Status of the LBT Interferometer - wiki.ilocater.nd.edu

Status of the LBT Interferometer

Philip M. Hinza, Teresa Bippert-Plymatea, Andy Breuningera, Tom Connorsa, Brian Duffya, SimoneEspositob, William Hoffmanna, Jihun Kima, Joe Krausa, Thomas McMahona, Manny Montoyaa,

Richard Nasha, Olivier Durneya, Elliott Solheida, Andrea Tozzib, Vidhya Vaitheeswarana

aSteward Observatory, 933 N. Cherry Ave, Tucson, AZ, USA 85721 bOsservatorio Astrosico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

ABSTRACT

The Large Binocular Telescope Interferometer, a thermal infrared imager and nulling interferometer for the LBT, iscurrently being integrated and tested at Steward Observatory. The system consists of a general purpose or universalbeamcombiner (UBC) and three camera ports, one of which is populated currently by the Nulling and Imaging Camera(NIC). Wavefront sensing is carried out using pyramid-based "W" units developed at Arcetri Observatory. The systemis designed for high spatial resolution, high dynamic range imaging in the thermal infrared. A key project for theprogram is to survey nearby stars for debris disks down to levels which may obscure detection of Earth-like planets.During 2007-2008 the UBC portion of the LBTI was assembled and tested at Steward Observatory. Initial integration ofthe system with the LBT is currently in progress as the W units and NIC are being completed in parallel.

Keywords: LBTI, Interferometer, Exoplanets,

1. INTRODUCTIONThe Large Binocular Telescope Interferometer is a nulling and imaging interferometric instrument designed for use inthe thermal infrared (>2.5 microns). LBTI is a modular instrument, with a general-purpose or universal beam combiner(UBC) that is capable of feeding three separate camera ports. The project is funded through NASA and is focused oncharacterizing nearby stars that may be the target of future direct imaging searches for Earth-like planets. LBTI isdesigned to carry out sensitive detection of zodiacal dust and image giant planets.

1.1 Components of LBTI

LBTI is made up of several components as shown in Figure 1. The beam combination is carried out via the UBC opticswhich consists of a single off-axis elliptical mirror and two fold flats1. The camera portion of LBTI is called the Nullingand Imaging Camera (NIC). The UBC provides a combined focal plane from the two LBT apertures which satisfy thesine condition, and with suitable degrees of freedom to maintain phase coherence and image overlap between the twobeams. NIC reimages this focal plane onto one of two science detectors, operating at 3-5 microns or 7-25 microns. Inaddition NIC uses light at 2-2.4 microns to sense phase variations between the apertures. Wavefront sensing is carriedout using pyramid wavefront sensors. These “W” or wavefront sensor units are a nearly identical design developed forthe LUCIFER cameras on the LBT. There is one unit for each of the two apertures on the LBT. The W units are fed byvisible light reflected from the titled entrance windows on each side of the LBTI cryostat.

The components of LBTI are held to each other and to the telescope with an external steel box frame assembly called thetelescope interface structure (TIS). The TIS has been designed to maintain the alignment of the UBC optic to within thespecified tolerances for the range of elevation angles and temperature variations encountered when operating on theLBT1.

*[email protected]; phone: 520-621-7866

Optical and Infrared Interferometry, edited by Markus Schöller, William C. Danchi, Françoise DelplanckeProc. of SPIE Vol. 7013, 701328, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.790211

Proc. of SPIE Vol. 7013 701328-1

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 01/31/2015 Terms of Use: http://spiedl.org/terms

InterfaceStructure

Lifting booms/Vacuum System

Beam Diverfer

Wave FrontSensor unit #2

(wLBTI

UniversalBeam

Combiner(U BC)

Wave FrontSensor unit #1

(wLBTI)

/HypotheticalInstrument

Combined /Imager/Nuller(Nil/NOMIC)

Instrument Bays

Fig. 1. The LBT Interferometer.

The various components of LBTI are at differing levels of completion. This paper describes the fabrication state andlevel of testing that has been carried out for the components described above. We focus in this paper particularly on theUBC, TIS, and W units. A separate paper2 describes the NIC design and fabrication status.

2. COMPONENT DESIGN FABRICATION AND TESTINGThe optics and components for the UBC have been tested over the past two years. We began by carrying out testing ofeach of the components, and then assembling the UBC portion, as well as the center portion of the TIS in a laboratory inSteward Observatory.

