LINC. Primary Mirrors. Secondary Tertiary. Elevation Structure. Azimuth Platform. Scale Comparison

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1 LINC: A Near Infrared Beam Combiner for the Large Binocular Telescope a T. M. Herbst, a,b H.-W. Rix, a P. Bizenberger, a and Marc Ollivier. a Max-Planck-Institut fur Astronomie, Konigstuhl 17, D Heidelberg, Germany bsteward Observatory, 933 N. Cherry Ave., Tucson, AZ, USA ABSTRACT The Large Binocular Telescope (LBT), currently under construction on Mount Graham in Arizona, will be the world's largest single telescope when it is completed in With its dual, 8.4 meter diameter primary mirrors and a maximum baseline of 23 meters, LBT will provide astronomers with an unprecedented combination of large collecting area, wide eld of view, and high spatial resolution. The common mount ensures constant entrance pupil geometry and therefore allows image-plane or \Fizeau" interferometry. Our simulations suggest that we will be able to achieve true imagery with 10 mas resolution over a eld of several tens of arcseconds in diameter with excellent sensitivity (for example, S/N of 10 on a 20 njy point source in 3 hours at K 0 ). Such performance enables a variety of fundamental, new science programs. For example, we anticipate pushing supernova cosmology studies to beyond redshift 3 and detecting Jupiter-mass planets around stars within 100 pc due to their reex, astrometric wobble. Building this instrument represents a signicant challenge, however. One of the primary diculties lies in adapting conventional optical design software to the dierent constraints of interferometry. In this paper, we briey review the status of interferometry on the Large Binocular Telescope and introduce LINC, a near-infrared beam combiner we have proposed for the LBT. Section 3 presents some key science programs, and we conclude with a discussion of how we are approaching LINC's optical design. Keywords: interferometry, Fizeau, infrared, imaging 1. INTERFEROMETRY WITH THE LARGE BINOCULAR TELESCOPE John Hill and Piero Salinari 1 give a complete review of the LBT project, and the current status is updated regularly at Here, we concentrate on those aspects of the telescope that drive the design of LINC. Figure 1 shows a three-dimensional rendering of the LBT based on the latest engineering model (see also Figure 4 in Hill and Salinari 1 ). The sti, open telescope structure holds the two, 8.4 meter diameter primary mirrors sideby-side with a 14.4 m center to center separation. This gives a maximum baseline of almost 23 m in a compact array, resulting in fairly complete (u; v) coverage. This is important for reconstructing accurate images from a small number of exposures. Light from the primary mirrors comes to a real focus, then is re-imaged by the concave (Gregorian), adaptive secondaries. These mirrors consist of a thin glass membrane driven by approximately 1000 actuators each. With sampling corresponding to approximately 20 cm at the primary mirror, the secondaries will provide full, adaptive correction of atmospheric distortion and optical path dierences. Under these conditions, the LBT will produce coherent, phased, planar wavefronts from the two individual telescopes. The interferometric instruments share the platform between the primary mirrors and are fed by the rotating tertiaries, allowing rapid switching between beam-combining and conventional instruments. In the remainder of this paper, we describe the design of a near infrared beam combiner, which will occupy one of the oset, shared focal positions (see Figure 1). Send correspondence to herbst@mpia-hd.mpg.de 1

2 LINC Primary Mirrors Secondary Tertiary Elevation Structure Azimuth Platform Scale Comparison Figure 1. The Large Binocular Telescope. In this rendering, the telescope is tipped over to observe an object at approximately 1.4 airmasses. Figure 4 in Hill and Salinari 1 shows the LBT pointing at the zenith. Note that the secondary mirror structures hold two sets of optics, the adaptive membrane secondaries and 45 mirrors for launching the laser guide stars. LINC is the light gray bent cylinder at the top of the platform between the primary mirrors. A person standing near the base gives the scale. Telescope data courtesy European Industrial Engineering. 2

