Ground vs. Space Telescopes
Ground Telescope: Design Optimization Challenge
LBT Project as a Response to the Challenge
Examples of LBT Science from Cosmology to Planet Formation
Simulated LBT Images of Jupiter's moon Io
Almost all astronomical research and discovery prior to 1950 was made by optical telescopes on the ground.
The leading observational advances in the period of 1900-1950 came from telescopes located in the southwestern
United States using photographic methods.
After World War II, the technologies developed for radar and rocketry were applied to astronomy, opening the whole electromagnetic spectrum to observation, as well as allowing direct exploration of the solar system. These advances, along with the development of electronic imaging, revolutionized astronomy at all wavelengths. However, no significant advances were made in ground-based telescope design; no telescopes more powerful than the Palomar 200-inch, completed in 1947, were built.
Today the pace of technological advances in space has slowed; future instruments in space, more powerful than the Hubble Space Telescope (HST) or similar observatories operating at other wavelengths, will be very expensive and slow in coming. It has been realized worldwide that, for optical and infrared observations, new advanced mountaintop telescopes offer huge potential for future discovery, at a much smaller cost. Light grasp more than an order of magnitude larger than HST can be obtained from the ground, allowing detailed analysis of fainter, more distant sources. Moreover, taking advantage of the latest sensors and fast computers adaptive optics allows the potential of correcting atmospheric blurring and enormously improving image sharpness to a level far exceeding that of the HST. Mountaintop telescopes are entering a new golden era.
The image above shows the spiral galaxy NGC 4535 moved to a redshift of 1.0 and synthetically imaged with various telescopes at a wavelength of 2 microns. The upper left frame shows the ground-based seeing limit under excellent conditions (0.4 arcsec). The upper right frame is the galaxy imaged by HST (a 2.4 meter telescope) with 0.2 arcsec resolution. The lower two frames show how the galaxy will appear with LBT. The lower left frame gives the appearance of the galaxy with full adaptive correction of a single 8.4 meter mirror (0.06 arcsec diffraction limit), while the lower right frame shows the detail attainable with a near infrared beam combiner which provides the diffraction limit of the full 22.8 meter baseline (0.02 arcsec). These images were provided by Drs. Hans-Walter Rix and Tom Herbst of MPIA Heidelberg who are part of a team studying the near infrared beam combiner design.
In attempting to optimize the performance of ground based telescopes, the following design drivers
were adopted and resulted in the LBT concept:
● Large total collecting area for sensitivity on faint objects.
● High imaging quality through passive, active and adaptive control of seeing
("seeing" is the term used by astronomers to describe the atmospheric
blurring of stellar images) and other aberrations.
● Clean, thermal infrared design to minimize background radiation.
● Versatile instrumentation and rapid interchange of optical configuration to
take maximum advantage of specific observing conditions.
● High ultimate spatial resolution by providing a relatively long baseline.
● Low construction and operations costs.
The Large Binocular Telescope will, thus, have a collecting area larger than any existing or planned single telescope.
It will provide unmatched sensitivity for the study of faint objects.
More important, the configuration allows essentially complete sampling of all spatial frequencies in the image up to 22.8 meters using interferometric imaging between the two 8.4-m pupils. This provides unique capabilities for high resolution imaging of faint objects; in the near infrared LBT will exceed the HST at its optimum wavelength by a factor of three. When combined with adaptive optics, the LBT interferometric mode offers high signal-to-noise imaging on even the faintest objects, over a relatively wide field.
The LBT design, with its "fast" focal ratio primary mirrors, permits very low obscuration from secondary structures, etc., and hence unrivaled thermal infrared performance. The optical configuration also permits incorporation of a one degree field of view (well matched to array detectors at f/4, for wide field imaging and multi-object spectroscopy at visible wavelengths. The LBT is unique among the very large telescopes (aperture of approximately 8-m or larger) in providing such performance.
The binocular structure will also provide a natural way of storing the secondary mirrors on-board the telescope and hence of interchanging them rapidly (10-15 minutes) to take advantage of changing observing conditions.
Finally, the revolutionary optical design, especially the f/1 borosilicate honeycomb primary mirrors, permit the entire LBT to be housed in a very compact and, hence, low cost structure. Compared to other designs such as Keck and the VLT, the rigid, honeycomb primaries also permit simple (and hence low operating cost) mirror support systems. LBT is the most cost effective project of the current generation of large telescopes under construction in terms of cost per collecting area (the primary measure of power in a telescope). LBT will cost $660,000 (1989 dollars) per square meter of collecting area. Other 8 to 10-m class, ground based telescope projects have or will cost significantly (two to four times) more while providing less capability per square meter.
