3.2  Personal overview: Hans Zinnecker (Astrophys. Inst. Potsdam, Germany)

The science case for ELTs: personal impressions from the first workshop.

1. Introduction

About 40 people gathered together at the Royal Observatory Edinburgh for two days (Sept 25-26, 2000) to discuss a possible science case for extremely large telescopes (30-100m class). The matter was subdivided into three topics/panels:

(a) stars and planets
(b) galaxies and stellar populations
(c) cosmology

I took part in the discussions in panel (a). Here, I briefly summarise my personal impressions, following the joint discussion of the individual panels.

2. Highlights of the Joint Discussion

(a) stars and planets (b) galaxies and stellar populations (c) NGST/ALMA follow-up spectroscopy

3. Conclusions

No hard conclusions were given, but some clues emerged. The stars/planets domain would probably benefit most from the imaging capability of OWL at very high angular resolution (2mas at 1 micron for 100m telescope diameter, i.e. 1 AU at the distance of the Orion star formation complex). On the other hand, cosmology would benefit most from the vastly increased collecting area compared to the VLT and Keck; by 2010-2015 there will be a need for follow-up spectroscopy of NGST imaging.

As for galaxies and stellar populations, the benefits did not appear to be so overwhelming, as the new thresholds depended only on optical observations, and left the audience with mixed feelings.

Building gigantic telescopes like OWL will make sense only, if the problems of multi-conjugate adaptive optics (MCAO) can be solved. The optimism expressed at the meeting, based on computer simulations, must be backed up by real systems. It was also noted that lack of wind would be a primary site selection criterion rather than the best seeing conditions. PS. With hindsight, it is unclear to me whether a fully filled circular aperture is the best solution for the new science listed above. For example, with the same collecting area one could also build a diluted aperture telescope (non-redundant baseline array) in which some baselines can be larger by a factor of two over the canonical circular diameter, thus increasing the diffraction limit and angular resolution correspondingly.