OIR Interferometry. Outline. OIR Interferometry. Astro 6525 Fall 2015 November Michelson s Interferometer

Transcription

1 Outline OIR Interferometry OIR Interferometry Michelson s Interferometer OIR Interferometers Aperture Masking Astro 6525 Fall 215 November Michelson s Interferometer 1921ApJ M 3 Image Plane / Pupil Plane (Fizeau/Michelson) 5

2 LBTI Interferometry Exozodis are difficult to detect Beam Combiner Fringe Spitzer/IRS% %Beichman%et%al.%26% he dust emission is faint and close to e star. nresolved photometry is the most Delay Line mmon method. en with infinite photometric curacy, limited to ~1% best case lative photometry by ability to predict e stellar MIR flux (~3 zodi, 1 ). Interferometry helps Status 2σ%stellar%model% uncertainty% Nulling Nulling'Measurement 1#μm#Fizeau#fringes.# patially separate the signal from the star nd the surrounding disk. main methods: NIR high-accuracyopen&loop#(seeing#limited)#nulling#(sep#212).# visibility (CHARA/FLOUR, VLTI/PIONIER). MIR Nulling (MMT/BLINC, KIN, LBTI). ee of modeling assumptions on the stellar ectrum. Best'6/1 Construc/ve'Images Best'6/1 Destruc/ve'Images /p1/lt'residuals'appear'to'dominate'nulled'images. No'a5empt'was'made'to'op/mize'null,'during'this'inital' a5empt.. Phase shift causes destructive interference on-axis 3" %by:%ciardi,%absil,%di%folco,%defrere,%%smith,%akeson,%liu,%stock,%stark,%millanmgabet,%mennesson%etc% %% 11" Layout Layout of of the the CHARA CHARA Array Array Telescopes Telescopes I.I. ~6 ft CAD by Laszlo Sturmann Photo by Steve Golden

3 Vacuum Vacuum Turning Turning Boxes Boxes Make Make66Lines LinesParallel Parallelwhile whilepreserving PreservingPolarizations Polarizations 46 TEN BRUMMELAAR ET AL. Vol. 628 Fig. 7. Delay line and beam management area. The PoPs, periscopes, OPLEs, BRTs, LDCs, and BSS system are clearly visible. Photograph by Steve Golden. A Simple -Baseline Interferometer Simple LongLong A Simple Long-Baseline Interferometer Delay DelayCompensator CompensatorMust MustMatch MatchPaths Pathsto towithin Within11micron micron Starligh t Starligh t to Sta r D e la y B s in Te l. #2 Te l. #1 B ase lin e B De la y Co mpe n s a tor F ring e P osition 1994). In this way, each system can be optimized for one task or another and development of the various beam combiners can go on in parallel. We are exploring several methods of fringe tracking, including packet tracking where one keeps the fringe packet centered in a long scan, group delay tracking where one keeps the center of the group delay in position, and phase locking where one tracks on a single fringe. It is expected that all three methods will be used under different conditions and for different science targets. For example, it will be possible to phase lock using the IR system and send a phased beam into the visible beam combiner. The reverse will, of course, also be possible. It will even be possible to divide the six telescopes between several beam combiners and thereby create separate interferoff-axis Paraboloid ometers working in parallel. Two high-speed CCDs have been purchased from Astronomical Research Cameras of San Diego for use in the visible beam combination area, one for the I- and R-band beam combiner and one for tip/tilt detection. We have also acquired a fiber coupling stage that breaks the full aperture into seven smaller apertures and couples each of these to a multimode fiber. The central beam includes the telescope secondary obscuration and Dewar is therefore used primarily forcamera alignment. This allows us to have (PICNIC detector at LN2 temp of 77K) seven small (3 cm on the sky) aperture systems feeding a fiberbased slit in the low-resolution spectrograph and process all seven systems in parallel (Ogden et al. 23). The construction of this visible light system is in progress, and we expect to have achieved first fringe on the sky by late 24. A higher spectral resolution spectrograph is also under construction and will use the same subapertures and fibers ( Hillwig et al. 22). This spectrograph is intended for combined high spatial and high spectral resolution measurements. The spectrograph is a modified Ebert-Newtonian based on the design used dielectric layer Folding Mirrors for the Georgia State Multi-Telescope Telescope (Barry et al. 1994). It is fiber fed and will produce coherent, spatially resolved spectra. The use of this spectrograph on the sky is delayed until phase locking is possible at the CHARA Array so that phased beams can be sent into the system. This should be possible 16 in late 25. The second camera is being tested as a tip/tilt detection upgrade. In lieu of a full-up visible light beam combiner, the visible channel short of 6 nm is presently used entirely for photomultiplier-based tip/tilt detection for which the high bandwidth limiting magnitude is V ¼ þ9:5 at 1 ms sample times, although in good seeing conditions that permit the use of longer integration times, we can track objects as faint as V ¼ þ12:. This tip/tilt detection scheme and correction algorithm are largely based on the system developed at the Sydney University Stellar Interferometer (SUSI; ten Brummelaar & Tango 1994). Light from 6 nm to 1 m is divided between an intensified CCD, allowing for image quality inspection and alignment, and the low-resolution spectrograph used for I- and R-band interferometry. The boresight laser and white-light sources are regularly used for the essential tasks of alignment and overall system finetuning. The ICCD also supplies us with a reference position defining the optical axis of the interferometer, and we regularly align the tip/tilt detectors to ensure that star images from each telescope lie on this optical axis. Folding Mirrors The layout of the current IR beam combiner, dubbed CHARA Classic, is that of a simple pupil-plane beam combiner (Sturmann et al. 23a). The two outputs from the beam splitter are separately imaged onto two spots on the beam combiner camera (described below), with fringes detected in a scanning mode provided by dithering a mirror mounted to a piezoelectric translation stage. The stage is driven with a symmetric sawtooth signal whose response to a given driving signal was mapped with a laser interferometer to provide a 1 khz data acquisition rate corresponding to five samples per fringe at a typical 2 Hz fringe frequency. Sample rates of 25, 5, and 75 Hz are also possible. The current K limiting magnitude for fringe detection with Dispersion Compensator Beam from Telescope 2 this system is K ¼ þ6:5 for raw visibilities of.5. CHARA CHARA Classic Beam Classic Classic Beam Combiner Combiner Folding Mirrors Dither Mirror Beam Splitter Beam from Telescope 1

