METIS – First Light Instruments for the European Extremely Large Telescope (E-ELT)
by Andreas Obereder, RICAM and MathConsult
After the sucessful research project Mathematical Algorithms for E-ELT Adaptive Optics , the Linz Adaptive Optics team is now involved the development of two first light instruments for the E-ELT In this blog entry, we focus on our contribution to METIS, the Mid-infrared E-ELT Imager and Spectrograph.
What is METIS?
In Greek mythology, Metis was the spouse of Zeus and mother of Athena, goddess of wisdom. METIS is also the proposed mid-infrared imager and spectrograph for the European ELT. Covering the L, M and N bands, METIS will offer imaging, coronagraphy and medium-resolution spectroscopy over the full wavelength range (3-19 microns), and high-resolution integral field spectroscopy in L and M bands (3-5 microns).
Why Adaptive Optics?
The need for powerful Adaptive Optics systems has been explained in a former blog entry explaining why more and more demanding systems have to be treated with increasing telescope sizes.
As METIS will be operating in the mid-infrared range, the wavefront wensors (WFS) will be operated at a longer wavelength (most likely K-Band, 2.2microns) as well. This poses additional challenges for the Team:
- Which type of wavefront sensor should be used, a Pyramid or a Shack-Hartmann sensor?
- Which control algorithms are suited for the desired/selected setup?.
Meeting the challenges
The Shack-Hartmann WFS is well known and utilized in almost all operated AO-Systems but has a significant drawback when used in very- or extremely large telescopes, the so called Low Wind Effect (LWE) introduced by „Spiders“, the mechanical support structures for the secondary mirror.
Those structures lead to less illuminated and even completely obstructed WFS subapertures and therefore disconnected domains on the sensor. We have developed new approaches (utilizing a low order Zernike polynomial basis to reconstruct the incoming wavefront from the sensor measurements) to reconnect, i.e. align those segments and control the deformable mirror, although a loss in quality has to be expected and accepted.
Hence the Pyramid WFS has gained popularity and is the sensor of choice for most AO-Systems in the First-Light instruments for all planned ELTs.
We have developed new reconstruction methods for the Pyramid WFS int the past and are now extending these prototype algorithms for the use in METIS. In cooperation with MPIA Heidelberg and the Observatory of the University of Leiden , we simulate the full Adaptive Optics System and the performance of our reconstrution methods in order to predict and optimize the system performance of METIS.
In a second development stage, METIS will be upgraded with a Laser Tomography (LTAO) module thus extending our tasks and challenges to tomography problems, as described here .