Because of the very small size of the isoplanatic patch resulting from high altitude, tropopause seeing, the use of the Solar Differential Image Motion Monitor (S-DIMM) for solar seeing observations at large zenith distances is significantly compromised. I examine the resulting limitations and attempt to correct for these in its use for the Advanced Technology Solar Telescope (ATST/DKIST) site survey. Suggestions are given about how one might correct for this altitude seeing by Dual Conjugate and Multi-Conjugate Adaptive Optics (DCAO/MCAO). I also comment on adaptive optics for corona observations with the use of Laser Guide Star aided wavefront sensing.
I compare recent site surveys for the future large 4-meter solar and 30-meter nighttime telescopes at the nearby
Haleakala and Mauna Kea sites respectively. They show that the outstanding early morning image quality at the solar
site corresponds indeed to that observed at the late night one at the nighttime site. That confirms the notion that daytime
solar site heating only shows itself later in the morning. The nighttime survey includes observations of the refractive
index structure function Cn2(h) to high altitudes from which the radius of the isoplanatic patch (Ɵ0) can be determined.
At zenith (ζ = 00) it equals 2.5 arcsec at 500 nm wavelength. For the early morning (best) seeing at the solar site, which
occurs at ζsun = 750 and the cos1.6(ζ) dependence of Θ0,that means an extremely small Ɵ0 (0.26 arcsec). Such small values
compromise Adaptive Optics (AO) solar correlation wavefront sensing for which areas are needed equal to about 8”× 8”
I suggest options for measuring Cn2(h), and therefore Ɵ0, during the day. These make use of the solar image as well as of
daytime images of bright stars and planets. Some use the MASS technique on stars; some use the SHABAR technique
using very large detector baselines on the Sun and shorter baselines on planets. It is suggested that these Cn2(h)
measurements are made also during regular solar observations. In that way optimal solar observations can be planned
using real-time Ɵ0 observations by image selection and optimization of the MCAO configuration.
In connection with the planning for Extremely Large Telescopes, I revisit a 2001 paper in which Cacciani and I describe
the use of Sodium Laser Guide Stars (LGSs) for diffraction limited daytime astronomical observations. The enabling
technology for seeing LGSs in broad daylight is the availability of very narrow band magneto-optical filters. Considering
the dominance of the atmospheric scattering of sunlight at wavelengths below 3.5 μm, daytime use is only indicated for
mid- and thermal IR observations. The launch of the 6.5 meter aperture James Web Space Telescope (JWST) appears to
be assured and planned for 2013, preceding the most optimistic projections for the completion date of the first ELT. The
projected thermal background of the JWST is very much less than that of ground-based telescopes so that any competing
ground-based observations are limited to those parameters not covered by the JWST: angular resolution (requiring
apertures > 6.5 meter) and spectral resolution (R>3000). I compare the benefits of daytime observations with Na-LGS
equipped telescopes and interferometers at moderate latitudes and in the Antarctic (specifically Dome C). In both cases
daytime observations extend the amount of observing time available for TIR observations. Antarctic observations have
the advantage of having very good seeing during the daytime, significantly better than nighttime seeing. In contrast the
seeing at moderate latitude sites significantly deteriorates during daytime resulting in lower quality observations than
during nighttime. In addition Antarctic sites are less hostile to maintenance and operations during daytime (summer)
observations as compared to nighttime (winter) observations.
The Advanced Solar Technology Telescope (ATST) is a 4-m solar telescope being designed for high spatial, spectral and temporal resolution, as well as IR and low-scattered light observations. The overall limit of performance of the telescope is strongly influenced by the qualities of the site at which it is located. Six sites were tested with a seeing monitor and a sky brightness instrument for 1.5 to 2 years. The sites were Big Bear (California), Haleakala (Hawaii), La Palma (Canary Islands, Spain), Panguitch Lake (Utah), Sacramento Peak (New Mexico), and San Pedro Martir (Baja California, Mexico). In this paper we will describe the methods and results of the site survey, which chose Haleakala as the location of the ATST.
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.
