Torben Andersen

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.

Diffraction hardware testbed and model validation

Optical systems, which operate over a wide range of Fresnel numbers, are often times performance-limited by diffraction effects. In order to characterize such effects at the 40-100 picometer level, a diffraction testbed has been built which has the capability of measuring diffraction effects at this level. Concurrently, mathematical diffraction modeling tools have been developed that propagate an input wavefront through an optical train, while retaining amplitude and phase information at a grid resolution sufficient for yielding picometer-resolution diffraction test data. This paper contains a description of this diffraction hardware testbed, the diffraction modeling approach, and a comparison of the modeled and hardware test results, which then serves as validation of the diffraction modeling methodology.

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.

Study of a 25-m telescope for optical wavelengths

A study of a 25 m class telescope is presented. The scientific case is described together with imaging and spectroscopy aspects. Spectroscopy is found possible also at high resolution with the telescope proposed. Light efficiency and mirror coating are discussed. The optical design and corresponding performance requirements are presented. With a spherical segmented f/0.9 primary mirror and an exit focal ratio of f/2.86, an on-axis four mirror system with segmented primary and quaternary mirrors is found optimal. It gives an image quality of 0.27 arcsec FWHM over a field of 90 arcsec. Mechanical design is based on a tripod configuration similar to those of radio telescopes. The alignment system proposed is discussed. Total alignment is divided into three main sub-tasks. First, low frequency alignment is established using a slow wavefront sensor. Second, a high frequency alignment is maintained with an internal laser measurement system. Third, a high frequency correction for wavefront tilt errors is made with a correlation tracker. The enclosure is co-rotating with two sections sliding apart for observations. It has adjustable wind screens and double skin panels with internal air circulation. Control facilities are installed in a thermal jacket and the observing floor is cooled.

Low turbulence--high performance

Results are presented on the evaluation of the site for the Nordic Optical Telescope (NOT) at its site at the Roque de los Muchachos Observatory. Basic design features of the NOT are described together with the parameters that define image quality, with special consideration given to the role of atmospheric turbulence and the factors taken into account in the selection of the telescope site. Attention is given to the optical elements of the NOT and to its mechanical structure as well as to the thermal control of the telescope, the enclosure, and the ancillary instrumentation. Results from first observations at the NOT point to excellent observing conditions in terms of transparency and extinction stability as well as image quality.

Modeling of structural cables

For many applications, including future giant telescopes, there is increased interest in structural cables. Usually integrated models, i.e. combined models of structures, optics and control systems are used for modeling of telescopes. Performance of structural cables is non-linear, so often a linearization is needed to include cable models in integrated models of telescopes. We present two different linear models describing static and dynamic performance of a cable near an equilibrium operating point. The models are plug-in models that in a black box form give end point forces as outputs, with end point movements as inputs. Using an example, we compare the two models and we outline the benefits of each of the models. Although we here focus on modeling of telescopes, the techniques are useful also for other applications.

Proposed 12-m submillimeter antenna design for the Large Southern Array

The planned Large Southern Array will have 50 - 100 radio antennas with apertures in the range 8 - 15 m. They will be placed in Northern Chile. To provide a basis for choice of the optical telescope aperture, a feasibility study of a 12 m submillimeter unit antenna has been carried out at European Southern Observatory. The antenna has a steel structure below, combined with a dish of carbon fiber reinforced plastic. The reflecting panels are precision machined from an aluminum alloy. The most difficult specification is related to pointing stability. The specification is 0.66. Representative calculations have shown that wind disturbances will cause pointing errors below 0.4 rms. A specification of 25 micrometer rms for the surface precision can be fulfilled. To provide a larger safety margin, or alternatively a larger dish, it is attractive to develop a simple active optics system to correct for pointing errors caused by deflections in the steel structure below the elevation axis for frequencies up to 10 - 15 Hz.