Max Planck Institute for Astronomy - Annual Report 2005

96 IV. Instrumental Development
research on the origins of our universe and the formation
of planets and life in our Galaxy. These activities place
MPIA in an excellent position to participate in ambitious
future missions
(Th. Henning (Co-PI), D. Lemke (Co-PI), S.
Birkmann, U. Grözinger, R. Hofferbert, A. Huber, U.
Klaas, O. Krause, S. Kuhlmann, H.-W. Rix, A. Böhm,
Monika Ebert, B. Grimm, S. Meister, J. Ramos, R.-R.
Rohloff, Alexandra Bohm, Hannelore Heißler)
IV.2 Novel Concepts for Extremely Large
Telescopes
With at least three large, international projects underway,
the era of the Extremely Large Telescope (ELT) is
dawning. Defined as ground-based telescopes with
diameters in the range of 20 – 100 meters, the ELTs represent
the next significant advance in the development of
ground-based telescopes.
The MPIA is playing a leading role in ushering in the
era of Extremely Large Telescopes. From involvement
in policy-making bodies to the evaluation of science
requirements and goals, to instrument design studies and
technology development, MPIA scientists and engineers
are positioning the Institute to take advantage of these
gigantic, yet exquisitely precise instruments.
Adaptive Optics (AO)
One of the fundamental challenges for extremely large
telescopes is adaptive optics, as the core science cases
for such telescopes require refraction-limited, rather than
seeing-limited imaging. Within the European Framework
Program 6 (FP6), a design study for an ELT was started
in 2005. Together with partners from iNaf (Bologna),
University of Durham, Technion Haifa, eSo, the University
of Galway and Lundt Observatory, we will investigate new
concepts of wavefront sensing techniques to overcome the
limitations current AO systems would have on an ELT.
cone effect Perspective Elongation
H = 100 km
AO depends on natural or artificial guide stars bright
enough to retrieve the information needed for atmospheric
turbulence correction. This limits the sky coverage for
AO observations. With an ELT, the limiting magnitude of
a guide star will not change, as the sub-aperture size used
in AO depends on the atmosphere and does not scale with
the diameter. Ideas to increase sky coverage with natural
guide stars and especially with a Laser Guide Star (LGS)
are being investigated. Laser Guide Stars could increase
the sky coverage to nearly 100 percent. Therefore,
the group is currently focusing on the LGS concepts.
However, LGS have certain inherent problems which
severely increase with telescope diameter.
These problems can be distinguished in the following
categories, which are illustrated in Fig. IV.2.1:
• Cone effect
• Spot elongation
• De-focus (extended focal depth – dynamic focal plane
– differential aberrations)
A solution studied at the MPIA is the Pseudo Infinite
Guide Star Sensor (pigS, see Fig. IV.2.2), which uses two
sensing devices, a mask in the infinity focus with annular
slits which senses the radial component of the wavefront,
and a reflecitve rod that senses the azimuthal part of the
wavefront. Multiplying the sensor and using it with several
LGS can overcome the cone effect. Intrinsically the
sensor does not have the problem with spot elongation
and extended focal depth; refocusing the sensor would
also remove the problem of the dynamic focal plane,
so that only the telescope aberation problem remains.
Currently we set up an experiment in the laboratory to
test the idea in a multi-guide star fashion.
Fig. IV.2.1: The finite distance and extended length in vertical
direction of the LGS leads to several problems within an AO
sensor: Cone effect (left): The LGS does not probe the full
atmosphere like a normal guide star. Spot elongation (middle):
Off axis projection of the LGS leads to an elongated spot on the
sensor plane. De-focus (right): The LGS has an extended focal
depth in the image space.
sodium layer
�h = 10 km
D = 30 – 100 m
ELT
de-focus