The location of the Advanced Technology Solar Telescope (ATST) is a critical factor in the overall performance of the telescope. We have developed a set of instrumentation to measure daytime seeing, sky brightness, cloud cover, water vapor, dust levels, and weather. The instruments have been located at six sites for periods of one to two years. Here we describe the sites and instrumentation, discuss the data reduction, and present some preliminary results. We demonstrate that it is possible to estimate seeing as a function of height near the ground with an array of scintillometers, and that there is a distinct qualitative difference in daytime seeing between sites with or without a nearby lake.
To obtain full sky coverage, astronomical adaptive optics systems require Na Sodium Beacons (SBs) (also referred to as Laser Guide Stars or LGSs) located at heights extending from 85 to 100 km. When viewed at the edge of large telescopes these SBs appear elongated. For the Euro50 50 meter aperture telecopes this elongation amounts to 6 to 9 arcseconds when the laser is launched from a point on the telescope axis. This is substantially larger than the -0.6 arcsec FWHM SB when viewed near the telescope center. This so-called "perspective elongation" substantially decreases the sensitivity of SB aided adaptive optics. We describe a way of removing this elongation when using pulsed lasers. It uses rapid (microsecond) refocusing of the telescope with the aid of birefringent lenses and polarization modulators. We present an outline of the SB wavefront sensor for the Euro50.
The Euro50 is a telescope for optical and infrared wavelengths. It has an aspherical primary mirror with a size of 50 meters and 618 segments. The optical configuration is of Gregorian type and the secondary mirror is deformable for adaptive optics. Observations can take place in prime focus, Gregorian foci, and Nasmyth foci using additional relay mirrors. The telescope provides seeing limited observations, partial adaptive optics with ground layer correction, single conjugate adaptive optics and dual-conjugate adaptive optics. For prime focus observations, a clam-shell corrector with a doublet lens is used. The primary mirror segments can be polished using the precessions polishing technique. "Live Optics" denotes the joint segment alignment system, secondary mirror control system, adaptive optics and main axes servos. An overview is given of the live optics architecture, including feedback from wavefront sensors for natural and laser guide stars, and from primary mirror segment edge sensors. A straw man concept of the laser guide star system using sum-frequency YAG lasers is presented together with a solution to the laser guide star perspective elongation problem. The structural design involves a large steel structure and a tripod of carbon fiber reinforced polymer to support the secondary mirror. Integrated models have been set up to simulate telescope performance. Results show that an enclosure is needed to protect the telescope against wind during observations. The enclosure is very large box-shaped steel structure.
In nighttime astronomy Vernin and co-workers have proposed and subsequently developed the so-called SCIDAR (SCIntillation Detection And Ranging) technique to probe Cn2(h). It makes use of the double shadow band (or scintillation) pattern formed on a telescope aperture by the two components of a binary star. We are developing a variant of this technique for solar astronomy. It uses pairs of small apertures on the solar image with diameters smaller than the isoplanatic patch (“artificial double stars”). Within the isoplanatic patch the complex amplitude (intensity and phase) of the atmospheric wavefront disturbances is constant. Solar SCIDAR (or S-SCIDAR) makes use of this. We will present the results of the first (inconclusive) experiments of this S-SCIDAR technique as used on the 76 cm aperture Dunn Solar Telescope (DST) and the 152 cm aperture McMath-Pierce facility (McM-P) of the US National Solar Observatory. It uses a 45 x 45 lenslet array placed in the solar image. The size of the lenslets corresponds to 2.25 x 2.25 arcsec at the DST and 1.67 x 1.67 arcsec at the McM-P; the separation of lenslet pairs on the DST (and hence of the separations of the artificial double stars) ranges from 2.25 arcsec to 140 arcsec. The lenslet array forms an array of pupil images on a CCD detector.
