Robert Hammerschlag, Sander Deelen, Pieter Hoogendoorn, Johannes N. Kommers, Thomas Sonner, Roberto Simoes, Olivier Grassin, Andreas Fischer, Simon Visser, Kristof Thewissen
These open-foldable very light-weight domes, based on very strong textile membranes highly tensioned between steel bows, are designed for bad-weather protection and maintenance of instruments for astronomical, meteorological and civil-engineering measurements and have extremely high wind stability. The domes of the GREGOR telescope and the Dutch Open Telescope are the two existing prototypes. Improvements were developed with all parts light-colored to remain cool in solar light. The new specially made connection parts (eyes) between the textile parts are made from white-colored PETP, a very strong and UV-stable synthetic, and have a better geometrical shape giving higher stability. The rubber seal tubes on top of the dome were of black-colored chloride rubber CR (neoprene), strong and UV stable, but very warm in sunlight. New UV-stable EPDM rubber tubes were produced in natural light color. To get this rubber stiff enough to give good sealing, a black-colored stiff EPDM rubber is put inside the light-colored one. Tests were performed and the forces necessary for compression of the rubber tubes were measured. An inside black tube with a circa 1.3 times larger compression force than the original black tubes was applied. The assembling of the black tubes into the light-colored tubes was successfully applied at the DOT and GREGOR domes.
Movies with fields-of-view larger than normal, for high-resolution telescopes, will give a better understanding of processes on the Sun such as filament and active region developments and their possible interactions. New active regions can serve as an igniter of the eruption of a nearby filament. A method to create a large field-of-view is to join several fields-of-view into a mosaic. Fields are imaged quickly, one after another, using fast telescope-pointing. Such a pointing cycle has been automated at the Dutch open telescope (DOT), a high-resolution solar telescope located on the Canary Island La Palma. The number and positions of the subfields are calculated automatically and represented by an array of bright points in the guider image which indicates the subfield centers inside the drawn rectangle of the total field on the computer screen with the whole-sun image. Automatic production of flats is also programmed. For the first time, mosaic movies were programmed from stored information on automated telescope motions. The mosaic movies show larger regions of the solar disk in high resolution and fill a gap between available whole-sun images with limited spatial resolution of synoptic telescopes including space instruments and small-field high-cadence movies of high-resolution solar telescopes.
Movies with fields-of-view larger than normal for high-resolution telescopes will give a better understanding of
processes on the Sun, such as filament and active region developments and their possible interactions. New active
regions can influence, by their emergence, their environment to the extent of possibly serving as an igniter of the
eruption of a nearby filament. A method to create a large field-of-view is to join several fields-of-view into a mosaic.
Fields are imaged quickly one after another using fast telescope-pointing. Such a pointing cycle has been automated at
the Dutch Open Telescope (DOT), a high-resolution solar telescope located on the Canary Island La Palma. The observer
can draw with the computer mouse the desired total field in the guider-telescope image of the whole Sun. The guider
telescope is equipped with an H-alpha filter and electronic enhancement of contrast in the image for good visibility of
filaments and prominences. The number and positions of the subfields are calculated automatically and represented by an
array of bright points indicating the subfield centers inside the drawn rectangle of the total field on the computer screen
with the whole-sun image. When the exposures start the telescope repeats automatically the sequence of subfields.
Automatic production of flats is also programmed including defocusing and fast motion over the solar disk of the image
field. For the first time mosaic movies were programmed from stored information on automated telescope motions from
one field to the next. The mosaic movies fill the gap between whole-sun images with limited resolution of synoptic
telescopes including space instruments and small-field high-cadence movies of high-resolution solar telescopes.
