Patent Application
20130284886
Generally, optical elements of long distance telescopes may be refractive or reflective. Refractive optical elements are generally effective in controlling and/or preventing aberrations, and may be used in a variety of applications, including star tracker applications. Reflective optical elements have been used in place of refractive elements in some system, for example, to provide large aperture optical systems. In long distance optical imaging systems the minimum number of optical elements is generally recognized to be three, to provide the minimum number of parameters that are necessary to correct for and/or prevent spherical aberration, coma, astigmatism and field curvature. An optical imaging system composed of three optical elements is known as a triplet.
Reflective optical triplets are generally constructed such that electromagnetic radiation enters the system from a distant object, is received on a primary mirror, is reflected onto a secondary mirror, is received on a tertiary minor, and finally, is focused on an image plane where an image of the distant object is formed. U.S. Patent Publication No. 2010/0110539 filed Nov. 4, 2008 and titled “REFLECTIVE TRIPLET OPTICAL FORM WITH EXTERNAL REAR APERTURE STOP FOR COLD SHIELDING” describes a reflective triplet configured such that the aperture stop of the optical system is located between the last optical element and the image plane.
Aspects and embodiments are directed to a rear-stopped radiation shielded reflective optical system that may provide up to 4-pi steradian radiation shielding for a detector.
According to one embodiment, a radiation shielded optical system comprises a labyrinthine housing having an entrance and defining a cavity, a detector positioned within the cavity of the housing, the housing configured to provide substantially 4-pi steradian radiation shielding for the detector, a rear-stopped optical sub-system having a rear aperture stop positioned proximate the entrance of the housing and configured to direct an optical beam through the rear aperture stop and the entrance into the housing, and a fold mirror positioned within the housing and configured to reflect the optical beam onto the detector.
In one example, the rear-stopped optical sub-system is a rear-stopped reflective optical sub-system. In one example, the rear-stopped reflective optical system is a reflective triplet. In another example, the reflective triplet includes a positive power primary mirror, a negative power secondary minor optically coupled to the primary mirror, and a positive power tertiary minor optically coupled to the secondary mirror. In one example the tertiary mirror receives the optical beam from the secondary mirror and reflects the optical beam through the rear aperture stop. The detector is positioned at an image plane of the optical system, and the rear aperture stop may be positioned approximately equidistant from the image plane and the tertiary minor along an optical axis of the optical system. In one example the detector is a focal plane array image sensor. In one example the housing comprises a graded-Z laminate material. In another example the housing comprises a tungsten steel laminate. In another example the housing comprises a material selected to shield the detector from ionizing radiation. The radiation shielded optical system may further comprise at least one additional fold minor positioned within the housing and configured to reflect the optical beam onto the first fold minor.
Still other aspects, embodiments, and advantages of these exemplary aspects and embodiments are discussed in detail below. Embodiments disclosed herein may be combined with other embodiments in any manner consistent with at least one of the principles disclosed herein, and references to “an embodiment,” “some embodiments,” “an alternate embodiment,” “various embodiments,” “one embodiment” or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described may be included in at least one embodiment. The appearances of such terms herein are not necessarily all referring to the same embodiment.
Various aspects of at least one embodiment are discussed below with reference to the accompanying figure, which are not intended to be drawn to scale. The figure is included to provide illustration and a further understanding of the various aspects and embodiments, and is incorporated in and constitutes a part of this specification, but is not intended as a definition of the limits of the invention. In the figures:
Aspects and embodiments are directed to an optical imaging system in which the detector is housed within a chamber designed to shield the detector from radiation. In certain applications, for example, long-distance space-based or airborne imaging systems, such as star tracker systems, it is desirable to shield the detector from radiation in order to limit noise and/or false readings that may degrade the imaging performance of the detector. This radiation may include electromagnetic radiation, such as stray light, thermal radiation, and/or gamma rays, as well as particle radiation, such as cosmic rays. Accordingly, in one embodiment, the detector is housed within a chamber composed of a suitable material or materials and having a configuration that shields the detector from such radiation, as discussed further below.