3 Zenith Airmass = 2 66 mas Figure 2. The K-band ( m) point spread function of the LBT at zenith (left) and at two airmasses (right). At non-zenith angles, atmospheric dispersion stretches the PSF perpendicular to the horizon and produces a stellar image that is wider at the bottom. Some sort of atmospheric dispersion compensation will likely be necessary for wavelengths shortward of 1.5 m. 2. THE LINC BEAM COMBINER As part of their in-kind contribution to the Large Binocular Telescope project, the MPIA in Heidelberg and its collaborators have proposed to build the LBT INterferometric Camera (LINC), a near-infrared beam combiner. In its rst incarnation, LINC will operate in the wavelength range 1.0 to 2.4 m. At the long wavelength end, this window is conned by atmospheric transparency, while at shorter wavelengths, the task of correcting atmospheric distortions over a signicant eld of view becomes increasingly dicult. These wavelengths are also well suited to the new generation of very large format infrared detector arrays. The shared, alt-azimuth mount maintains constant entrance pupil geometry for all pointing directions. A beam combiner which preserves this geometry allows Fizeau-type interferometry, in which the input beams interfere at the nal image plane. Fizeau interferometry has an enormous advantage over traditional amplitude (pupil plane or Michelson) interferometry in terms of eld of view. In the case of the LBT, the region of sky over which coherent combination occurs is limited only by the performance of the adaptive optics system. With LINC, we expect to achieve elds of view of one arcminute or more, several thousand times larger than the 1" eld available with Michelson type interferometers such as VLTI and Keck. A Fizeau interferometer is a true imager with a point spread function (PSF) given by the Fourier transform of the pupil. For a conguration with two 8.4 m mirrors on a 14.4 m center-to-center baseline, the PSF resembles a classic Airy disk crossed by Young's fringes (Figure 2). The LBT's maximum spatial resolution corresponds to the frequency of these fringes and is 10 mas at 1.0 m and 20 mas at 2.0 m. The alt-azimuth mount enables \earth rotation synthesis" to ll the (u; v) plane in the same manner as for radio interferometers, and the relatively compact array means that we can produce high delity imagery with a small number of parallactic angles. 3 For observations limited by background noise statistics, a reduction in the size of the PSF corresponds directly to an increase in sensitivity. Table 1 summarizes LINC's expected photometric performance. Given the nature of the image reconstruction process, we cite separate sensitivities for detecting point sources using single baselines and for producing fully reconstructed images. Detecting diuse emission has traditionally been a weak point of imaging interferometers, since the time to achieve a target signal to noise ratio increases with the baseline/aperture ratio to the fourth power. 2 In this respect, LINC enjoys a signicant advantage over other, sparser interferometric arrays such as VLTI and Keck. Experience has also shown that as spatial resolution improves, many \diuse" objects break up into the point sources for which interferometers are ideally suited. 3. SAMPLE SCIENCE PROGRAM The performance outlined in the previous section will enable breakthrough science in a number of elds. In this section, we describe a couple of key programs in which we plan to invest signicant observing time in the early years 3

4 Table 1. Anticipated photometric performance of LINC. All sources detected with S/N=10 in three hours at 2.2 m (K'), assuming a sky background of 13.8 mag/square arcsec, a single-aperture Strehl ratio of 0.8, and an overall system eciency of 0.5. Observation Limiting mag Point Source Detection 26.3 Point Source Reconstructed 25.2 Extended Source Reconstructed 19.6 a a per square arcsecond with 20 mas spatial resolution. of LINC operations. Perhaps the most remarkable thing about these programs is that they represent conventional astronomy pushed to the next level by improved capability. It is a central goal of the LINC project to enable and improve the kinds of science astronomers wish to do. LINC will not be an interferometer in search of a science program Supernova Cosmology Light curves of high redshift type Ia supernovae (SN Ia) provide a powerful probe of the cosmological parameters, mass and. The technique depends on the observation that SN Ia have intrinsic luminosities that can be predicted from their light-curves. Therefore, measurement of both the light curve and spectroscopic redshift can give the luminosity distance to a given z, a mapping which depends on mass and. The recent success of ongoing SN Ia searches, suggests that the luminosity distance to z 1.2 will soon be mapped to 5% accuracy, limited ultimately by systematics. 