The completely open-foldable dome of the GREGOR telescope is a further development of the DOT dome, respectively
9 and 7 meter in diameter. New technical developments are implemented and tested at the GREGOR dome, that are
important for the design of the much larger dome for the EST, which will be 28 meter in diameter. The GREGOR dome
is the first with more than one clamp working simultaneously for closing the dome and bringing the membranes on the
required high tension for storm resistance. The storm Delta with 245 km/h 1-minute mean maximum at the location of
the GREGOR gave no problems nor did the storms afterwards. Opening and closing experiences are up to wind speeds
of 90 km/h without problems. Good observing circumstances never occur with higher wind speeds. A double layer of
membranes is applied in the GREGOR construction whereas the DOT dome is equipped with a single layer.
Simultaneous climate measurements inside and outside the dome have proven the thermal-insulation capability of this
double-layer construction. The experiences with the GREGOR showed that the elongation by tensioning of the prestrained
membrane material is much lower than originally expected. In the meantime, more strong and stiff membrane
material is available and applied in the EST design. As a consequence, the clamps of the EST can have a relatively much
shorter length and there is no need anymore for simultaneous operation of the clamps and the main actuators in low
speed with help of a frequency inverter. The clamps can close after the main bow operation is finished, which simplifies
the electrical control.
There is growing interest in measuring seeing at existing and prospective telescope sites. Several methods exist to quantify
seeing, one among them is by measuring the scintillation of solar or lunar light using a photodiode. A shadow band
ranger (SHABAR) analyses the covariance of the signals from an array of such photodiodes, which allows for the spatial
resolution of the index of refraction above the SHABAR device. This allows one to estimate the index of refraction
structure parameter as a function of height, C2n(h).
Although a SHABAR has a limited range compared to a differential image motion monitor (DIMM) or the latest
wavefront sensors, the advantage is that it does not need telescope optics to work. A SHABAR device can be made very
compact and can operate independent of other instruments. We describe the design of such a SHABAR device with six
photodiodes that can operate virtually indefinitely without requiring human intervention.
An inversion algorithm is used to convert the raw scintillation signals of the photodiodes to the desired C2n(h) profile
and a value for the Fried parameter r0 at height zero. We show that it is possible to perform inversions of 10 s periods in
real time on relatively low-end hardware, such as an Intel Atom based computer, which allows the results to be presented
live to astronomers, who can use this information to help make decisions about their observation schedule.
EST (European Solar Telescope) is a 4-m class solar telescope, which is currently in the conceptual design phase. EST
will be located in the Canary Islands and will aim at high spectral, spatial and temporal resolution observations in the
photosphere and chromosphere, using a suite of instruments that can produce efficiently two-dimensional
spectropolarimetric information of the thermal, dynamic and magnetic properties of the plasma over many scale heights.
The pier is defined as the construction that supports the telescope and the enclosure. It needs a certain height to minimize
daytime ground turbulence. At the bottom of the pier a large instrument lab is located, 16 m in diameter and 10 m high.
To the pier is attached a service building that accommodates all auxiliary services, possibly together with a separate
building.
Solid concrete- and open framework piers are compared, in terms of stability, thermal properties and flow characteristics
and building structures in terms of construction issues. FE and CFD analysis are used to give qualitative insight in the
differences between the alternatives. The preferred alternative is a cone shaped pier surrounded by an open framework.
As part of a larger project for measuring various aspects of foldable domes in the context of EST and with support of the
Dutch Technology Foundation STW, we have collected over a year of continuous temperature and humidity measurements,
both inside and outside the domes of the Dutch Open Telescope (DOT) on La Palma5 and the GREGOR telescope on
Tenerife.6 In addition, we have measured the wind field around each dome.
Although the structure of both domes is similar, the DOT dome has a single layer of cloth, and is situated on top of an
open tower. In contrast, the GREGOR dome has a double layer of cloth, and is situated on top of a tower-shaped building.
These differences result in large differences in temperature and humidity insulation when the dome is closed. We will
present the changes in temperature and humidity one can expect for each dome within one day, and the statistics for the
variations throughout a year.