2.1 Optics

The optics of the UBC consist of three mirrors for each arm of the interferometer. The first mirror is an off-axis portionof an ellipse (OAE). Four OAEs (two mirrors and two spares) were generated from a parent Zerodur ellipse polished atthe University of Arizona Optical Sciences Center. The final radius (R=2568.29 mm) and conic constant (k=-0.202).Four sections were cut from the parent with a nominal diameter of 200 mm and off-axis distance of 152.7 mm. All foursections were measured in double pass and found to have an RMS figure error of less than 0.09 RMS over a 180 mmclear aperture diameter. Surface roughness was measured to be less than 1.6 nm RMS on sample patches for eachsection.

The second mirror in each arm is a flat 80 mm diameter circular mirror. These mirrors are light-weighted silicon carbidemirrors, since one arm of the interferometer uses this mirror as a fast pathlength corrector (FPC). These mirrors werefabricated by Coorstek, and were delivered with an initial figure of < 0.125 RMS. Three invar inserts are bonded tothe back of the mirror, and tested under cryogenic conditions. After several thermal cycles, these bonds were found tocreate small dimples on the front surface of the optic. The amplitude of the bumps were measured, using a 4-Dinterferometer to be approximately 150 nm in amplitude. A new bonding scheme is planned for the final set of optics



that will correct for this slight error. The current mirrors will provide spares for the UBC while the new optics withbetter figure quality will be the

The final flat optic is a 4.25 inches tall x 4.375 rectangular section with a tapered end to allow these two mirrors to beplaced close together, near the centerline of the instrument and fold the beams downward. The optics were fabricatedby TORC using zerodur blanks. The mirrors have an RMS surface quality of approximately 0.07 waves.

2.2 Fast Path-length Corrector (FPC)

The fast path-length corrector is designed to be able to correct for variations in path-length between the two apertures aswell as residual image overlap between the arms. For the LBT baseline, we expect, in mediocre seeing conditions toexperience a peak-to-peak pathlength variation of approximately 60 microns.

The FPC uses Polytec PI high voltage PZT stacks with a nominal room temperature stroke of 180 microns. These PZTstacks are rated for cryogenic operation and have an expected throw at 100 K of approximately 60-90 microns. Since thePZTs are mounted on a nearly-normal incidence mirror, the corrected path-length for this rating is 120-180 microns. Ifwe allocate half of the stroke for tilt correction, the minimum stroke still can match that needed for atmosphericvariations.

Fig. 2. The Fast Path-lengthCorrector. The lightweight silicon carbide mirror is visible on the left. The white cylinders are the PZT stacks drivingthe mirror. Two of the capacitive sensors can be seen on the top left and lower left side of the assembly.

Fig. 3. Dynamic Response testing of the FPC. Sinusoidal and square wave inputs of varying amplitude and frequency weredriven on the FPC and sensed with the capacitive sensors. The FPC successfully performed well above modelvariations expected from atmospheric perturbations.



The FPC utilizes capacitive sensors co-located at the mirror edge to determine PZT positioning for each actuator. Thesewere utilized to characterize dynamic performance, as illustrated in Figure 3. The finite element analysis of the fullstructure predicts resonant modes at 310 and 1035 Hz. Dynamical testing verified this analysis.

The mirror of the FPC is connected via copper straps to the liquid nitrogen cooled base plate of the cryostat, allowingcooling of the mirror to below 120 K. The whole assembly is designed to shrink with a TCE similar to the PZT stacksfrom PI. This results in the ability to use the FPC both at room temperature and at 100K without the need to readjust thepreload on the PZTs or adjust the gaps in the capacitive sensors.

2.3 The Slow Path-length Corrector (SPC)

The Slow path-length corrector is a three-axis actuated mount which holds the right side pupil mirror in the UBC. Themirror mount is designed to provide slow (<1 Hz) correction of phase and image overlap variations between theinterferometer arms. The amount of range of the SPC is 10 mm, providing 20 mm of total path-length variation.

The actuators for the SPC are driven by external stepper motors with 25000 steps per revolution. These are connectedvia right angle joints to three lead screws with 1 mm/revolution. This results in approximately 40 nm resolution for onemotor step.

Tests were carried out to determine the smoothness of actuator movement. The most difficult movement is that neededfor scanning in pathlength. Image motion of approximately 1.5 mm were seen for long scans in path-length. This wouldresult in image misalignment of approximately 0.9 arcsec ath the LBTI focal plane, much larger than the image FWHM.We are exploring the source if this image wander with the SPC and may carry out a refurbishment of the SPC if theimage movement remains above 1 mm after further refinements.

2.4 Cryostats

The UBC optics, baffles and radiation shielding are cooled by three liquid nitrogen vessels, one at the center and one oneach side of the horizontal section of the UBC. The sizing of each of these vessels (12.75 L for the side vessels and 24 Lfor the center) is scaled to provide a minimum of 24 hours hold time for the liquid nitrogen. The rectangular cryostatsare connected via stainless steel vacuum tubes and large aperture gate valves to allow access to a portion of the UBCwithout needing to re-evacuate the whole UBC.

