Challenges for the next generation stellar interferometer. Markus Schöller European Southern Observatory January 29, 2009

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1 Challenges for the next generation stellar interferometer Markus Schöller European Southern Observatory January 29, 2009

2 VLTI Four 8.2m telescopes (UTs) All equipped with AO (MACAO) Six Baselines 47m-130m Four 1.8m telescopes (ATs) Movable to 30 stations Baselines 8m-202m Six delay lines PRIMA dual feed facility IRIS lab tip/tilt tracker FINITO fringe tracker MIDI/AMBER/VINCI

3 The mountain top

4 Put the light in the one place at the one time. J. Spyromilio

5 VLTI Scheme

6 Elements of an interferometer Telescopes (including adaptive optics) Delay Lines Beam Combiner Optical trains Tip/tilt (or angle) tracker Fringe tracker

7 The VLTI Telescopes

8 Main challenge - vibrations VLTI 1.8m telescopes are virtually free of vibrations VLT 8m, Keck 10m, and LBT 8m suffer from severe vibrations, on the order of 250 to 900 nm rms on the VLT, much higher according to word of mouth on the others. As a consequence, control loops have to run faster, science integration times have to be decreased, sensitivity is poor. Vibrations are the main reason that sensitivity limits are not according to expectations, except for 10µm. Vibrations on the large telescopes are very likely a result of tradeoffs during telescope design rather than a natural limit.

9 Vibration suppression By design remove as much as possible the sources: On the VLT these are mainly close cycle coolers for the MIR/NIR instruments on the individual telescopes. Use accelerometers on the critical optical surfaces. Use laser metrology to track fast OPD changes.

10 The need for adaptive optics Operating telescopes with diameters beyond the size of a turbulence cell, one needs adaptive optics to optimize the number of photons available for interference. Even for 10µm observations with an 8m telescope, AO is increasing the sensitivity limit.

11 Adaptive Optics Constraints on VLTI Visual magnitude 1<m V <17 Seeing < 1.5 τ 0 > 1.5 ms Airmass < 2 Natural guide star up to 57.5 from science target ATs: -1.7 <m V <13.5

12 Delay lines

13 Optical Delay Requirements Maximum Delay necessary to be controlled: ±B * sin(60) = 866m for a 1km baseline Assuming observations down to airmass=2 and design that balances static OPD Maximum Delay speed: B * 15 / s = B * 75 x 10-6 / s = 75 mm/s for a 1km baseline With an accuracy much better than 100nm.

14 Fiber Delay Lines OHANA - Optical Hawaiian Array for Nanoradion Astronomy also: Family OHANA uses optical fibers to combine the large telescopes on Mauna Key and has obtained fringes between the two Kecks, with the CFHT and Gemini next in line Challenges: Dispersion and birefringence in the fibers Need for delay lines outside of the fibers

15 Beam combiner

16 Coaxial beam combination

17 VINCI fringes

18 Multiaxial beam combination

19 Simulated AMBER PSF

20 Beam arrangement and MTF

21 Beam Combination If combining all possible combinations of telescopes in an array with N elements, one has to split the photons of each telescope into N-1 subsets, diminishing the number of photons per combination pair by a factor much smaller than 1/(N-1) due to losses in the optics needed for these combinations. The actual realization of a beam combiner will probably not use the pairwise combination, but the signals will be of the same order. Pairs: N*(N-1)/2 One way out of this is heterodyne detections, which does work at 10µm, but not at shorter wavelengths.

22 VLTI Science Instrumentation Bands # telescopes spectral resolution limiting magnitude (UTs/ATs/ UTs+FINITO/ ATs+FINITO) AMBER J,H,K 3 35, 1500, 12,000 7,4,1.5/ 5.1,1.6,-/ 7,7,6/ 5,5,5 MIDI N 2 30, (1Jy), 2.8/ 0.7,0.3

23 Fringe Tracking Constraints on VLTI Seeing < 1.2 τ 0 > 2.5ms Airmass < 1.5 Correlated magnitude H < 7 (8m) or H<3 (1.8m) Visibility > 10% Unlike the science beam combiner, for fringe tracking only N-1 pairs have to be combined. Yet, in the studies for the 4 telescope fringe tracker of Gravity, very likely an all-on-all scheme will be implemented.

24 AMBER MRK

25 Large Baselines require Large Telescopes When observing resolved objects, the amount of light from a certain element of the object is determined only by its black body temperature, unless one observes NLTE phenomena. Thus, at a certain baseline all objects up to the sensitivity limit and a given spectral type are resolved out. So either one observes hotter objects, or NLTE objects, or has to increase the sensitivity of the interferometer, ie the size of the telescope.

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27 Some acronyms OPL - optical path length OPD - optical path difference ZOPD - zero optical path difference UT - Unit Telescope (8.2m) AT - Auxiliary Telescope (1.8m) MACAO - Multi Application Curvature Adaptive Optics STRAP - System for Tip-tilt Removal with Avalanche Photo diodes VINCI - VLT INterferometer Commissioning Instrument AMBER - Astronomical Multiple BEam Recombiner MIDI - MID Infrared interferometric instrument FINITO - Fringe sensing Instrument NIce TOrino IRIS - InfraRed Image Sensor ISS - Interferometer Supervisor Software VCM - Variable Curvature Mirror