In short the LBT offers a very wide range of superlative performance characteristics (sensitivity, spatial resolution, wide field) for low capital and operating cost. It is applicable to almost all areas of astronomical research; some typical examples are given below.
Creating Pictures Showing Fine Angular Detail
Since the high angular resolution faint object imaging capability is such a key element in the LBT project, some further
discussion of this aspect of the design is given below.
Measuring the finest distinguishable angular detail requires the telescope to be used with beam-combining of the light collected from the separate mirrors. However, the detail that can be seen depends on the rotation of the sky with respect to the baseline provided by the telescope and on the full range of possible baselines covered. These two criteria lead to the desired condition called "full coverage of the UV plane".
At the University of Arizona, the technique of "tomographic" image reconstruction has been explored using simulated observations at just three position angles (just as in a medical "CAT" scan, an image of a slice of the brain is obtained by looking through the head from different directions). These show that it is possible to reconstruct images from the LBT with the angular resolution of a 22.8- meter telescope.
The gains are illustrated in this figure to the left, which shows at top left an image as it would be resolved with a
22.8-meter telescope. At the bottom left, this picture is seen as it would be seen with a single eight-meter telescope.
The loss of all the detail is very obvious. At the bottom right the same picture appears as observed with one position
angle at the LBT. It shows how the LBT places fringes on each point-like portion of the image. When we combine pictures
taken with these fringes at three different angles, the fringes cross and give information about the exact placement of
the point of light, distinguishing other points of light close to it. It is the crossings of these fringes that allow us
to reconstruct a high resolution image. The reconstructed image is shown at top right. Comparison of it with the image
yielded by a 22.8-meter telescope shows that the LBT provides the same detail. The improvement over the performance of a
single eight-meter telescope is shown dramatically by comparing the top right and bottom left images.
Another strength of the LBT design is the large field of view that can be imaged with optimal quality. Imaging with the other telescopes is limited to a much smaller patch of sky because in general the telescope baseline is not perpendicular to the direction of the incoming light. The net result is a requirement for complex path length correction optics which dramatically limit the field of view. Thus the effective LBT field is limited only by the atmosphere (approximately 2 arc minutes at 2.2 microns) while that of the other telescopes is restricted to a few arcseconds.
While the mirror spacing in the VLT and Keck telescope arrays is larger, they do not have full coverage of the UV plane, so imaging will always be ambiguous. On the other hand, the apertures are wider apart than at the LBT, so for some specialized work the other two telescopes will achieve higher resolution. The most likely examples are objects with simple structure such as binary stars in which the only quantities to be measured are separation and relative brightness. Thus, the LBT will be unique in making high resolution true images even on faint objects while the VLT and Keck pair will complement this capability by extending the angular resolution to simple structures.
The study of phenomena at early epochs in the expansion of the Universe has been a major driver for astronomy since
early in this century. Slipher and Hubble first discovered the expansion and Einstein, Friedman, Lemaitre and de Sitter
interpreted it in terms of General Relativity. Indeed the Palomar 200-inch telescope was originally built to address
this problem and to determine the value of Hubble's constant (the proportionality between distance and velocity which
determines the age of the universe) and of the deceleration parameter (which measures the amount of matter or energy in
the universe). This goal was not realized because the galaxies were found to manifest substantial evolution in their
properties - an important discovery in itself. The "Palomar generation" of telescopes also led to the discovery of
quasars, the demonstration that the luminous material in the universe is not uniformly distributed even on scales as
large as several hundred million light-years, and that most of the material in the Universe could not be observed directly
(the problem of the "missing mass"). Also connected with these questions is the problem of the origin of the chemical
elements which, apart from hydrogen and helium, were formed almost entirely after matter began to condense into discrete
objects such as galaxies and stars. This latter process was already well advanced by the earliest epochs accessible to
The LBT will play a major role in solving these problems, which in essence address the questions of how the material content of the universe evolved from the postulated uniform distribution of the "hot big bang" to the current distribution of galaxies, stars and planets of composition capable of supporting life. The stage was set at such early epochs that the important processes are beyond the reach of current telescopes. In its wide field mode the LBT will permit the identification of galaxies in the process of formation and its multifiber spectroscopy mode will allow analysis of their composition and radial motion. The development of the "cluster and void" distribution of present day galaxies can then be documented and will in turn provide insight into the nature of the "missing matter" which determines this evolution through its gravitational effects. The results may have important implications for particle physics as well as cosmology. The high spatial resolution capability will permit study of galaxy morphology and hence of the way galaxies form and the role of rotation in the process. This capability will also permit study of the structure of galaxy nuclei and the giant black holes which seem to power the quasars and radio sources.