4 These observations can constrain a combination of mass and, but not their individual values. However, observations of SN Ia at higher redshift can break the cosmological parameter degeneracy. Such supernovae (e.g. 1992A) have a peak ux of 400 njy at z 2:5. Hence, their brightness can be measured with LINC to 10% accuracy within 2 hours, even three magnitudes below peak. Spectroscopic redshift determination will be feasible for these objects close to peak brightness with other instruments mounted on the telescope. An LBT key program could survey twenty adjacent circumpolar elds (each with 30 minute exposure time), probing 10 5 Mpc 3 z (for z 2), and detecting any SNIa at z < 3. Since time dilation slows the light curves, revisiting these areas every month over 2 years will provide light curves for about 20 SNIa's between z = 1 and 3, with a total expenditure of 25 nights of observing. These measurements will permit a determination of the total mass content of the universe, mass, to 5% accuracy Extrasolar Planets Current search strategies for extrasolar planets concentrate either on the Doppler shifts in stellar spectral features arising from the reex motion due to unseen planets, or on photometric changes arising from planetary transits. Both these techniques are biased toward detecting more massive (Jupiter-like) objects close to the host star; the majority of the 30 or so \hot Jupiters" discovered to date have semi-major axes below 0.5 AU, forcing a re-evaluation of planetary formation theories. The LINC interferometric beam combiner oers the possibility of pushing these searches into the regime of \real" Jupiters, namely 1 MJup objects orbiting at 5 AU from a solar-type star. The technique is a very old one: measuring the astrometric wobble imposed on the parent star by the gravitational tug of the planet. It is a pure coincidence that the barycenter of the Sun-Jupiter system lies almost exactly at the solar surface. and the total excursion of the sun over a 12 year period due to Jupiter is 1.5 million kilometers, or 0.01 AU. On bright sources such as nearby stars, it should be possible to achieve 0.1 mas astrometric precision with LINC, sucient to detect the reex motion of Jupiter on the Sun out to a distance of 100 pc. LINC's wide eld of view increases the likelihood of multiple reference stars, improving the precision of relative astrometry. Having several references also allows an unambiguous identication of the star hosting the planet { it will be the one showing periodic reex motion with respect to multiple neighbouring stars. The preceding paragraphs focused on planets that orbit normal stars. However, formation theories predict that there may be large numbers of isolated planets either created alone or expelled from binary and multiple systems. 4

5 Refractive Concept Reflective Concept Collimator Cold Pupil Cryostat Window Camera Detector Array Derotator to follow sky Figure 3. Refractive (left) and reective (right) design concepts for LINC. Searches for free-oating planets are ideally suited to the greater sensitivity and wide eld of view of the LINC beam combiner. 4. LINC CONCEPTUAL DESIGN Obtaining high angular resolution over a wide eld of view imposes certain constraints on the optical layout of the telescope and beam combiner. Specically, the telescope and instrument must obey the \sine condition," which requires that the entrance and exit pupils of the interferometer be geometrically similar or homothetic (i.e. one is a scaled version of the other 2 ). In the case of the LBT, the geometry of the primary mirrors denes the entrance pupil of the telescope / interferometer. The optics within the beam combiner must produce a scaled-down version of this entrance pupil then re-focus the combined beams onto a detector array. Figure 3 shows both refractive and reective design concepts for LINC. Building on several earlier eorts, 5{8 we are currently rening and optimizing both types of designs with the aim of selecting a nal conguration in early summer Here, we describe the refractive concept only, since the mirror-based beam combiner operates in a similar manner. Light from the f/15 adaptive secondaries is reected by the 45 folding tertiaries to the platform area between the primary mirrors. There are three shared foci on this platform. We plan to use one of the oset locations, since the central one will likely hold the nulling interferometer, 2 an instrument far more sensitive to asymmetric polarization eects. The beam-combining optics and detector will be housed in a large vacuum cryostat (see Figure 1). This