Patent Description:
<NPL>" teaches that the LOTIS Collimator provides scene projection within a <NUM> diameter collimated beam used for optical testing research in air and vacuum. Diffraction-limited performance (<NUM> to <NUM> wavelength) requires active wavefront control of the alignment and primary mirror shape. A hexapod corrects secondary mirror alignment using measurements from collimated sources directed into the system with nine scanning pentaprisms. The primary mirror shape is controlled with <NUM> adjustable force actuators based on figure measurements from a center-of-curvature test. A variation of the Hartmann test measures slopes by monitoring the reflections from <NUM> small mirrors bonded to the optical surface of the primary mirror. The Hartmann source and detector are located at the //<NUM> Cassegrain focus. Initial operation has demonstrated a closed-loop 110nmrms wavefront error in ambient air over the <NUM> collimated beam.

<CIT> discloses a variable power type telephotographic optical system that includes: a dioptric system including a refraction surface having a first optical axis; a reflection optical system including a reflective surface having a second optical axis arranged in parallel with the first optical axis; a folding mirror arranged behind the dioptric system and bending the first optical axis toward the second optical axis; and an optical path-switching mirror arranged behind the reflection optical system on an intersection between the bent optical axis and another optical axis, and selectively taking a first state where it turns only light from the dioptric system toward a back optical path and a second state where it turns only light from the reflection optical system toward the back optical path. The dioptric system includes a lens using glass material having a temperature dependent coefficient dn/dT of a refractive index that is negative. In the reflection optical system, a main mirror and a sub mirror constituting the reflective surface are connected through a coupling member using material having a linear expansion coefficient α that is negative.

<NPL>" teaches that highly stable but lightweight structural materials are essential for the realization of spaceborne optical instruments, for example telescopes. In terms of optical performance, usually tight tolerances on the absolute spacing between telescope mirrors have to be maintained from integration on ground to operation in final orbit. Furthermore, a certain stability of the telescope structure must typically be ensured in the measurement band. Particular challenging requirements have to be met for the LISA Mission (Laser Interferometer Space Antenna), where the spacing between primary and secondary mirror must be stable to a few picometers. Only few materials offer sufficient thermal stability to provide such performance. Candidates are for example Zerodur and Carbon-Fiber Reinforced Plastic (CFRP), where the latter is preferred in terms of mechanical stiffness and robustness. We are currently investigating the suitability of CFRP with respect to the LISA requirements by characterization of its dimensional stability with heterodyne laser interferometry. The special, highly symmetric interferometer setup offers a noise level of <NUM> pm/√Hz at <NUM> and above, and therefore represents a unique tool for this purpose. Various procedures for the determination of the coefficient of thermal expansion (CTE) have been investigated, both on a test sample with negative CTE, as well as on a CFRP tube specifically tuned to provide a theoretical zero expansion in the axial dimension.

In one aspect, the present disclosure provides a mirror system, comprising: a base having a near-zero coefficient of thermal expansion (CTE); a primary mirror strut having a negative CTE; a primary mirror coupled to, and supported by, the base via the primary mirror strut, the primary mirror strut having a non-zero CTE; and a secondary mirror supported relative to the primary mirror, wherein the primary mirror and the secondary mirror have different coefficients of thermal expansion.

In another aspect, the present disclosure provides mirror system, comprising: a support structure having a base with a near-zero coefficient of thermal expansion (CTE), and primary and secondary mirror struts extending from the base, the primary mirror strut comprising a negative CTE; a primary mirror coupled to the base by a the primary mirror strut, the primary mirror being supported by the base and having a non-zero CTE; and a secondary mirror coupled to the base by the secondary mirror strut and having a near-zero CTE.

An initial overview of the inventive concepts are provided below and then specific examples are described in further detail later. This initial summary is intended to aid readers in understanding the examples more quickly, but is not intended to identify key features or essential features of the examples, nor is it intended to limit the scope of the claimed subject matter.