Star and Planet Formation:
Observations with existing telescopes have shown that star (and presumably planet) formation is a continuing process within dense interstellar clouds in our own and other galaxies. However, very little is known about the nature of the process, especially the role of angular momentum, magnetic fields and initial turbulence on fragmentation (the final mass spectrum), the formation of pre-planetary disks, and the formation of binary stars. The difficulties arise both because of the obscuration of visible light by interstellar dust in star forming regions and because, during the stages of interest, the condensations are sufficiently cool that they are quite faint and radiate mainly in the infrared, where the resolution and sensitivity of existing telescopes is simply too low to solve the problems. The LBT will provide almost an order of magnitude (factor 10) gain in both quantities and should permit major advances to be made. Some examples are given below:
Presence of low mass stars and substellar objects in star forming regions set important observational constraints on the mechanisms for fragmentation of interstellar clouds and the accretion processes leading to star formation. Searches for these objects have been frustrated by their intrinsically low luminosity and low surface temperatures. However, recent infrared imaging has revealed possible members of young, embedded clusters that appear to be very low mass stars or brown dwarfs. Such observations begin to fill in the missing link between stars and planets and also bear on the missing mass problem. These faint sources are currently beyond the limit of infrared spectroscopy, which is required to establish conclusively their nature. The large aperture of the LBT together with the improved image quality afforded by adaptive optics will allow, in the nearest embedded clusters, study of these faint sources.
Very young, solar mass stars often appear to be surrounded by a disk of gas and dust that may eventually evolve into a planetary system. The mid-infrared emission of this disk dominates the output of the system for the first few million years, but then fades as the material is expelled by stellar winds (and possibly partially condenses into planets). Recent HST imaging in the Orion nebula has confirmed the existence of a few such disks, but most such systems are so rich of dust that they are only accessible in the near and mid infrared. There is an urgent need for high spatial resolution images of these disks in the infrared. The closest embedded cluster suitable for such studies is the Rho Ophiuchi cloud, where the diffraction limit of the LBT is such as to permit observations of great scientific interest. Several other isolated, very young systems that also show evidence for preplanetary disks become accessible to study by the LBT. Thus, the LBT is capable of resolving preplanetary disks in the first stages of evolution, and on a scale that would probe the subsequent formation of planetary systems similar to the solar system.
The Direct Detection of Planets Orbiting Nearby Stars:
One of the long standing questions in astronomy has been whether planets exist around other stars. Since 1995 using indirect methods (Doppler velocity techniques) astronomers have detected 17 cases of planetary systems around other stars. The next step is to make direct detections, i.e., to obtain images of these planets as the orbit their parent stars. Thus far, direct observations of planets have been impossible because of the presence of the very bright stellar image blurred by atmospheric "seeing". Since such a Jupiter-like planet would be roughly a billion times fainter than the star, it would be impossible to detect using traditional techniques.
The LBT, equipped with adaptive optics (atmospheric blurring compensation) and fully exploiting the expected superb quality of its mirror surface (already achieved in 3.5-m mirrors), will permit formation of images of sufficient sharpness (diffraction-limited) that the planet could be detected against only a low surface brightness halo of residual scattered light. In this manner, a Jupiter-like planet could be detected, if present, around some fifty of the nearest stars. The interferometric mode will enhance the planet/background contrast even further, thus increasing the number of candidate stars and the sensitivity of the survey. The direct detection of such a planet would surely be counted as one of the major steps forward in determining the likelihood of life existing elsewhere in the Universe and in understanding our place in it.
Jupiter's Moon Io
The simulated image to the left was created by E. K. Hege and J. R. P. Angel as an example of the kind of image LBT would
make of a familiar object. Their starting point was a Galileo spacecraft image of Jupiter's moon Io taken in visible light.
Io is one of Jupiter's Galilean moons and has more volcanoes than any other world.
The upper image "a" is a simulated LBT image created by convolving the spacecraft image with the LBT point spread function. This image corresponds to a single LBT exposure. The resolution was modeled for wavelengths of 0.8, 1.25 and 1.65 microns in the near-infrared (displayed here as blue, green and red). Because of the binocular aperture of LBT, this image is sharper in the left-right direction (resolution corresponding to 22.8 m baseline) than in the top-bottom direction (resolution corresponding to 8.4 m baseline). The point spread function (PSF) of LBT can be seen in the image at the right of this page created by Serge Corriea of Arcetri Observatory. The PSF can also be seen in the simulated star image just to the lower left of Io's disk. These images assume that the LBT adaptive optics system is correcting the blurring caused by turbulence in Earth's atmosphere.
By taking several such images through a night, the astronomer can deconvolve the images to recover the image quality of a 22.8 m circular aperture. This resolution is demonstrated by the "b" image of Io in the lower panel. The lower image is the result of combining three LBT images taken at different angles in the computer.