layout has minimal thermal background for infrared operations, since there are only three warm reections and one window prior to the cold optics. The layout of the beam combiner itself is a \classic" collimator{camera design. A cold eld-limiting aperture lies in the focal plane of the telescope some distance inside the dewar window. The individual telescope beams diverge from the focal planes and are subsequently collimated by a pair of lenses deeper within the cryostat. These lenses form an image of the telescope entrance pupil on a cold (Lyot) stop for suppression of radiation outside the f/15 primary beams. The folding mirrors direct the radiation downward, parallel to the optical axes of the telescopes, and a single camera then combines and focuses the interfering wavefronts onto the detector array. The result is a wide-eld image whose point spread function resembles that shown in gure 2. Executing this concept in a real interferometric optical system requires specialized design steps, which are described in the following paragraphs. 5

6 4.1. Interferometric Optical Design Any optical design process has three fundamental stages. The rst is to establish an overall layout which satises the basic requirements of the instrument in terms of image scale, pupil position, eld of view, etc. The second stage involves optimization of the design for maximum performance, usually within the constraints imposed by available materials, test plate radii, and so forth. The nal, critical step is to \tolerance" the design by introducing and measuring the eects of various perturbations such as tilts of optical elements, changes of radii, and component spacing. This last step determines whether a particular design is buildable, and may often force a return to the rst and second stages. Proper tolerancing is particularly important for interferometric instruments, where shifts of a fraction of a wavelength can mean the dierence between an instrument that works and one that does not. At the time of this writing, we are at the rst stage, developing optical layouts for LINC based on both refractive and reective optics. Making proper design decisions depends critically on the ultimate output of the instrument, namely wide-eld, interferometric images, and traditional criteria for judging optical systems do not necessarily apply. For example, we can tolerate eld distortion, as long as it is identical in the two arms of the interferometer, thereby allowing the astronomical images to overlap and interfere. Pure optical path dierence (piston), on the other hand, does not inuence traditional cameras, but is fatal to an interferometer. Clearly, we need tools to assist the design process based on real, interferometric requirements. A number of commercial software packages are available which allow the astronomical instrument builder to complete the design, optimization, and tolerancing stages. Unfortunately, these packages are not well suited to interferometric instruments in general, and to image-plane interferometers in particular. A fundamental shortcoming has been their inability to ray-trace and interfere the wavefronts of multiple, parallel optical axes. This has led to a somewhat ad-hoc approach to interferometric optical design in the past. For example, one common strategy was to treat each arm of the interferometer as a single, separate instrument, and to design to a semi-arbitrary performance target (for example, a certain Strehl ratio). Writing a custom, multi-axial optical design program does not represent an attractive alternative, since it means giving up (or reproducing) the powerful editing, optimization, and tolerancing capabilities of commercial programs, (to say nothing of integrating lens, glass, and test plate catalogs). We have adapted one such commercial program, Zemax, 9 to do true interferometric, multi-axial optical design. Our strategy is based on the fact that the output point spread function of any imaging system, including a Fizeau interferometer, is the Fourier transform of the wavefront at the exit pupil. Zemax allows externally compiled C-language modules to be linked into the infrastructure of the program. Our module treats each arm of the interferometer as a separate conguration of a single optical design. Multiple congurations are typically used in the design of photographic zoom lenses. Zemax allows any or all optical parameters to vary between congurations, including the location of the optical axis (see Figure 4). The program will also trace any number of rays specied by the external module and return the resulting