To obtain full sky coverage, astronomical adaptive optics systems require Na Sodium Beacons (SBs) (also referred to as Laser Guide Stars or LGSs) located at heights extending from 85 to 100 km. When viewed at the edge of large telescopes these SBs appear elongated. For the Euro50 50 meter aperture telescopes this elongation amounts to 6 to 9 arcseconds when the laser is launched from a point on the telescope axis. This is substantially larger than the ~0.6 arcsec FWHM SB when viewed near the telescope center. This so-called "perspective elongation" substantially decreases the sensitivity of SB aided adaptive optics. We describe a way of removing this elongation when using pulsed lasers. It uses rapid (microsecond) refocusing of the telescope with the aid of birefringent lenses and polarization modulators. We present an outline of the SB wavefront sensor for the Euro50.
We describe a solar seeing monitor used for the site testing for the 4 meter US Advanced Technology Solar Telescope and the 1 meter Yunnan Observatory Solar Telescope. It has two parts: a solar Differential Image Motion Monitor (S-DIMM) and a linear array of 6 solar scintillometers (SHABAR= SHAdow BAnd Ranger). The results obtained by both methods are compared on the basis of observations obtained in February 2002 at the Yunnan Observatory Fuxian Lake solar station. Analysis showed that these two ways of measuring the Fried parameter give consistent results. We confirm earlier observations that showed that the boundary layer seeing over lakes is strongly suppressed. The amount of this boundary layer seeing depends on the temperature difference between lake and air and on the wind velocity. We have also carried out seeing observation along a 9.15 km horizontal path across the lake. The Cn2 values derived from these is consistent with the solar observations. They confirm the dependence of Cn2 on the lake-to-air temperature difference. From the SHABAR we find a typical scale height for the boundary layer seeing of 20 meters and from inter-comparison of the S-DIMM and SHABAR observations we derive an outer scale of turbulence of about 50 meters.
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.
In this paper I review and reflect on the contributions given at this conference and place them in a broader context. Emboldened by the recent successes of 8 to 10-meter class telescopes and by the success of adaptive optics in making these telescopes diffraction limited, astronomers and engineers are now embarking on the quest for giant telescopes. Are these plans realistic? Are we overreaching ourselves?
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.
The scheme presently envisaged for the EURO50 adaptive optics is presented. The Euro50 adaptive optics will primarily work with laser guide stars (LGSs) and control of either one or two deformable mirrors (SCAO and DCAO respectively), but operation using a natural guide star (NGS) is also foreseen. The point spread function (PSF) for SCAO operation using a single NGS is evaluated. An algorithm for optimal control of the deformable mirrors (DMs) using LGSs and Shack-Hartman wavefront sensors is presented and commented upon. It is an extension of a recently developed algorithm for optimal control using NGSs and working in the spatial Fourier domain. In addition the concept of a virtual wavefront sensor is introduced to overcome the difficulty in transmitting a large number (37) of LGSs to the final DCAO focus with both adequate field and adequate aberrations. The expected performance is estimated in form of maps of the Strehl ratio versus field angle using a standard seven layer atmospheric model for the Observatorio del Roque de los Muchachos (ORM) site on la Palma for the case of the outer scale being either 20 m (nominal for ORM) or infinity (Kolmogorov - most pessimistic case).
Based on the principle of correlation tracking algorithm, the effectiveness of Cross Correlation coefficient and Absolute Difference algorithms for the low contrast extended objects such as solar granulation and the sunspot is studied. The tilt signals computed by computer post- processing are presented for the successively acquired solar granulation images. Moreover, the contrasts of the long exposure images of the solar granulation and sunspot without and with tilt removal are compared.
In 1987 I described a technique call Multi-Conjugate Adaptive Optics (MCAO) as a way of increasing the size of the area on the sky over which Adaptive Optics corrects for atmospheric wavefront distortions. An essential component of MCAO is the estimation of this wavefront distortion at different heights in the atmosphere. The technique proposed to do so was called 'Atmospheric Tomography,' or AT, since it uses tomographic techniques using the wavefront distortions at the telescope entrance pupil of objects observed in a number of different directions in the sky to infer the 3-D wavefront behavior. This paper describes a program to do so using the small scale structure on the solar surface (sunspots, pores and granulation). The Sun has the advantage of being an extended object on which the wavefront can be observed in a large number of directions using correlation Hartmann-Shack wavefront sensing. The AT experiment described in this paper uses the 76 cm Dunn Solar Telescope at NSO, 69 sub-apertures, a 2 X 2 arcmin2 field-of-view and a wavelength of 411 nm. The MCAO-AT system is being developed for the future 4 meter aperture Advanced Solar Telescope.