Vacuum testing of the complete system, including the connecting vacuum tubes and gate valves was carried out in late2007. After bakeout of the system and six days of pumping a vacuum of approximately 3x10-4 Torr was reached,sufficient to allow cooling. Activated charcoal getters are located in each cryostat to carry out cryo-pumping once theliquid nitrogen is added.

The initial cool down of the full UBC system took approximately 12 hours to achieve a steady-state temperature on theoptics. The cool-down required approximately 160 liters of liquid nitrogen. The optics in the center cryostat weremeasured to have a temperature of 111 K, the SPC settled to 100 K and the FPC reached 106 K. The temperature ofthese optics is well below the temperature required for their emissivity to contribute to the overall background, thussmall temperature variations such as this are acceptable.

Once temperature equilibrium was reached, a final fill of the cryostats was carried out. All three cryostats retainedliquid nitrogen for 26 hours, at which point the hold time monitoring was discontinued.

2.5 Wavefront sensor units (WLBTI)

The wavefront sensor units for the LBTI are nearly-identical designs as the “W” units developed for LBT first-light AOsystem3. The W units being developed specifically for the LBTI have been designated WLBTI. The design interceptsthe light reflected from a long pass dichroic window tilted at 15 degrees with respect to the incoming telescope beam.Optics are mounted on a small breadboard that can be positioned with a very stiff three-axis translation stage. This stage



allows setup of the W unit on a star anywhere in a rectangular field-of-view patrolled by the translation stages. The sizeof the FOV is 3.6 arcminutes in elevation and 2.2 arcminutes in azimuth.

The W units for the LBTI re-image the incoming f/15 beam to an f/45 beam. An atmospheric dispersion corrector isplace in the beam after the re-imaging lens. A portion of the light is reflected, via a selectable beamsplitter to a E2VCCD47 detector with a 7.2 arcsec FOV. For the WLBTI units an insertable lens in this path will increase this FOV to14.4 arcsec for object acquisition. The wavefront sensing portion of the WLBTI unit will use pyramids fabricated atArcetri Observatory4. These pyramids use a back-to-back double pyramid design to ease the fabrication tolerances.

The WLBTI units are designed to allow chopping of the adaptive secondaries with AO correction in both chop beams.To implement chopping with the WLBTI units we are planning fabrication of a side-by-side set of two pyramids. Thepyramids will be separated in elevation by approximately 5 arcseconds. The implementation of pyramids for choppingutilizes a side-by-side design with a separation of 9 mm between the tips of the two pyramids. The pupil reimaging lens,located after the pyramids, will form images of the pupils in the same location on the detector, whether the light comesfrom the left or the right pyramid. Slight differences in separation between the throw of the chop in the left and rightarm of the interferometer can be compensated using the PZT modulation mirrors which are used for pyramid tipmodulation in each W unit.

3. THE LBTI TELESCOPE SIMULATORTo simulate light from a single, coherent source, imaged by two separate telescopes, we have developed a set of opticsresiding on the rear top of the LBTI, called the telescope simulator. The optics are all reflective to allow testing of thesystem in both the optical and infrared.

Fig. 4. LBTI Telescope Simulator and UBC Design. An initial OAP collimates the light and sends it to opposite sides ofthe UBC. A second set of OAPs on each side create an f/15 which feed the beam to the UBC optics.



The telescope simulator uses an off-axis parabola with a focal length of 508 mm to create a collimated beamapproximately 50 mm in diameter. This beam is relayed to a set of back-to-back right angle prisms which create the leftand side beams to be fed to the inputs of the UBC. These collimated beams are directed to the ends of the UBC and overto the entrance window using two fold mirrors in each beam. The final beam that directs the light into the UBC isanother OAP that creates an f/15 beam at the Gregorian focus locations on each side of LBTI. The focal length of thesecond OAP is 254 mm resulting in a magnification between the source and the Gregorian focus of 0.5.

Fig. 5. LBTI test setup for interference testing. The telescope simulator breadboard is underneath the LBTI structure at thebottom center of the picture.

4. INTERFERENCE TESTINGInitial testing with the telescope simulator utilized a Helium-Neon laser as a source and a video camera to record thecombined image. This was useful for initial alignment of the TS and UBC, and allowed easy monitoring of fringevariation and image quality during alignment. The setup used was, as shown in Figure 5, with the UBC assembled on itsside, and the TS hanging underneath the UBC. The UBC was mounted on isolators to minimize effects of vibrationsfrom the lab. With the beamcombiner closed, the fringes were stable, when viewed on the video camera. Roughly, thiscorresponded to pathlength stability of < 100 nm. Such stability is near the final goal of 30 nm for the UBC. This setupwas used to measure vibrations introduced by vacuum pumping (which was significant) and cryogrenic operation.During croygen fills, the fringes were noticeably disturbed, but during normal boil-off no excess fringe vibration wasnoticed.