In addition, we will show that the main advantage of a foldable dome is the near instantaneous equilibration of the air
inside the volume originally enclosed by the dome and that of the environment outside the dome. This property allows
one to operate a telescope without needing expensive air conditioning and dome skin temperature control in order to limit
dome and shell seeing effects.
The measurements give also information about the weather fluctuations at the sites of the domes. It was observed that
on small time scales the temperature fluctuations are significantly greater during the day than during the night.
The European Solar Telescope (EST) is a 4-m class solar telescope, which is currently in the conceptual design phase.
EST will be located in the Canary Islands and aims at observations with high spectral, spatial and temporal resolution of
the solar photosphere and chromosphere.
The main purpose of the enclosure is to protect the telescope and instruments from severe weather conditions. An
enclosure is also often needed for reducing wind buffeting on the telescope and primary mirror cell, but on the other
hand enclosures are generally considered to degrade local seeing. In this contribution we will present the conceptual
design of the enclosure for EST. Two different concepts have been studied in more detail: the first being a dome concept
with vent gates to enhance local flushing, the other being a retractable enclosure, with an optional windshield.
Technically both alternatives seem feasible, but we conclude that the retractable enclosure is the less risky solution, since
it allows easier local seeing control and allows the use of a reflecting heat stop in the primary focus. A windshield is
effective in reducing wind load on the primary mirror; although preliminary analysis indicate that there are feasible
solutions to keep the deformation caused by wind buffeting within the requirements.
In the context of the EST design study for a 4m-class solar telescope and a study for large open-foldable domes of the
Dutch Technology Foundation STW, a design is made for the 20 to 30m diameter range. Detailed designs are made for
three specific diameter sizes: 23, 28 and 33m. Smaller-size open-foldable domes based on tensioned cloth and in use at
the Dutch Open Telescope (7m) and the GREGOR (9m) have proven to be all-weather stable and very effective for good
seeing conditions for solar telescopes. The cloth has shown no degradation over the past 14 (DOT) resp. 6 (GREGOR)
years of experience and no permanent elongation with the frequent de-tensioning and tensioning during opening and
closing. The application of cloth permits a dome design leaving, when opened, the telescope completely free without any
structure over the telescope and no massive structures besides or under it. Basis for the new design is the available prestretched
stable cloth, which is nowadays produced in much stronger qualities than used for DOT and GREGOR. The
larger curvature radius requires larger tension in the cloth, but combination with stronger cloth fits for the upscaling.
Calculations show that the steel construction geometries of the GREGOR dome can be upscaled with a few adjustments.
Bearings and drives remain within normal sizes. Cost calculations show that open-foldable domes of this size are
remarkably lower in price than closed domes. In addition, an interesting option is presented for a semi-transparent
windshield of which the position can be adapted to the wind direction. This shield gives an effective wind protection of
the region around the primary mirror without disturbing the wind flows above the shield and without stagnant air or big
eddies behind it. It is storm safe and the costs are only a fraction of the open-foldable dome costs.
A wide-field birefringent filter for the barium II line at 455.4nm is developed in Irkutsk. The Barium line is excellent for
Doppler-shift measurements because of low thermal line-broadening and steep flanks of the line profile. The filter width
is 0.008nm and the filter is tunable over 0.4nm through the whole line and far enough in the neighboring regions. A fast
tuning system with servomotor is developed at the Dutch Open Telescope (DOT). Observations are done in speckle
mode with 10 images per second and Keller-VonDerLühe reconstruction using synchronous images of a nearby bluecontinuum
channel at 450.5nm. Simultaneous observation of several line positions, typically 3 or 5, are made with this
combination of fast tuning and speckle. All polarizers are birefringent prisms which largely reduced the light loss
compared to polarizing sheets. The advantage of this filter over Fabry-Perot filters is its wide field due to a large
permitted entrance angle and no need of polishing extremely precise surfaces. The BaII observations at the DOT occur
simultaneously with those of a fast-tunable birefringent H-alpha filter. This gives the unique possibility of simultaneous
speckle-reconstructed observations of velocities in photosphere (BaII) and chromosphere (H-alpha).
