The Spectral Instruments 900 Series imager shown here illustrates
the concept of the imager planned to be used in Project ASTEROID for the
1.5-m telescope. A far larger and more sophisticated imager would be
used on the 2.5-m telescope.
The goal for the optical designer will be to obtain a corrected field of view of 2/3 to 1.5 degrees across a detector with detector pixels optimized for local seeing. The Project Scientist, in consultation with the Science Advisory Committee, will determine the best pixel scale and optimum detector size. Our initial vision before any detailed study is to use a mosaic of edge-buttable 2k by 4k arrays such as those available from e2v. Such large focal plane arrays come at the expense of data reduction efficiency, and the cost, manufacturability, and technical risk of the optics. Eventually, one begins to lose light and total optical throughput as the secondary mirror and corrector optics grow in size to accommodate the increased field size to feed the larger detector. One also risks having the telescope fail to meet specifications if the optical design pushes manufacturing tolerances too far. As f-ratios become faster to obtain wider fields, depth of field decreases, and the location of sharp focus becomes both more difficult to find and even more difficult to maintain.
Another possibility for focal plane real estate allocation is to use a smaller array as the main science detector, and to surround it with smaller detectors connected to a controller capable of rapid sub-array readout. In this configuration, the surrounding chips would be used for guiding, focus, and possibly for collimation (assuming a hexapod mounted secondary assembly).
The issues of detector selection, pixel scale, focal plane layout, and similar matters that directly affect the science will be assessed as part of a collaboration among the Science Advisory Committee, the Project Scientist, and the optical designer during the Design Phase of the project. To reduce the effect of waste heat dumped into the dome, active cooling of the Dewar to remove heat from the optical path is essential. To reduce life cycle operations costs, it would be less expensive to use a chilled coolant and Peltier cooling system, or a gas cooler (e.g., a Joule-Thompson cycle "Cryo-Tiger") as opposed to liquid cryogens.
To obtain the maximum benefit from the large aperture, the detectors will be thinned and backside illuminated with maximum quantum efficiency near or exceeding 90 percent. Again, the design will optimize key parameters according to direction from the Science Advisory Committee, e.g., perhaps the ability to detect faint objects at the maximum signal-to-noise ratio.
During the study phase of the project, the Project Scientist will consider the long-term science programs for the telescope in the era after the new professional survey telescopes now under development begin providing high-quality NEO astrometry to the NEO community. He and the Systems Engineer will study the possible use of filters and grisms to help characterize NEO's, and how best to place them in the optical train of the telescope and imager (e.g., the placement of pupils and collimated space within the imager).
Last modified: January 3, 2008.