Arne Ardeberg

The usage of Stromgren photometry in studies of local group dwarf spheroidal galaxies - Application to Draco: a new catalogue of Draco members and a study of the metallicity distribution function and radial gradients

Aims. In this paper we demonstrate how Stromgren uvby photometry can be efficiently used to: 1. identify red giant branch stars that are members in a dwarf spheroidal galaxy; 2. derive age-independent metallicities for the same stars and quantify the associated errors. Methods. Stromgren uvby photometry in a 11 x 22 arcmin field centered on the Draco dwarf spheroidal galaxy was obtained using the Isaac Newton Telescope on La Palma. Members of the Draco dSph galaxy were identified using the surface gravity sensitive c(1) index which discriminates between red giant and dwarf stars. Thus enabling us to distinguish the (red giant branch) members of the dwarf spheroidal galaxy from the foreground dwarf stars in our galaxy. The method is evaluated through a comparison of our membership list with membership classifications in the literature based on radial velocities and proper motions. The metallicity sensitive m(1) index was used to derive individual and age-independent metallicities for the members of the Draco dSph galaxy. The derived metallicities are compared to studies based on high resolution spectroscopy and the agreement is found to be very good. Results. We present metallicities for 169 members of the red giant branch in the Draco dwarf spheroidal galaxy (the largest sample to date). The metallicity distribution function for the Draco dSph galaxy shows a mean [Fe/H] = -1.74 dex with a spread of 0.24 dex. The correlation between metallicity and colour for the stars on the red giant branch is consistent with a dominant old, and coeval population. There is a possible spatial population gradient over the field with the most metal-rich stars being more centrally concentrated than the metal-poor stars. (Less)

Novel concept for large deformable mirrors

Large, high-bandwidth deformable mirrors (DMs) with thousands of actuators for adaptive optics are of high interest for existing large telescopes and indispensable for construction of efficient future extremely large telescopes. Different actuation and sensing principles are possible. We propose a novel concept using commercially available voice coil actuators attached to the back of the mirror with suction cups and using LVDT sensors on the actuators for local stabilization. Also, a new low-cost sensor for easy measurement of DM displacement or velocity has been developed. It has a sensitivity better than 20 nm and a bandwidth wider than 20 to 1000 Hz. Finally, studies are in progress of global, hierarchical mirror form controllers based on many parallel multiple-input, multiple-output regulators of low order.

From Euro50 towards a European ELT

With Euro50 as a convenient telescope laboratory, the Euro50 team has continued development aiming at a European extremely large telescope (ELT). Here, we give a progress report. The needs of science and instrumentation are briefly discussed as is the importance of photometric stability and precision. Results are reported from work on integrated modelling. Details are given concerning point-spread functions (PSFs) obtained with and without adaptive optics (AO). Our results are rather encouraging concerning AO photometry and compensation of edge sensor noise as well as regarding seeing-limited ELT operation. The current status of our development of large deformable mirrors is shown. Low-cost actuators and deflection sensors have been developed as have hierarchic control algorithms. Fabrication of large thin mirror blanks as well as polishing and handling of thin mirrors has been studied experimentally. Regarding adaptive optics, we discuss differential refraction and the limitations imposed by dispersive optical path differences (OPDs) and dispersive anisoplanatism. We report on progress in laser guide star (LGS) performance and a real-time online experiment in multi-conjugate AO (MCAO). We discuss ELTs, high-resolution spectroscopy and pupil slicing with and without use of AO. Finally, we present some recent studies of ELT enclosure options.

Matching and improving the best atmospheric turbulence conditions with very large telescopes

Abstract Some programmes in astrophysics, in early progress and being planned, needing telescopes with high light-collecting power and excellent image quality, are presented and discussed. Special references are made to a study of star formation in, and heavy element abundance and evolution of, the bar of the Large Magellanic Cloud. Passive and active optical systems as well as live optics and adaptive optics are considered. Applications to telescopes of different sizes are commented on. Main features of telescopes with mirror diameters larger than 25 metres are described, as are possible optical and mechanical layouts. Examples of live optics systems are demonstrated and some quality estimates commented on. Different types of corrections for effects of atmospheric turbulence are compared. Steerable mirror segments and their possible locations in extremely large telescopes are debated together with considerations regarding the size of such segments as well as implications regarding image quality and project cost. Finally, some potential problem areas and corresponding possible remedies are considered.

