Adaptive Optics

In theory, the resolving power of a telescope, i.e., its capability to produce separate images of two objects ly-ing close together, increases with the diameter of the primary mirror. In practice, however, atmospheric turbu-lences blur long-exposure images to such a degree that the resolution is one half to one arc second at its best, regardless of the mirror size. Astronomers and en-gineers at MPIA, together with colleagues from the MPI für extraterrestrische Physik (MPE), have built a so-cal-led adaptive optics system for the near-infrared spectral range (ALFA) for the Calar Alto Observatory that corrects image fluctuations during the exposures (cf. Annual Report 2000, p. 31). In this way, the theoretically possible resolution, i.e. the diffraction limit, can be achieved.

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IV Instrumental Development

Fig. IV.1:The prototype of PARSECmounted in the MPE labora-tory. (Image: MPE)

On Calar Alto, it was demonstrated that ALFAcan also be operated using an artificial laser guide star. Such an in-strument consists of a laser pointed parallel to the telesco-pe’s optical axis towards the sky. At an altitude of about 90 kilometers, the laser beam excites atmospheric sodium atoms, which then start glowing. The emission typically extends over a cylindrical volume about one meter in dia-meter and seven kilodia-meters long. The spot of light created in this way serves as a reference star for the adaptive op-tics system. The experience gained with this instrument will now be utilized at the ESO Very Large Telescope (VLT) and at the Large Binocular Telescope (LBT).

The VLT 8 m telescope YEPUNwill be the first to be equipped with a laser-guide-star device. Here, a sodium laser, named PARSEC, will produce a continuous beam of 10 to 15 watts power. This instrument is being developed jointly by researchers at MPIA and MPE (Fig. IV.1).

MPIA is contributing a so-called LIDAR(Light Detecting and Ranging) – a pulsed laser that can be used to measu-re the altitude of the atmospheric sodium layer and the concentration of sodium atoms there.

The LIDAR is based mainly on experience acquired with ALFA. It was shown that a laser guide- star can only be used efficiently during the observations if the atmos-pheric conditions are known in detail. Experiments with a LIDARat the 3.5 m telescope on Calar Alto had turned out to be successful, so it was decided to build a similar devi-ce for PARSEC.

The LIDARdeveloped at MPIA is capable of measuring the altitude of the sodium layer with 150 m resolution and of determining the magnitude of the laser guide star inde-pendently from the adaptive optics. Acceptance by ESO

will be in fall of 2003 in Garching. The first phase of commissioning will be early 2004 on Paranal. In the be-ginning, the LIDAR will be operated together with the NACOinstrument (see below). In 2004, SINFONI, the se-cond focal instrument on VLT-YEPUN, is planned to be put into operation. It is a 3D-spectrograph with an adapti-ve optics systems of its own that will use the laser guide star, too.

For the LBT, a somewhat different instrument is cur-rently being tested – a SCIDAR (Scintillation Detection and Ranging), which helps to optimize the adaptive optics system of the LBT. Adaptive optics systems can only par-tially compensate image distortions due to atmospheric turbulences. This mostly affects objects outside the cen-tral correction axis of the adaptive optics. The strength of this so-called anisoplanicity effect depends mainly on the vertical structure of the atmospheric turbulence. If there are several bright stars in the field of view during an ex-posure with adaptive optics, the strength of this effect can be estimated afterwards to improve photometric and astrometric measurements. But bright stars are not always available and, furthermore, these estimates are highly un-certain. This is where SCIDARwill be put into action.

SCIDARobserves a binary star, producing a defocused image of it (actually it is an image in the pupil plane).

From the intensities of the pupil images of both stars the vertical structure of the atmospheric turbulence can be de-termined up to an altitude of about 20 km. While measu-rements of the phase distortions over the pupil cannot yield information about the vertical structure of the turbu-lence, the strength of the scintillation depends on the di-stance between a turbulence layer and the observational plane. Thus, the brightness distribution over the pupil contains information on the vertical distribution of the turbulence.

The SCIDAR hardware was built at Steward Observatory while MPIA contributed the data analysis software. The instrument was successfully tested on the Vatican Advanced Technology Telescope. In mid-2004, it will be used during First Light on the LBT.

PYRAMIR

PYRAMIRis a new kind of wavefront sensor for the ne-ar infrne-ared spectral range which will be used with ALFA. It is intended to replace the system’s old tip-tilt sensor.

The sensor will be completely reside within a dewar and mounted on the existing second sensor platform in ALFA. The design of a pyramid wavefront sensor had been proposed for the first time in 1996. Its special feature is to create four pupil images which are then used to measure the gradient of the wavefront. This instrument has an im-portant advantage over the customary Shack-Hartmann wavefront sensor in being able to control the amplificati-on according to the observatiamplificati-onal camplificati-onditiamplificati-ons. Optically, this is achieved by a reflecting pyramid that splits the be-am focused onto its tip into four parts. A zoom lens then creates four pupil images on the detector (Fig. IV.2).

