Alexander Gontcharov

The Euro50 Extremely Large Telescope

Euro50 is a proposed optical telescope with an equivalent primary mirror diameter of 50 m. Partners of the collaboration are institutes in Sweden, Spain, Ireland, Finland, and the UK. The telescope will have a segmented primary mirror and an aplanatic Gregorian configuration with two elliptical mirrors. For a 50 m telescope there would be no economical advantage in going to a spherical primary. The size of the primary mirror segments (2 m) has been selected on the basis of a minimization of cost. An adaptive optics system will be integrated into the telescope. The telescope will have three operational modes: Seeing limited observations, single conjugate adaptive observations in the K-band, and dual conjugate observations also in the K-band. An upgrade to adaptive optics also in the visible down to 500 nm is foreseen. There will be an enclosure to protect the telescope against adverse weather and wind disturbances. Integrated simulation models are under development. The project time will be 10 years and the cost some 591 MEuros.

Intrinsic apodization effect in a compact two-mirror system with a spherical primary mirror

In so-called compact two-mirror focusing and afocal systems the secondary mirror is situated at the paraxial focus of the spherical primary mirror. The peculiar conelike shape of the secondary is chosen to eliminate the spherical aberration. Thus the systems are stigmatic, but they have a great offence against the sine condition (OSC), creating uneven distribution of energy on the surface of the outgoing wavefront. There is a substantial concentration of energy toward the optical axis, similar to apodization of the pupil. Calculations of the point spread function (PSF) show that in such systems, the Airy disk is slightly enlarged (at most 20%) and the second maximum (the first bright ring) can be completely suppressed. The most effective suppression of the second and higher maxima in the diffraction pattern is obtained with an imposed central obscuration ratio of 0.5 and a primary mirror focal ratio of 2. Compact two-mirror afocal and focusing systems have intrinsic apodization, enabling modification of the diffraction pattern without the use of a variable transmission filter or coating at the pupil.

Adaptive optics for the Euro50: design and performance

The optical design for the proposed Euro50 extremely large telescope with integrated adaptive optics (AO) is presented. For atmospheric turbulence correction, we propose using single and dual-conjugate AO systems working with natural and laser guide stars. The corrective shape of the deformable mirrors (DMs) is derived from an analytical algorithm based on minimization of the sum of the residual power spectra of the phase fluctuations seen by guide stars after correction. Predictions for performance of the Euro50 ELT with Dual-conjugate AO are given for the K band using a seven layer atmospheric model for the atmosphere at the Observatorio del Roque de los Muchachos (ORM) on La Palma. The average Strehl ratio is used to quantify the system performance for different values of actuator pitch and DM conjugation altitudes. The influence of the outer scale and telescope pointing on the RMS stroke of the DMs is presented. It is concluded that construction of such a system is feasible and that there is a need for development of a simulation tool to verify the analytical calculations. Precise knowledge of the outer scale of the atmosphere at the ORM is needed to establish the dynamical range of the mirrors.

Aplanatic four-mirror system for optical telescopes with a spherical primary mirror

An exact analytical solution for the design of an optical system without spherical aberration that satisfies the sine condition is presented. The system consists of two parts. The first two mirrors form a strictly afocal system with a spherical primary and the second convex aspherical mirror. The next two mirrors form a focusing system that compensates the coma aberration of the afocal system. A complete description of the optical design, with system data, is given. The spherical primary mirror has a diameter of 10 m. The focal length of the entire system is 152 m, length is 13.5 m, angular field is 7 arcmin, and its theoretical resolution is 0.03 arcsec.

Some consequences of atmospheric dispersion for ELTs

Previously the effect of atmospheric dispersion on telescope performance has attracted only relatively little attention. This may be due to the fact that the dispersion effects have been evaluated in relation to the size of the diffraction limited resolution angle of current telescopes, or to seeing limited telescopes. However, since the resolution angle is inversely proportional to telescope diameter, dispersion and dispersion compensation becomes increasingly important for extremely large telescopes (ELTs). In this paper we present a simple model for the dispersion effects in telescopes with adaptive optics (AO). The model addresses the expected loss in Strehl ratio when the atmospheric wavefront error is measured at a wavelength different from the wavelength of observation. Also, the bandwidth over which the correction will be of a given quality is evaluated. Related to AO performance, the consequence of using laser guide stars (LGSs) for probing the atmosphere may be that the measured wavefront error must be rescaled to the wavelength of observation. This places special demands on the AO control loop. Since linear atmospheric dispersion compensation need not cover a larger bandwidth than the AO compensation, an atmospheric dispersion compensator (ADC) can be designed for narrow band operation. As an example of the benefits to be obtained from this, we briefly present the proposed ADC for the Euro50.

Compact two-mirror schemes for telescopes with a fast spherical primary

Two types of optical schemes are considered for telescopes with a fast spherical primary: the first for an afocal system, and the sec- ond for a focusing system. Both are strictly corrected for spherical aber- ration. In the extreme case the relative aperture of the spherical primary is 1/0.35 and the length of the system is 2.8 times smaller than the diameter of the primary. Exact analytical expressions describing a profile of the aspheric secondary are presented. The field of view of the system is highly limited by coma. It is shown that this drawback might be slightly compensated by field scanning by means of rotation of the secondary about either its vertex or the center of curvature of the fixed primary. Such schemes can be used for many different purposes, such as studies of point sources, laser-beam transformation, transmission and reception of infrared emission, and indication of telescope pointing errors.