument of this complexity is a major task; the following is just a high-level summary. Most of the real-time control is implemented using VME-based systems with Power-PC processors running VxWorks. The delay lines and starlight detector systems employ an object-oriented software framework (RTC
20), running on top of VxWorks, that was developed at JPL for interferometry applications. Motion control and auto-alignment use EPICS, which is a Keck standard, employed widely at the observatory for telescope and instrument interface. CORBA interfaces to RTC and EPICS subsystems allow uniform access by the interferometer
sequencer for observing automation, by engineering and operator GUIs, and by the data archiver.
Control system team at summit: (Front to back) Sam Crawford, Andrew Booth, Erik Hovland, Mark Colavita, and Mark Swain.
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