In 1996 we started a seeing survey of a number of existing and potential solar observing sites using solar scintillometry. This paper reports the result of the first year of that survey. It confirms earlier reports about the superior observing conditions of lake sites. I also describe the first results of atmospheric structure constant (Cn2) probing using a solar scintillometer array.
This paper is an interim report of a feasibility study which is in progress for a large 400 cm aperture solar telescope (`CLEAR'). Unlike other large solar telescopes constructed in the last three decades, CLEAR does not use the concept of evacuated telescopes to eliminate internal seeing. The requirement for full access to the far infrared spectral region (> 2.5 micrometers ), and for low scattered light, eliminates the use of the entrance window which evacuated telescopes require. Instead, CLEAR avoids internal seeing by carefully controlling the internal thermal environment of the telescope by a number of means: (1) thermal control of the primary mirror; (2) flow of ambient air over the primary mirror surface and in the telescope; (3) locating the primary focus outside the telescope beam and enclosure where the heating resulting in concentrated sunlight can be managed better (this requires the use of an off-axis primary mirror); and (4) the use of a prime focus heat stop/absorber. In addition to controlling the internal seeing, such a configuration produces a telescope with very low scattered light characteristics, allowing quality observation of regions outside the solar limb and of sunspots. By eliminating the need for a large entrance window, the CLEAR concept therefore opens up the possibility of larger aperture solar telescopes. Notwithstanding its off-axis configuration, the Gregorian telescope produces excellent images (< 0.1 arcsec) over a 5 arcminute diameter field-of-view at the f/130 Gregorian focus. In addition to the four instrumentation stations near the Gregorian focus (i.e., direct Gregorian, Nasmyth, two `folded Gregorian'), the design provides for extensive instrumentation locations in a coude area. By means of a 3- level rotating coude platform, large instruments can be located at respectively f/30, f/45 and f/60 foci.
Sometimes Fabry-Perot, and other narrow-band filters, are used for astronomical imaging in the so-called telecentric mode. In it the pupil is collimated through the filter, resulting in different incidence angles on the filter for rays coming from different parts of the objective. This results in variations of the central transmission wavelength, which broaden the effective filter bandpath. In addition each wavelength within this filter bandpath sees a different illumiatnon of the pupil when viewed from behind the fitler. This causes the diffraction limited point-spread-function to vary with wavelength. With the advent of diffraction limited imaging using adaptive optics, this can cause complications. In this paper, which elaborates further on research published elsewhere, I examine the magnitude of this effect.
The need for atmospheric dispersion correction on large telescopes is well known. Therefore it was decided to implement atmospheric dispersion correctors for FORS, the focal reducer/spectrographs of the ESO very large telescope. The boundary conditions at the VLT Cassegrain foci excluded however all previously known ADC concepts and therefore we were forced to design a new one, the longitudinal atmospheric dispersion corrector (LADC) consisting of two thin prisms with variable distance. This design has several advantages compared to the 'classical concepts:' among others it avoids tilting the pupil axis and uses only one material (silica) which has a very high transmission over the operating wavelength range of FORS (330 - 1000 nm).
The European Southern Observatory is planning to construct its Very Large Telescope on Cerro Paranal, a site with superb astronomical observing conditions located in the Atacama Desert of Northern Chile some 100 km south of the town of Antofagasta and 16 km from the Pacific Ocean. This region of Chile is know for its rich mineral resources. One of these, Nitrates, are mined by surface, strip mining causing extensive area air pollution. To estimate the effects of a Nitrate mine which may be started 25 km SSE from Cerro Paranal a model for this pollution, based on estimates of aerosols resulting from similar mines north of Antofagasta, was developed. These estimates are based on measurements of the solar aureole brightness. The expected deteriorating effects on atmospheric extinction, sky thermal emissivity and night-time sky brightness appear acceptable.