Weather effects on foldable domes, as used at the DOT and GREGOR, are investigated, in particular the correlation between
the wind field and the stresses caused to both metal framework and tent clothing. Camera systems measure contactless the
displacement of several dome points. The stresses follow from the measured deformation pattern. The cameras placed
near the dome floor do not disturb telescope operations. In the set-ups of DOT and GREGOR, these cameras are up to 8
meters away from the measured points and must be able to detect displacements of less than 0.1 mm. The cameras have
a FireWire (IEEE1394) interface to eliminate the need for frame grabbers. Each camera captures 15 images of 640 × 480
pixels per second. All data is processed on-site in real-time. In order to get the best estimate for the displacement within the
constraints of available processing power, all image processing is done in Fourier-space, with all convolution operations
being pre-computed once. A sub-pixel estimate of the peak of the correlation function is made. This enables to process the
images of four cameras using only one commodity PC with a dual-core processor, and achieve an effective sensitivity of
up to 0.01 mm. The deformation measurements are well correlated to the simultaneous wind measurements. The results
are of high interest to upscaling the dome design (ELTs and solar telescopes).
In the near future ELTs (Extreme Large Telescopes) will be built. Preferably these telescopes should operate without
obstructions in the near surrounding to reach optimal seeing conditions and avoid large turbulences with wind-gust
accelerations around large obstacles. This applies also to future large solar telescopes. At present two foldable dome
prototypes have been built on the Canary Islands: the Dutch Open Telescope (DOT, La Palma) and the GREGOR
Telescope (Tenerife), having a diameter of 7 and 9 meter, respectively. The domes are usually fully retracted during
observations. The research consists of measurements on the two domes. New camera systems are developed and placed
inside the domes for precise dome deformation measurements within 0.1 mm over the whole dome size. Simultaneously,
a variety of wind-speed and -direction sensors measure the wind field around the dome. In addition, fast sensitive air-pressure
sensors placed on the supporting bows measure the wind pressure. The aim is to predict accurately the expected
forces and deformations on up-scaled, fully retractable domes to make their construction more economically. The
dimensions of 7 and 9 meter are large enough for realistic on-site tests in gusty wind and will give much more
information than wind tunnel experiments.
Two prototypes of fully retractable enclosures with diameters of 7 and 9 m have been built for the high-resolution solar
telescopes DOT (Dutch Open Telescope) and GREGOR, both located at the Canary Islands. These enclosures protect
the instruments for bad weather and are fully open when the telescopes are in operation. The telescopes and enclosures
also operate in hard wind. The prototypes are based on tensioned membrane between movable but stiff bows, which fold
together to a ring when opened. The height of the ring is small. The prototypes already survived several storms, with
often snow and ice, without any damage, including hurricane Delta with wind speeds up to 68 m/s. The enclosures can
still be closed and opened with wind speeds of 20 m/s without any problems or restrictions. The DOT successfully
demonstrated the open, wind-flushing concept for astronomical telescopes. It is now widely recognized that also large
future telescopes benefit from wind-flushing and retractable enclosures. These telescopes require enclosures with
diameters of 30 m until roughly 100 m, the largest sizes for the ELTs (Extreme Large Telescopes), which will be built in
the near future. We discuss developments and required technology for the realization of these large sizes.
It was in the years around 1970 that during site-test campaigns for JOSO masts were erected up till 30 m height with
sensors at several heights for the measurement of temperature fluctuations. Cornelis (Kees) Zwaan discovered that the
fluctuations decrease drastically at heights from about 15 m and upward when there is some wind. The conclusion from
this experience was the open telescope principle: the telescope should be completely free in the air 15 m or more above
the ground. The Dutch Open Telescope (DOT) was the pioneering demonstrator of the open-telescope technology. Now
that larger high-resolution telescopes come in view, it is time to analyze again the principle: (i) the essentials for proper
working of the open principle; (ii) the differences with nighttime observations particularly concerning the seeing; (iii)
the design consequences for the new generation of high-resolution solar telescopes.