25-m Live Optics Telescope

A 25 m four mirror live optics telescope is studied. M1 is spherical with 141 segments and f/0.96. M1 is re-imaged onto M4, also with 141 segments. Image FWHM is less than 0.10 arc sec over greater than 20 arc min. A horseshoe solution with a simple azimuth platform is applied. M1 segments are supported by a fine meniscus form truss structure, tied to the horseshoes by a coarser mesh. A FEM with 104 dof was developed and applied. Live optics control M1 and M4 segments (the latter with potentially high bandwidth), M1/M4 segment balancing and servos. Correction signals in tilt, coma and defocus are traced. A correlation tracker and a laser guide star system are included. Low and high wind speed regimes are studied. An end-to-end simulation model is developed, based on modal representation of our FEM. Image quality dependence on wind load is studied from segment piston and tilt deflections. Eigenmodes are recorded. Using wind time series, we study dynamic effects and image quality resulting from the 141 segment spots. Automatic segment control at a bandwidth of only 1 Hz gives excellent image quality. We foresee to reach a bandwidth greater than 50 Hz, securing a system partly adaptive, with effects of atmospheric wave front tilt removed through M4 segment tilting at high frequency. Further progress include optimization of mechanical design and end-to-end simulation model, wind tunnel testing and studies of wave front sensor, correlation tracker and instruments. A fully adaptive system is tentatively studied as is coherent operation at IR wavelengths.

Science with the Euro50

ELT science drivers stress aperture, Strehl ratio, PSF definition and stability, field of view, wavelength range, flexibility, low polarisation and thermal emittance, auxiliary instruments, site and infrastructure. Applicable science categories are planets and planetary systems, stars and stellar systems, galaxies and galaxy clusters, and cosmology. ELT observations are needed for our own and other planetary systems. The study of planetary disks and formation requires ELT data. ELT results, with emphasis on PSF quality and stability, are crucial to the search for earth-like planets, especially those favourable for life. Investigation of star formation and stellar evolution requires ELT performance, as does the study of final stellar stages. ELTs are necessary for extremely high time resolution, details of stellar surfaces and astroseismology. Galaxy formation studies will benefit dramatically from ELT data, as will studies of large-scale development of galaxies and galaxy clusters over cosmological time scales. Detecting active galactic nuclei requires ELTs. ELT data are crucial for examining the structure and evolution of the universe. Observations of supernovae and other standard sources over very large distances are necessary for mapping the expansion of the universe and determining its acceleration or deceleration. Comparisons of Euro50 with VLTs and HST show a dramatic gain. The complementarity of Euro50, NGST and ALMA is noted.

Optical design of a 25-m telescope for optical wavelengths

Results from the design of a 28 m optical telescope with a spherical segmented primary are presented. The telescope is a four mirror configuration reimaging M1 on M4. The wish for a small and compact structure resulting in a need for controlling high order aspherical mirror coefficients has initiated development of a design procedure satisfying Fermat principle and Abbes Sine condition. Thus the only remaining point aberration will be astigmatism. For a given shape of M4, the design procedure delivers the Taylor expansion coefficients for the shapes of M3 and M4 to be directly used for optical analysis by software capable of handling the needed number of coefficients.

Some aspects of science with a 50-m AO telescope

Among the science challenges of the Extremely Large Telescopes (ELTs), four object types are studied for performance with a 50 m ELT with adaptive optics (AO), Euro50. Emphasis is on planetary systems and very distant objects. For planetary systems and their evolution, we examine high resolution imaging of the nuclei of comets and high-resolution imaging, photometry and low and intermediate resolution spectroscopy of Kuiper-Belt objects. Imaging of Earth-like planets is discussed. The very high contrast imaging necessary for these purposes is discussed together with the relevant error sources. Finally, photometry and classification of supernovae is discussed and examined. The performance of a 50 m AO ELT is compared to corresponding data obtainable with current VLTs equipped with AO.

Clusters as probes of distance, evolution, and chemistry of galaxies

Stellar clusters are highly useful as tools for determination of distances, ages and abundances of heavy elements of galaxies, also at larger distances. Their utility for these purposes has, so far, been severely limited, mainly due to image crowding. The introduction of Extremely Large Telescopes (ELTs) with full adaptive optics (AO) and near diffraction limited performance should imply a drastic improvement concerning the usefulness of clusters and the limiting distances of high quality data. We have made a study of stellar clusters as probes of distance, evolution and chemistry of galaxies at distances from one to twenty Mpc. From data on the Stromgren uvby system, partly from direct measurements taken from the literature, we have synthesized test clusters, one open and one globular, as well as galactic backgrounds. The clusters have been embedded in the backgrounds and located at distances between one and twenty Mpc. Here, vby data have been measured, reduced and analyzed. Color-magnitude diagrams (CMDs), metallicity diagrams (MDs) and luminosity functions (LFs) have been constructed. They have been evaluated absolutely and compared to the corresponding template data. We conclude that with a 50 m AO ELT, for open as well as globular clusters, MDs are of high quality for clusters out to and beyond 5 Mpc and useful out to 10 Mpc. CMDs are of very high quality well beyond 5 Mpc. They are of high scientific value out to and beyond 10 Mpc and valuable for clusters even out to 20 Mpc. LFs are highly informative well beyond 10 Mpc and still rather valuable at 20 Mpc. With sufficient measurement data available, LFs are useful for clusters in galaxies even beyond 20 Mpc.

Science drivers of a 50-m AO telescope

Some leading science programs undertaken with Very Large Telescopes and challenges driving the progress of the Extremely Large Telescopes are discussed together with the corresponding requirements. They concern expolanets, Earth-like planets, habitable zones, formation of stars and galaxies, first stars and cosmology. A description is attempted.