Advanced Concepts Studies

Advanced Interferometric
Space Telescope (AIST)

The studies of large aperture telescopes in space indicated that launch and assembly problems would dominate the selection of optical configurations in the foreseeable future. Thus investigations of advanced telescope concepts were being based on deployable systems and on-orbit assembly concepts utilizing the Space Shuttle Cargo Bay. Ground-assembled subapertures of up to 4 m could be carried for on-orbit mating to build an interferometer or phased array of very large baseline dimensions.

The Coherent Optical System of Multiple Imaging Collectors, an initial module with the beam-combining telescope, could easily be carried in the Space Shuttle. Modular additions could then be docked during follow-on missions.

Sparsely filled aperture arrays of afocal interferometric telescopes, not requiring rotation to build up an image are the Golay Systems.

Erectable Cassegrain type telescopes with a deployable primary mirror structure supporting multiple subapertures were also analyzed under the category of deployable telescope systems.

Two afocal interferometric telescope concepts were selected for further technology investigations: The one-dimensional COSMIC and the "Golay 9" phased array . The Golay 9 array concept is optimum from the standpoint of providing an autocorrelation function of maximum compactness. The concept has the advantage of being relatively compact, not requiring rotation to form an image, and allowing later expansion of the baseline by the addition of more telescopes that would feed the same beam combiner. However, these types of systems suffer from low UV throughput due to the eight or more reflections needed to bring the collected light to focus. In addition, the useful field of view (FOV) is very narrow, only a few tens of arcseconds. This may not be too critical, however, since even a 10-microrad FOV will require a detector array with 1 million resolution elements (for 10-nanorad telescope resolution). Furthermore, considerably less cross sectional area is exposed to the micrometeoroid flux compared to large, contiguous aperture systems. The probability of micrometeroid impact with the mirror surfaces is therefore reduced, allowing smaller light shields made from rigid material (micrometeoroid bumper). This enhances the ability to avoid straylight increases during the telescope lifetime.

In order to investigate the technology requirements for a contiguous partially filled primary mirror telescope, the configuration was selected. In this concept the primary mirror consists of eighteen off-axis segments of a single parent primary mirror feeding light to a common secondary mirror and thence to a focal plane behind the primary in a Cassegrain or Ritchey-Chretien arrangement. The non redundant mirror pattern provides the widest two-dimensional aperture separation that avoid zeros in the optical transfer function. The dilute aperture configuration requires some degree of image processing to obtain diffraction limited imagery. The unfilled aperture Cassegrain type telescope with a common secondary mirror, has a large field of view. It allows for later increasing the sensitivity by adding segments to the primary and has a minimum number of reflections and thus a high UV throughput. The major disadvantage of this approach is that a very large central structure is required to place the secondary mirror at the proper distance from the primary. In this diluted aperture design the secondary mirror structure and mount was envisioned as an erectable unit, which also serves as the assembly and mirror deployment fixture. The system will only function as a complete telescope for the first time when the entire system is deployed in space. Because of the large dimensions of the lightweight support structure, complete assembly and checkout on Earth is probably impossible or meaningless, at least form the standpoint of verifying optical performance.

Technology assessments for this type of telescope indicate that the key technology requirements are driven by the structural tolerances and stability. At the time of the concept study in the mid 1980’s off-axis aspheric mirrors appeared to be a technology driver. However this technology is now well understood and and would pose no difficulty for such an advanced telescope.

Spherical primary elements could also be used with aberration correction by a deformable tertiary or quartenary mirror ( Korsch three and four mirror aplanats). This would offer a great advantage because radial placement and rotational alignment of the sub aperture mirror segments on the plane of the primary is no longer a sub-micron precision task, since the mirrors are all identical subapertures forming a contiguous primary.