High-resolution telescopes require a mechanical stability of fractions of an arc second. Placing such a telescope on top of a tower will improve the local seeing. An open transparent tower of framework minimizes the upward, temperature disturbed air flow. The tower platform has to be extremely stable against rotational motions, which have to be less than fractions of an arc second, unusual in mechanical engineering. Active systems can improve the stability. However, they need sensors for position measurements, active actuators and a control loop. The performance is limited by the available signal-to-noise ratio. Consequently, improvement of the passive stability of large tower structures will significantly contribute to the final stability. Special geometries in steel framework can reach extreme passive stability of a tower platform, particularly against rotational motions. There are several groups of basic geometries, which lead to solutions and we will give a systematic description. The proposed towers can be welded or screwed together from smaller parts. This makes a construction in adverse environments like the Antarctic region within good reach.
The 1-meter Swedish Solar Telescope (SST) obtains images of the solar surface with an unprecedented resolution of
0.1 arcsec. It consists of a relatively slender tower with on top only the vacuum turret for reflecting downward the solar
beam and no protective dome. This is a favourable situation to get good local seeing. Just in the case of some wind,
seeing is best for daytime observations, therefore the precision bearings and drives of the elevation- and azimuth axis
of the turret have to be stiff against wind. This requires line contact between the meshing teeth of the large gear wheel
and the pinion. High preload forces to achieve line contact are not allowed because of appearing stick-slip effects. To
reduce the risk on stick-slip a special design of the teeth for high stiffness combined with low friction and smooth
transition from one tooth to the next was made. Furthermore, extreme precision in the fabrication was pursued such that
relatively small contact forces give already line contact. This required a special order of the successive fabrication steps
of the combination of bearing and gear teeth. An additional problem was the relatively thin section of the bearings
required for a compact turret construction, needed for best local seeing and minimum wind load. Solutions for all these
problems will be discussed. For the large gears the exceptional good DIN quality class 4 for the pitch precision and
straightness plus direction of the teeth faces was achieved.
KEYWORDS: Telescopes, Space telescopes, Optical instrument design, Motion models, Control systems design, 3D modeling, Device simulation, Adaptive optics, Domes, Control systems
Studies by Mark Swain and a colleague at the Max Planck Institut fur Astronomie, coupled with results from past and ongoing projects at Harvey Mudd College, strongly suggest that it may be possible to achieve imaging performance comparable to the Hubble Space Telescope at relatively low cost using available, commercial products. This is achievable by placing a 2.4 m telescope, with readily available adaptive optics, on a 30 m tower located at a high-elevation geological "dome" in Antarctica. An initial project surveyed relevant tower design approaches, then generated and evaluated six concept designs for telescope towers. Using data for typical and extreme wind at Dome C to generate wind loads, finite element analysis yielded lateral deflections at the top of 0.3 mm for typical winds and 12.1 mm for extreme gusts, with the lowest resonant frequency at 0.7 Hz; some tower concepts are innovative and allow for easy shipment, setup, and relocation. A subsequent project analyzed a tower designed by Hammerschlag and found fundamental resonance frequencies at 4.3 Hz for bending and 5.9 Hz for torsion; this project also designed and simulated an active telescope control system that maintained 17 milliarcsecond pointing error for the telescope atop the tower during typical wind conditions.
