Paul Giordano

Active Optics II. Results of an Experiment with a Thin 1 m Test Mirror

Abstract Part I of this paper [1] presented the theory, and practical basis, of a complete system for active control of optical quality, with particular application to large telescopes. Reference was made to an experimental verification using a thin model mirror of 1 m diameter. Part II now gives a complete account of these experiments which consisted essentially of two parts: the verification of the theoretical modal calibrations and their orthogonality in practice, and the application of the calibrations to achieve the active modal correction of the mirror. Both aspects of the experiment were successful, with a precision higher than expected. The residual errors left after final correction are, by definition, of a higher order than the five corrected low spatial frequency terms and are small. However, one residual term is dominant (fifth-order astigmatism) but this was omitted in the correction process as being probably too high a mode to appear in practice. A calibration for this term will be performed...

The VLTI – A Status Report

The Very Large Telescope (VLT) Observatory on Cerro Paranal (2635 m) in Northern Chile is approaching completion. After the four 8-m Unit Telescopes (UT) individually saw first light in the last years, two of them were combined for the first time on October 30, 2001 to form a stellar interferometer, the VLT Interferometer. The remaining two UTs will be integrated into the interferometric array later this year. In this article, we will describe the subsystems of the VLTI and the planning for the following years.

VLTI technical advances: present and future

The Very Large Telescope Interferometer (VLTI) on Cerro Paranal (2635 m) in Northern Chile reached a major milestone in September 2003 when the mid infrared instrument MIDI was offered for scientific observations to the community. This was only nine months after MIDI had recorded first fringes. In the meantime, the near infrared instrument AMBER saw first fringes in March 2004, and it is planned to offer AMBER in September 2004. The large number of subsystems that have been installed in the last two years - amongst them adaptive optics for the 8-m Unit Telescopes (UT), the first 1.8-m Auxiliary Telescope (AT), the fringe tracker FINITO and three more Delay Lines for a total of six, only to name the major ones - will be described in this article. We will also discuss the next steps of the VLTI mainly concerned with the dual feed system PRIMA and we will give an outlook to possible future extensions.

Active Optics: III. Final Results with the 1 m Test Mirror and NTT 3·58 m Primary in the Workshop

In ‘Active Optics II’, the active correction of a 1 m test mirror was described in detail. A residual aberration term was found which still required correction in order to achieve the aim of the experiment: removal of all systematic defects leaving only statistical or high-frequency noise. The present paper describes the interferometric confirmation of the term and this final correction stage of the 1 m experiment.

Survey of airborne particle density and the aging of mirror coatings in the open air at the VLT Observatory

A long term survey of airborne particles was initiated in 1992 at the VLT Observatory of Cerro Paranal to establish the cleanliness of the telescope area before the start of construction work. The results presented in this paper shows large variations with time of the density of inhalable particles (< 10 micrometers ), and a very low density of the larger aerosols when compared to clean room industrial standards. In parallel with the aerosol survey, an analysis of the damage caused to mirror coatings was conducted by periodically exposing sets of mirrors outdoors at 10 m above ground level for periods of two weeks. A follow-up of the evolution: with time and meteorological conditions of the reflectivity, diffusion and scattering coefficients gives insights on mirror maintenance requirements in modern telescopes using natural air flushing to right local seeing. From the study of damages caused during wind storms, some new constraints on operational limits are discussed, in particular with regards to the protection of an unbaffled secondary mirror.

Latest developments of active optics of the ESO NTT and the implications for the ESO VLT

The latest developments of active optics of the ESO NTT include the reduction of friction in the lateral supports of the primary mirror and in the positioning system of the secondary mirror. The most important remaining problem is the local air condition. The implications for the ESO VLT and the latest developments in the design of its active optics are discussed.

Dust contamination and in-situ cleaning of ground-based telescope optics: the VLT approach

It seems to be a fundamental law of nature that optical surfaces become dirty. On-site contamination has been recorded at the ESO La Silla Observatory and at the VLT site over a period of six years. Measured data are presented, and the efforts made at ESO since 1990 to define suitable on-line monitoring and preventive maintenance are detailed. In-situ cleaning techniques, existing equipment and procedures are reviewed. Emphasis is put on the carbon-dioxide snowflake cleaning technique and the integrated cleaning device of the 3.5 m NTT telescope is described. The preliminary cleaning and protection test conducted on the first finished 8 m mirror at the optical manufacturers site is presented as well, and plans for the in-situ cleaning of the VLT mirrors are explained.

ESO Active Optics System: Verification On A Lm Diameter Test Mirror

Active optics, as envisaged for the ESO NTT, is based on four principles: 1. The linear relationship between the applied support force and the deformation of the mirror resting on a force based support system. 2. The orthogonality of the corrected aberrations. 3. The restriction to correct only those aberrations for which small forces are necessary, i.e. long wavelength aberrations as e.g. astigmatism and spherical aberration, which are also those modes induced by errors in the system. 4. The use of precalibrated forces for these corrections. Using the Shack-Hartmann test method for the analysis of the wavefront, we have confirmed these principles in an experiment with a lm diameter spherical mirror with a thickness of 18,9 mm and 78 supports.

Progress in the optical maintenance of the VLT

Since February 2000, the Washing & Coating Unit of Paranal Observatory, one of the master piece in the coating and re-coating of the VLT mirrors, has been continuously improved. Details of those works will be described as well as the washing process revealed. Measured data will be presented and the efforts made at ESO since 1998 to define suitable on-line monitoring and preventive maintenance will be detailed. In-situ techniques, existing equipment and procedures will be reviewed. Emphasis will be put on the CO2 snow-flake technique and the re-enable of the peelable lacque (XL Clean 5) will be described. The plans for the long-term optical maintenance of the VLT and VLTI mirrors will be exposed.

Evaluation of the progress in the optical maintenance of the VLT mirrors

Two years after the first light of UT1 a summary of the On- site optical maintenance situation seems necessary. A comparison with the Maintenance Program developed in Europe will be commented. Measured data on the large mirrors and the efforts to maintain and improve the on-site contamination at a low level will be presented. Data and pictures will illustrate the CO2 in-situ cleaning technique, advantage and disadvantage. Methods and results of the AMOS washing unit, in the preparation of mirrors before coating, will be presented. Emphasis will be put on the reflective Al coating quality of the LINDE coating unit and the achieved reflectivity data demonstrated.