Trade.Information

Details of Ground-based telescopes.

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.