PYRAMIRwill be the first sensor of this kind worldwi-de operating in the near-infrared range. Thus, it will be working in a wavelength region where a highly efficient correction is achieved by the adaptive optics system.

Theoretically, pyramid sensors should be able to work with fainter guide stars than comparable Shack-Hartman systems. However, up till now, no sensor exists to prove this in practice. Currently, PYRAMIR is the only project that will test its performance at a telescope. Because of its

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(3) (4)

Fig. IV.2:The pyramid splits the beam focused onto its tip into four. A zoom lens then creates four pupil images on the detec-tor.

Adaptive Optics

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higher sensitivity, PYRAMIR increases the sky coverage for certain classes of objects, such as young stars or high-ly reddened objects in general, in the Galactic plane from 7 percent to nearly 50 percent.

The internal design review of the project, taking place in December 2002, arrived at a positive result, with the re-striction that the project has to be carried out as a techno-logical development. The system is expected to be used in adaptive optics systems on 8 m class telescopes, such as the PLANETFINDERon VLT (see below). PYRAMIRentered the construction phase limited to one year. In spring 2004, it is planned to be put into operation on Calar Alto.

The PLANET-FINDERproject CHEOPS

PLANETFINDERwas announced by ESOas a VLT in-strument of the second generation. An adaptive optics sy-stem of extremely high imaging quality is asked for that will be combined with an instrument achieving particu-larly high contrasts in the immediate vicinity of bright ob-jects. In the end, this instrument is intended for the detec-tion and imaging of extrasolar planets.

MPIA has assembled an international consortium of German, Italian, Swiss, Dutch and Portuguese institutes and in February 2002 submitted to ESOa preliminary pro-posal for such an instrument. It is called CHEOPS

(Characterizing Extrasolar Planets by Opto-infrared Polarimetry and Spectroscopy). It records difference sig-nals between star and planet and uses the different spec-tral properties of the two objects as well as their different degrees of polarization. A similar second proposal was submitted by a consortium led by the French. Thus ESO

has asked both consortia to each perform a complete Phase-A study. The study started in March 2003 and will take 18 months. After that, a further selective review will take place.

Adaptive Optics as a Teaching Subject at the Universität Heidelberg

Because of the growing importance of adaptive optics MPIA, since winter semester 2002, offers a new experi-ment within the scope of the advanced practical course to all students of the Department of Physics and Astronomy at the Universität Heidelberg. During four afternoons, the students can set up a modern analyzer to examine the op-tical quality of light waves and determine opop-tical aberra-tions such as, e.g., astigmatism and coma. The experiment is carried out in the newly established laboratory for ad-aptive optics at MPIA (Fig. IV.3).

(S. Hippler, M. Feldt, P. Bizenberger, W. Brandner, D.

Butler, J. Costa, B. Grimm, Th. Henning, U. Neumann, W. Rix, R.-R. Rohloff, C. Unser)

Multiconjugate Adaptive Optics (MCAO)

Adaptive optics systems always need a reference star of a certain minimum brightness. Furthermore, optimum correction is only possible within a certain angle around this star. Beyond, the image becomes increasingly blur-red. In the future, this limitation will be avoided by using so-called multiconjugate adaptive optics (MCAO).

To make this technique practicable for the first time, a special team was established at MPIA. On a long-term ba-sis, a MCAOis also to provide the LBT with diffraction-limited images in the combined focus – not only in the ne-ar infrne-ared range but also at wavelengths down to 800 nm.

At an observing wavelength of 1 m, the diffraction limit of the LBT is 9 milli-arc seconds. The goal is to get a dif-fraction-limited image over the entire field of view, which has a size of 1 arc minute.

With the classical adaptive optics, only one direction within the field of view is corrected. With MCAO, this technique is applied to several directions and reference stars, assuming the atmosphere can be represented by on-ly a few thin turbulent layers.

During the next three years, MCAO will be coupled with the LBT LINC-NIRVANAcamera (see below) that is also being built at MPIA. Light coming from one LBT mirror is divided by a beam-splitter. One part travels to a wavefront sensor, which controls the 672 actuators of the adaptive secondary mirror of the corresponding LBT pri-mary mirror. The portion of light that passes the beam splitter is directed by two flexible mirrors with 349 actua-tors each and by several additional mirrors to the focus.

The light beam coming from the second LBT primary mirror is subjected to the same procedure. Wave trains in phase then interfere in the joint focus.

In this instrument, a total of six wavefront sensors as well as six adaptive mirrors with a total of 2740 actuators will be used – a hitherto unique concept that will render the spatial resolution of ground-based telescopes almost independent of atmospheric influences over a large field of view. Moreover, the wavefront sensors’ large fields of view of one to two arc minutes facilitate the selection of reference stars of sufficient brightness for the adaptive optics. This is crucial for conducting as many scientific projects as possible with this instrument.

(T. Herbst, D. Andersen, P. Bizenberger, H.

Böhnhardt, W. Gässler, S. Kellner, Ch. Leinert, R.

Ragazzoni, H.-W. Rix, R.-R. Rohloff, R. Soci, W. Xu)

2. LUCIFERand LINC-NIRVANAfor the Large