To detect faint signals in the presence of a high background, differential techniques are often used. In IR astronomy this has led to the so-called chopping and nodding techniques. The introduction of array detectors especially for imaging in the thermal IR region of the astronomical spectrum requires an adaptations of these techniques which also takes into account the pixel nonuniformity of the array detector. I describe one possible imaging algorithm and the associated requirements for the chopping mechanisms of an 8-m telescope.
Heating of the air in astronomical telescopes is known to have a deteriorating effect on the image quality. Main heat sources are the primary mirror and localized electrical components in or near the light beam. To evaluate the effects of these localized heat sources we measured the image quality deterioration at the focus of the ESO-La Silla 2.2 meter telescopes with variable amounts of heating of a dummy electronics box and of a bar simulating one of the secondary mirror spiders. The effects on the FWHM of the image profile turned out to be remarkably small. Most of the effect of the heating showed up in the removal of energy of the core into the far wings of the image profile. For a heat input of 560 Watts (resulting in an excess temperature of 60 K) the amount of energy removed amounted to 13%. This behavior can be explained by a model in which the heating destroys the wave-front over only a part of the aperture while leaving it unaffected over most of the aperture. With such a model we predict the amount of energy removed by a 500 Watt heat source in an 8 meter telescope to be only 1%.
In this paper I review shortly the status of high angular resolution imaging with astronomical telescopes and arrays of telescopes. Emphasis is given on the topics to be discussed in the Satellite Meeting to this conference on "Active and Adaptive Optics" in Garching, Germany.
The factors contributing to fringe contrast decrease for telescopes working near their diffraction limits are summarized. These factors include variations with time, such as atmospheric variations, vibrations, pathlength drift, and fringe tracking noise; and variations accross the pupil; variation with wavelength and factors relating to polarization effects; unequal beam intensities; detector resolution; and pupil transfer geometry. The effects on multispeckle images are also considered. The resulting error budget for the Very Large Telescope Interferometer (VLTI) is derived. It is concluded that the total random error in the fringe contrast is 4.2. The total calibratable systematic error amounts to 34 percent (27 percent due to the instrument, 9 percent due to the atmosphere).
The Very Large Telescope, being build by the European Southern Observatory, has an
interferoetric node in which the light of the four 8 eter aperture telescopes, and of
a number of smaller telescopes, will be combined coherently to give iilliarcsecond level
angular resolution at optical wavelengths on very faint objects. This paper describes
current plans to impleMent this mode.
The application of full adaptive optics to astronomical telescopes in the foreseeable future is likely to be limited to infrared wavelengths greater than 1 micron both because of the limited number of bright enough wavefront sensing objects at visible wavelengths and because of the complexity and expense of making an adaptive optics system for large telescopes with the large number of elements required at visible wavelengths. Adaptive optics designed for infrared wavelengths do, however, improve the image quality at wavelengths shorter than the design wavelength, thus improving the sensitivity of interferometric imaging at those wavelengths.
This paper discusses the requirements posed on the ESO Very Large Telescope (VLT) Interferometer by the applications that require a field-of-view larger than the Airy disk of the individual telescopes. The most essential requirement for such wide field-of-view use of interferometric arrays is the maintenance of the pupil configuration, which applies to all the details of this configuration. Not meeting this requirement leads to path-length differences among the rays of each of the telescopes composing the array. An error budget for the optical design parameters of the VLT Interferometer is derived.
The Very Large Telescope Interferometer (VLTI) is one of the operating modes of the VLT. In addition to consisting of the four stationary 8-meter-diameter telescopes, it includes a number of movable Auxiliary Telescopes which both complement the (u,v) plane coverage of the large telescopes and provide a powerful interferometric facility by itself (available 100 percent of the time). The current plans for the implementation of the VLTI are described. These plans will be finalized after the choice of the VLT site in 1990.