In order to increase the fidelity of hardware-in-the-loop ground-truth testing, it is desirable to create a dynamic scene of multiple, independently controlled IR point sources. ATK-Mission Research has developed and supplied the steering mirror systems for the 7V and 10V Space Simulation Test Chambers at the Arnold Engineering Development Center (AEDC), Air Force Materiel Command (AFMC). A portion of the 10V system incorporates multiple target sources beam-combined at the focal point of a 20K cryogenic collimator. Each IR source consists of a precision blackbody with cryogenic aperture and filter wheels mounted on a cryogenic two-axis translation stage. This point source target scene is steered by a high-speed steering mirror to produce further complex motion. The scene changes dynamically in order to simulate an actual operational scene as viewed by the System Under Test (SUT) as it executes various dynamic look-direction changes during its flight to a target. Synchronization and real-time hardware-in-the-loop control is accomplished using reflective memory for each subsystem control and feedback loop. This paper focuses on the steering mirror system and the required tradeoffs of optical performance, precision, repeatability and high-speed motion as well as the complications of encoder feedback calibration and operation at 20K.
The challenging target position calibration task was accomplished in the recently upgraded AEDC 7V Chamber by use of a newly developed instrument termed the 7V Alignment Monitor System (7V-AMS). The 7V-AMS is essentially a resident, reference sensor consisting of a high quality imaging telescope, staring focal plane array (FPA), and FPA frame data acquisition and image processing system. A sub-pixel image centroiding routine permits accurate and precise determination of target image position. Unorthodox operation of the otherwise off-the-shelf FPA and frame data acquisition system results in a radiometric sensitivity that permits the 7V-AMS to 'see' the high to midrange radiometric levels of all 7V calibration and target simulation sources. This sensitivity range, coupled with its high quality imaging capability, allowed the 7V-AMS to inspect the radiation patterns of the newly activated radiometric calibration and target simulation equipment, to pinpoint sources of stray radiation, and to detect and display faint ghost images. As part of its primary task, target position calibration, the 7V-AMS was used to precisely define the coordinate axis relationships of all equipment capable of controlling target position. This paper describes the 7V-AMS salient design features and also presents some results of its first application in the 7V Chamber.
The calibration of low background IR sensor systems require cryogenically cooled collimators. The need to characterize the output beam angle as a function of pointing position, cryogenic pressure, temperature and liquid level has always been difficult. Historically, manual theodolites have been used to measure these parameters. Because accuracy and repeatability of the manual measurement device is dependent upon user interpretation and manual coupling, the ability to measure small drifts with the thoedolite is not sufficient. The time required to manually characterize the output beam angle as a function of pointing position is so long that the drift of the beam angle begins to dominate the measurement error. A Charge Injection Device (CID) camera with very uniform focal plane pixel spacing and quality optics, can be used to very accurately monitor output beam angles in short intervals. This is accomplished by using sub-pixel centroiding techniques. Resolving a reimaged spot to 1/40 pixel accuracy is accomplished by using a weighted center of area calculation. Automated measurements have two advantages: they are faster, allowing for large numbers of measurements and higher resolution as well as removing human error, resulting in a better understanding of the pointing system. Using a 512 X 512 CID camera, angular resolution of 5 (mu) rad is achieved for a field of view of 6 degrees full angle. The potential for absolute accuracies of the same resolution is achievable if the stability and nonuniformities of the CID camera and optics are calibrated. Typical results obtained from a cryogenic system taken with the CID camera will be presented.
The Low-background Scanning Point Source (LSPS), a field-portable low-background target generator designed to allow vehicle-level ground test and checkout of infrared seekers, is discussed. The LSPS can be used to simulate a room-temperature point source moving at a constant rate against an exoatmospheric background or as a point-source calibration change. It incorporates a cryogenically cooled off-axis Gregorian collimator and a dual-axis scan mirror viewed through a zinc selenide window. An analysis is presented of its optical and radiometric performance, and its integration to the vehicle is discussed.
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