Ambient-to-cold focus and alignment of cryogenic space...

Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive

Reexamination Certificate

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C250S332000

Reexamination Certificate

active

06833547

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to infrared sensors. More specifically, the present invention relates to systems and methods for focusing and aligning infrared detectors at cryogenic temperatures.
2. Description of the Related Art
Infrared space sensors are used in many acquisition and tracking applications such as remote sensing, weapons guidance, and target tracking. These systems need to operate at cryogenic temperatures (150° K to 20° K). Unfortunately, the process of focusing and aligning infrared detectors at these low temperatures is presently a complex and extremely expensive operation.
The ability to focus and align the detectors at cryogenic temperatures is seriously affected by thermal shift in any substrate material used to house the detectors. The very low temperatures at which these sensors operate produce contractive forces on the optical path which are difficult or impossible to simulate using traditional optical design programs. The shrinkage problem manifests itself as a focus problem with the infrared imaging systems.
Infrared detectors operating in the 3-12 &mgr;m range—including short wave, medium wave, long wave, and visible radiation—often have a very tight focusing requirement. If the sensors are not focused properly, i.e. the set of detectors—generally a detector array or focal plane array (FPA)—are located off of the focal plane, then the output image will be blurred, resulting in a loss of effectivity of the entire system. Focusing an imaging system generally includes either adjusting the optical elements in order to move the point of focus (focal plane) onto the detectors, or moving the detectors into the point of focus established by the fixed optical elements. For many applications, it is more desirable from an engineering perspective to move the detectors rather than the optics.
The infrared detectors presently being used for space applications are operable only at cryogenic temperatures. They do not function and therefore cannot be focused under ambient conditions. Furthermore, space sensors typically do not incorporate moving focus adjust mechanisms due to risk issues. Hence, the focus cannot be adjusted after the sensor is deployed into space. The correct location of the focal plane under space conditions must be determined on earth, and the detectors fixed to that location prior to deployment.
Current focusing procedures involve cycles of manually adjusting the detectors at ambient temperatures followed by performing a focus check at cryogenic temperatures to determine the accuracy of the focus. The focus is measured by placing the sensors in a thermal vacuum chamber where the solar loads and radiative heat losses of space are simulated. These tests typically require large support teams (typically over 30 people) working 7 days a week, 24 hours a day for two to four weeks. Between outgasing and thermal equalization, it can take over a week to just be prepared to perform a focus test. Once the proper test conditions are achieved, the focus of the infrared sensors is measured using modulation transfer techniques or other methods.
In general, the detectors are not properly focused during the first focus test. Since space sensors typically do not incorporate moving mechanisms, the focus can only be measured, not adjusted. Hence, the data from the test must be used to guide the manual realignment of the detectors once the sensor has been brought back to room temperature. With the sensor at room temperature, the manual realignment begins with using the focus data to build a set of mechanical metallic shims. The sensor is disassembled to expose the focal planes and each focal plane is individually shimmed to the calculated focus point. The sensor is reassembled and the multi-layered insulation (MLI) reinstalled. The sensor is then placed back into the vacuum chamber for a second focus check. This cycle of adjusting the detectors tinder ambient conditions followed by checking the focus under cryogenic conditions is repeated until the system is focused. It is typical that 3 to 5 of these focus cycles may be necessary to achieve the required focus of infrared systems at the correct conditions.
Making the problem even more difficult is the fact that each sensor is typically covered with multi-layered insulation (MLI). The MLI has a strong relationship with the thermal performance of the sensor. Each time the detectors are readjusted, the MLI must be altered and removed. This adds variability to the focus response and hence lessens the chances of achieving an optimal focus of the optical train.
Consequently, conventional focusing procedures are time-consuming (on the order of several weeks) and are quite expensive.
Hence, a need exists in the art for an improved system or method for focusing infrared detectors operable at cryogenic temperatures which is less expensive than current conventional approaches.
SUMMARY OF THE INVENTION
The need in the art is addressed by the system and method for focusing infrared detectors operable at cryogenic temperatures of the present invention. The invention includes a sensor for detecting electromagnetic energy comprising a first detector operable over a first temperature range and a predetermined number of auxiliary detectors operable over a second temperature range, wherein the auxiliary detectors are mounted in the same optical plane as the first detector.
In the illustrative embodiment, the energy is infrared or visible light, the first temperature range is a range of cryogenic temperatures, and the second temperature range is a range of ambient temperatures. The first detector is a focal plane array and the auxiliary detectors are uncooled detector arrays. In the preferred embodiment, the focal plane array and the uncooled detectors are disposed on a common substrate.
In accordance with the teachings of the present invention, the novel sensor can be used to focus an optical system at cryogenic temperatures. The inventive method includes illuminating energy onto the sensor through the optical system at ambient temperatures and then adjusting the position of the sensor until maximum illumination is received by the auxiliary detectors. This determines the location of the focal plane of the system at ambient temperatures. The location of the focal plane at cryogenic temperatures can then be calculated using the location of the focal plane at ambient and a model of the thermal characteristics of the system.


REFERENCES:
patent: 5149970 (1992-09-01), Whitney
patent: 5391875 (1995-02-01), Cederberg et al.
patent: 5600143 (1997-02-01), Roberts et al.
patent: 6137107 (2000-10-01), Hanson et al.
patent: 6583416 (2003-06-01), Villani
patent: WO 02 25741 (2002-03-01), None

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