Since these initial tests we have procure a high speed video camera capable of 200 Hz frame acquisition. Analysis offringe location in series of frames has allowed us to carry out quantitative tests similar to the above tests for stabilityduring croygenic operation. Detection of even small levels of vibration, at the level of 10 nm should be detectable withthis new system.

The demonstration of visible light interference was followed up by replacing the optical laser light source with abroadband infrared source. The source used was a small length of 25 micron thick nickel-chromium wire, baffled toonly use ~1 mm length of the wire, creating an effective point source.

The detection of the IR source was carried out using MIRAC4-BLINC. This instruments is a mid-infrared camera andnulling interferometer used on the MMT to develop nulling techniques in preparation for LBTI. The camera is mountedto the bottom portion of the LBTI.

Unlike detection of the laser fringes, the detection of the broadband fringes requires the light from the source to travelnearly equal path-lengths. Using a 10% bandwidth filter at 10 microns, the coherence length is approximately 100



LBTI image of a lab-created broadband point source at 10 microns

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150

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microns. Initial alignment of the TS lead to beam paths which differed by approximately 10 mm. Once this offset wascorrected, we were successful in demonstrating interference. Figure ?? shows the light source, imaged by MIRAC4.The central Airy pattern of the PSF is modulated by three interference peaks, which is the characteristic PSF of the dual-aperture LBT. The minima of the PSF do not extend to zero, since the size of the source used in this demonstration issignificantly non-point-like. The visibility measured is approximately 30%.

Fig. 6. Image at 10 microns wavelength from LBTI lab testing. The source used was a hot nichrome wire, and a 50% filterat 10.6 microns. The visibility of this initial imaging experiment was approximately 30%, due to the size of the source.



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Fig. 7. The LBT Interferometer, assembled upright and ready for shipment to the telescope. The bottom portion of thetelescope interface structure is not attached in this picture, but was separately test fit to check the complete assembly.

5. STATUS AND SCHEDULEThe system level testing of the UBC in spring 2008 has verified that the beam combination portion of LBTI is ready foron-sky testing. The image quality is suitable for the observations, the alignment mechanisms have the required precisionand range, and an end-to-end test of the UBC and our prototype infrared imager was successfully performed.

An initial installation of the LBTI structure is planned for late July 2008. This will verify mechanical interfaces to theLBT structure, allow testing of the vibrational environment of the facility with different telescope subsystems active, andtest the process of installation for the LBTI and the science cameras, including MIRAC4-BLINC initially, followed byNIC as it becomes available.

Following the summer test fit, the LBTI will be reassembled in Tucson and integrated with the WLBTI units starting infall 2008. The schedule of future telescope testing, including on-sky testing will be driven by the schedule of theadaptive secondary mirrors, currently being completed at Arcetri Observatory.

It is possible that on-sky tests could be carried out using the fixed secondary mirror in one arm and the first adaptivesecondary in the other. Observations carried out with this setup would need to focus on brighter objects for whichspeckle interferometry techniques could be used. However, this setup would allow early testing of the full end-to-endoptical train of LBTI. Such observations, in addition to being scientifically interesting, could result information relatedto the power spectrum of atmospherically-induced phase variations, as well as any phase variations introduced byvibrations from the telescope.



REFERENCES

[1] Hinz, P.M., Connors, T., McMahon, T. Cheng, A., Peng, C.Y., Hoffmann, W., McCarthy, D.J.,Angel, R., "Large Binocular Telescope Interferometer: the universal beam combiner" Proc.SPIE 5491, 787 (2004).

[2] Hinz, P.M., Solheid, E., Durney, O., Hoffmann, W.F., “NIC: The Nulling and Imaging Camerafor the LBT” Proc. SPIE 7013, in press (2008).

[3] Esposito, Simone; Tozzi, Andrea; Puglisi, Alfio; Fini, Luca; Stefanini, Paolo; Salinari, Piero;Gallieni, Daniele; Storm, Jesper “Development of the first- light AO system for the largebinocular telescope” Proc. SPIE 5169, 149 (2003).

[4] Esposito, Simone; Tozzi, Andrea; Puglisi, Alfio T.; Pinna, Enrico; Stefanini, Paolo;Giorgetti, Gabriele; Camiciottoli, Fabrizio; Salinari, Piero; Bianchi, Paolo; Storm, Jesper“ Integration and test of the first light AO system for LBT” Proc. SPIE 5490, 228 (2004).



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