In addition, in many imaging applications where it is desirable to shield the detector from radiation, it is also desirable to minimize the size and weight of the optical system. In one embodiment, the optical system includes a reflective or refractive optical system having a rear aperture stop. In one example, using all-reflecting optical elements, the optical system is configured such that the aperture stop of the optical system is between the last optical element and the image plane. These types of optical systems are referred to as rear-stopped optical systems. With the aperture stop in this position, the image plane, and therefore the detector, may be shielded from radiation more effectively and efficiently than with conventional reflective optical systems having a forward aperture stop or refractive optical systems. In one example, the reflective optical system has a reflective triplet optical form; however, other rear-stopped optical forms may also be used, as discussed further below.
It is to be appreciated that embodiments of the methods and apparatuses discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The methods and apparatuses are capable of implementation in other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use herein of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms.
Referring to
According to one embodiment, the reflective optical sub-system 130 is an all-reflecting, non-relayed optical sub-system. As discussed above, in one embodiment, the reflective optical sub-system 130 is a reflective triplet. As shown in
U.S. Patent Publication No. 2010/0110539 filed Nov. 4, 2008 and titled “REFLECTIVE TRIPLET OPTICAL FORM WITH EXTERNAL REAR APERTURE STOP FOR COLD SHIELDING,” which is herein incorporated by reference in its entirety, describes and provides an optical prescription for an example of a reflective triplet which may be used for the reflective optical sub-system 130. However, as discussed above, numerous other rear-stopped reflective or refractive optical forms may be used.
As discussed above, a defining aperture stop 140 of the optical sub-system 130 is located between the tertiary minor 136 and the image plane at the detector 110. In this position, the detector 110 may be placed inside the housing 120 to be substantially completely shielded from radiation. The external rear aperture stop location naturally results in the virtual entrance pupil 170 shown in
Still referring to
The combination of the rear-stopped reflective optical sub-system 130, labyrinthine housing 120 and fold minor 160 may provide an efficient shielding configuration that may protect the detector 110 from stray radiation from all directions (4-pi steradian). As illustrated in
The housing 120 may be constructed from a material suitable for shielding against electromagnetic radiation and/or particle radiation. For example, the housing may include a tungsten steel laminate. In one example, the housing may be constructed in accordance with a graded-Z laminate approach. Graded-Z shielding is a laminate of several materials with different Z values (atomic numbers) designed to protect against ionizing radiation. Graded-Z shielding may provide more effective shielding compared to single-material shielding of the same mass. In one example, a graded-Z housing may include multiple layers of materials from an outer layer of a high-Z material, such as tantalum, through successively lower-Z materials (e.g., tin, steel, copper), to an inner layer of a low-Z material, such as aluminum, polypropylene or boron carbide, for example. Thus, the overall structure has a Z-gradient from the outer layer through to the inner layer. The high-Z outer layer effectively scatters protons and electrons, and may also absorb gamma rays, which produce X-ray fluorescence. Each subsequent layer absorbs the X-ray fluorescence of the previous material, eventually reducing the energy to a level that is not harmful to the detector 110. The shielding provided by the housing 120 may further enhance the inherent radiation hardness of CMOS-based detectors, for example, and provide protection for detectors in applications where radiation hardness may be an important consideration.
The above-discussed aspects and embodiment relate to shielding an optical detector 110 from radiation. However, embodiments of the optical system may also be implemented where the detector is to be cold shielded in addition to radiation shielded. For example, the housing 120 may incorporate or may be included within a cold shielding chamber, such as a cryo-vac Dewar. The fold minor 160 may introduce a light path that may be undesirable in cold shielded applications. Accordingly, baffling may be introduced around the fold mirror(s) or detector for stray light rejection within the housing 120. The fold minor may also include a coating or other surface treatment to reduce out-of-band reflections.
Having described above several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only, and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.