position, angles, and optical path dierence (OPD { in waves) for each ray in the focal plane. Given these capabilities, generating an interferometric PSF within the Zemax environment is straightforward. The external module instructs the main program to switch to conguration 1 and trace a series of rays which uniformly ll the entrance pupil. The OPD's returned by Zemax give the wavefront shape in the exit pupil of the rst arm of the interferometer. The module then repeats this procedure for conguration 2, which shares some components with conguration 1 but has a separate optical axis. The module then inserts the wavefront information returned by Zemax for each arm of the interferometer into a large array with the correct pupil separation and orientation. The interferometric PSF is the Fourier transform of this array. Not only can we display the resulting point spread function (see gure 4), but also we can return a gure of merit based on the PSF to the main program. This means that all three stages of the optical design process, layout, optimization, and tolerancing, can be carried out based on true interferometric performance. A couple of aspects of these calculations deserve elaboration. First, determining a reasonable gure of merit based on the type of PSF shown in Figure 2 requires some care. For example, we can tolerate some shift in the location of the central fringe with respect to the circular Airy disk for diering eld positions, but we do not want to lose any spatial resolution or fringe contrast. An interferometric equivalent of the Strehl ratio would penalize the former situation more severely than the latter. Ultimately, the true merit of a particular PSF depends on our ability to reconstruct accurate, high signal-to-noise ratio images, and we are now using simulations to determine the ideal gure of merit. The second aspect of these calculations concerns the sampling of the exit pupil. We insert a uniform grid of rays in the entrance pupil, and there is no guarantee that these rays emerge with uniform sampling of the exit 6

7 Figure 4. Sample screen output of the Zemax optical design program showing each arm of the beam combiner as a separate conguration, and the resulting interferometric point spread function. 7

8 pupil. In fact, distortion of the pupil imagery is a primary concern. The ray-trace data returned by Zemax includes the direction cosines of the rays in the focal plane, however. These correspond directly to the ultimate locations of the rays in the exit pupil, allowing us to properly resample the data to reect the true output wavefront. 5. CONCLUSIONS We have proposed to design and build a near-infrared interferometric beam combiner for the Large Binocular Telescope. This instrument, named LINC, will exploit LBT's unique potential to achieve very high spatial resolution to faint ux limits over a wide eld of view. LINC extends our current capabilities into new regimes, and will enable breakthrough science in a variety of elds. Interferometric optical design presents a unique set of requirements and challenges, however, and we are now adapting commercial software packages to allow us to layout, optimize, and tolerance the LINC design. ACKNOWLEDGMENTS T. M. H. extends thanks to Steward Observatory and the LBT Project Oce, which hosted him for a sabbatical leave during which this paper was written. REFERENCES 1. J. M. Hill and P. Salinari, \The Large Binocular Telescope Project," Paper , this conference, J. R. P. Angel, J. M. Hill, P. A. Strittmatter, P. Salinari, and G. Weigelt, \Interferometry with the Large Binocular Telescope," SPIE 3350, E. K. Hege, J. R. P. Angel, M. Cheselka, and M. Lloyd-Hart, \Simulation of Aperture Synthesis with the Large Binocular Telescope," SPIE 2566B, p. 144, Garnavich, P. M., et al., \Constraints on Cosmological Models from Hubble Space Telescope Observations of High-z Supernovae," ApJ, 493, 53, P. Byard and D. Bonaccini, \Optical Design for Interferometry with the Large Binocular Telescope," SPIE 2200, p. 446, P. Salinari, \The Large Binocular Telescope Interferometer," SPIE 2871, p. 564, J. M. Hill, \Strategy for Interferometry with the Large Binocular Telescope," SPIE 2200, p. 248, McCarthy, D. W. Jr., Hege, E. K., Freeman, J. D., Blanco, D. R., Sjogren, J. C., Janes, C. C., Montgomery, J. W., and Shaklan, S. B., \Interferometry with the Columbus Telescope: Design Considerations Based on MMT Experience and Imaging Simulations," in Very Large Telescopes and Their Instrumentation, Ulrich, M.-H. (ed.), p. 787, Zemax Optical Design Program, version 8.1, Focus Software Inc