4 Measuring s Diameter Star Measuring aa Star Star s Diameter Then, Then,Bandpass BandpassFilter Filterthe thepower PowerSpectrum Spectrumand andinvert Invert The visibility arising from the angular diameter of each component is: V(b) = 2[J1(.6.4 Semi-amplitude is the fringe visibility M k g = 2.13 m b/ ) k Before we do such fits, we must derive actual visibilities from what we measure with an interferometer. This involves a straightforward calibration 8 process using other stars. Visibility.2 b/ )]/( where is the angular diameter (in radians), 1 b is the baseline, and is the effective wavelength of the.8 observed spectral pass band. J1 is the first order Bessel function. = 1.6 mas.4 = 5. mas Baseline Table 1 Days of observation of Algol with CHARA/MIRC Imaging Stars and Planets with the CHARA Array MIRC: Michigan Infrared Combiner UT Date Telescope configuration Calibrators 26 Oct 9 26 Oct Oct Oct 4 27 Nov Aug Aug Aug 2 28 Aug Aug 1 29 Aug Aug Aug Aug Aug Aug 2 29 Aug Aug Aug 6 21 Aug 8 S1-W1-W2 S2-E1-W1-W2 S2-E1-W1-W2 37 And, 37 And 37 And,, 37 And, 1 Aur, 1 Aur UV sampling Basic Capabilities: 1) Designed for imaging -- currently combines 4 telescopes at once 2) micron wavelength coverage (in this talk, all results are H band, 1.65 microns) 3) Spectral modes: R~4,15,4 Figure 1. Typical uv coverage for one of our Algol split data set when using the telescope configuration (left) and (right). (Monnier et al. 24) Sagan Symposium 29 Nov Table 2 Calibrator sizes and 1σ Error in Milli-arcseconds. 22 Calibrator Uniform Disk Size (mas) Error (mas) Reference 1 Aur 37 And b a,b a, b, c b References. (a) Kervella et al. (28) ; (b) Barnes et al. (1978) ; (c) Bonneau et al. (26). Imaging Stars and Planets with the CHARA Array First image of a main-sequence star (besides the Sun ) Altair (α Aql, V=.7) Nearby hot star (d=5.1pc, A7V, T=785 K) Rapidly rotating (v sin i = 24 km/s, ~9% breakup) Monnier et al. 27 Sagan Symposium 29 Nov 12-13

5 Dual Star Interferometry Interferometry Wavefront Wavefront Interferometer With Fringe Tracker Interferometer Phase (Position) Amplitude (Visibility) Fringe Fringe Dual Star Interferometer Gravity Beam Combiner 6.5 Beam Combiner The two beam combiners for the reference star and the science object are implemented in integrated optics. The beamcombiner chips are directly fed by the single mode fibers, and provide instantaneous pair-wise combination of all six baselines for the four telescopes. Internal phase-shifter and splitter provide the instantaneous ABCD sampling of each interferogram. As such the integrated optics beam combiner provide 6 (baselines) 4 (samples) = 24 outputs for the four inputs (figure 16, [37]). Figure 16: Top: Beam combination principle for the 4-telescope, 6-baseline integrated optics beam combiner. Bottom: Prototype beam combiner, available in the lab. 28 Closure Phase Aperture Masking 9 hole mask 36 baselines 84 closure triangles (28 independant) φ(2 1) = φ(2 1) φ(1 3) = φ(1 3) φ(3 2) = φ(3 2) 29 φ2 φ1 φ1 φ3 φ3 φ2 measured phase = intrinsic instrumental Integrated Optics (Photonics) Quadrature interference (spatial multiplexing vs temporal multiplexing)

6 Sensitivity vs Conventional AO But, But, But All those precious photons 32 But, But, But All those precious photons Rayleigh Criterion δθ=1.22 λ/d Michelson Criterion δθ=.5 λ/d JWST AMI PSF/Power Spectrum Redundancy 36

7 Figure 3 from LkCa 15: A Young Exoplanet Caught at Formation? Adam L. Kraus and Michael J. Ireland 212 ApJ doi:1.188/4-637x/745/1/5 37 Composite Hα, Ks, and L image. S Sallum et al. Nature 527, (215) doi:1.138/nature15761