The Dutch Open Telescope (DOT; http://dot.astro.uu.nl) on La Palma is a revolutionary open solar telescope, on an excellent site, on top of a transparent tower of steel framework, and uses natural air flow to minimize local seeing. The DOT is a high-resolution multi-wavelength imager capable of long-duration time series aiming at magnetic fine structure, topology and dynamics in the photosphere and low- and high chromosphere. In this paper we describe the latest addition to the multi-wavelength imaging system: a Lyot H-alpha camera channel operating at a wavelength of 656.3 nm, being of major interest for high-chromospheric phenomena. The channel is operated strictly synchronous with the other channels and all data are speckle reconstructed. The channel permits profile sampling and delivers Dopplergrams in a 15 second time cadence, up to several hours long and adding up to a total data amount of 1.6 Terabyte/day. A dedicated computer (DSP, DOT Speckle Processor) has been built for processing the data overnight.
KEYWORDS: Mirrors, Telescopes, Solar telescopes, Point spread functions, Adaptive optics, Space telescopes, Diffraction, Off axis mirrors, Optical components, Modulation transfer functions
A concept is presented for an extremely large high-resolution solar telescope with an aperture of 11 m and diffraction limited for visual wavelengths. The structure of GISOT will be transparent to wind and placed on a transparent stiff tower. For efficient wind flushing, all optics, including the primary mirror, will be located above the elevation axis. The aperture will be of the order of 11 m, not rotatively symmetrical, but of an elongated shape with dimensions 11 x 4 m. It consists of a central on-axis 4 m mirror with on both sides 3 pieces of 2 m mirrors. The optical layout will be kept simple to guarantee quality and minimize stray light. A Coudé room for instruments is planned below the telescope.
The telescope will not be housed in a dome-like construction, which interferes with the open principle. Instead the telescope will be protected by a foldable tent construction with a diameter of the order of 30 m, which doesn’t form any obstruction during observations, but can withstand the severe weather circumstances on mountain sites.
Because of the nature of the solar scene, extremely high resolution in only one dimension is sufficient to solve many exciting problems in solar physics and in this respect the concept of GISOT is very promising.
The Dutch Open Telescope (DOT; http://dot.astro.uu.nl) on La Palma is a revolutionary open solar telescope, on an excellent site, on top of a transparent steel tower, and uses natural air flow to minimize local seeing. The aim is long-duration high-resolution imaging with a multi-wavelength camera system. In order to achieve this, the DOT is equipped with a diffraction limited imaging system and uses the speckle reconstruction technique for removing the remaining atmospheric turbulence. The DOT optical system is simple and consists currently of a 0.45m/F4.44 parabolic mirror and a 10x enlargement lens system.
We present our plans to increase the aperture of the DOT from 0.45m to 1.4m. The mirror support and telescope top shall be redesigned, but telescope, tower, multi-wavelength camera system and speckle system remain intact. The new optical design permits user selectable choice between angular resolution and field size, as well as transversal pupil shift introducing the possibility to use obstruction free apertures up to 65cm. The design will include a low order AO system, which improves the speckle S/N substantially during moderate seeing conditions.
The design characteristics of a large open telescope (LOT) are: (i) an open tower with only pure translations of the platform under wind load; (ii) an open telescope construction with extremely stiff geometry and drives; (iii) simple optics with easy aligning and testing, but nevertheless suitable for large auxiliary equipment like spectrographs.
The Dutch Open Telescope (DOT) is an innovative solar telescope, completely open, on an open steel tower, without a vacuum system. The aim is long-duration high resolution imaging and in order to achieve this the DOT is equipped with a diffraction limited imaging system in combination with a data acquisition system designed for use with the speckle masking reconstruction technique for removing atmospheric aberrations.
Currently the DOT is being equipped with a multi-wavelength system forming a high-resolution tomographic imager of magnetic fine structure, topology and dynamics in the photosphere and low- and high chromosphere. Finally the system will contain 6 channels: G-band (430.5 nm), Ca II H (K) (396.8 nm), H-α (656.3 nm), Ba II (455.4 nm), and two continuum channels (432 and 651 nm).
Two channels are in full operation now and observations show that the DOT produces real diffraction limited movies (with 0.2" resolution) over hours in G-band (430.5 nm) and continuum (432 nm).
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