Although mirror system designs incorporating all near-zero CTE components are effective in eliminating sensitivity to environmental variations, ULE glass mirrors are relatively heavy and difficult to produce. The low stiffness to weight ratio of ULE glass material results in self-weight deflection in large ULE glass mirrors (e.g., primary mirrors in Cassegrain reflector mirror systems), which complicates alignment and testing on the ground. In addition, mirror system designs incorporating all SiC components, which is less expensive than ULE glass, but has a relatively high CTE, requires excellent thermal control to minimize the effects of environmental variations or a continuous refocus mechanism to adjust for the effects of such environmental variations.

Accordingly, a mirror system is disclosed that provides image quality that meets high resolution requirements at lower cost than near-zero CTE mirror systems and with shorter manufacturing schedules, and provides superior image quality than all SiC mirror systems at nearly the same cost and manufacturing schedule. In one aspect, the mirror system can be configured with different CTE at front and back ends to specifically address the environmental variations experienced at those locations. The mirror system can include a primary mirror, and a secondary mirror supported relative to the primary mirror. The primary mirror and the secondary mirror can have different CTE.

A negative CTE strut is also disclosed. The negative CTE strut can include a main body portion. The negative CTE strut can also include a first coupling portion and a second coupling portion disposed opposite one another about the main body portion and defining a strut length. The first and second coupling portions can each be configured to interface with an external structure. In addition, the negative CTE strut can include an offsetting extension member having a first end coupled to the main body portion and a second end coupled to the first coupling portion by an intermediate extension member. The first end can be between the first coupling portion and the second end. The first and second ends can define an offset length parallel to the strut length. When the negative CTE strut increases in temperature, the offset length can be configured to increase due to thermal expansion of the offsetting extension member sufficient to cause the strut length to decrease.

To further describe the present technology, examples are now provided with reference to the figures. With reference to <FIG>, one example of a mirror system <NUM> is schematically illustrated in a side view. The mirror system <NUM> can include a support structure <NUM>. The mirror system <NUM> can also include a primary mirror <NUM> supported by the support structure <NUM>. In addition, the mirror system <NUM> can include a secondary mirror <NUM> supported by the support structure <NUM>. In some examples, the primary mirror <NUM> and the secondary mirror <NUM> form a Cassegrain reflector, which may be used in optical telescopes (e.g., high resolution imaging systems) and radio antennas.

The support structure <NUM> can comprise a metering structure, which can include a base <NUM> (e.g., a bench that forms the structural base of the mirror system <NUM>) coupled to the primary mirror <NUM> by one or more primary mirror struts <NUM>. The support structure <NUM> can also include one or more secondary mirror struts <NUM> extending from the base <NUM> and coupled to the secondary mirror <NUM>. For example, the secondary mirror struts <NUM> can support a secondary mirror mount <NUM>, which can be coupled to the secondary mirror <NUM>. A primary mirror assembly <NUM> can include the base <NUM>, the primary mirror struts <NUM>, and the primary mirror <NUM>. A secondary mirror assembly <NUM> can include the secondary mirror struts <NUM>, the secondary mirror mount <NUM>, and the secondary mirror <NUM>.

In a typical space-based application (e.g., one in which the mirror system is mounted on a satellite), the mirror system <NUM> can be subjected to environmental variations over the course of their orbit of the earth, which result in thermal transient conditions that can vary for different parts of the mirror system <NUM>. For example, the mirror system <NUM> will typically be mounted to a satellite by the base <NUM> at a "back end" <NUM> of the mirror system <NUM>, with a "front end" <NUM> of the mirror system <NUM> protruding from the satellite. In this configuration, the relatively large mass at the back end <NUM> of the mirror system <NUM> may be thermally insulated compared to the relatively small, exposed mass at the front end <NUM> of the mirror system <NUM>. The components at the front end <NUM> (e.g., the secondary mirror assembly <NUM>) can therefore experience greater thermal variation that can change the thermal loading on a daily basis (e.g., day/night, changing inclination of the satellite relative to the earth (i.e., beta angle of orbit), etc.). As a result, the components at the back end <NUM> (e.g., the primary mirror assembly <NUM>) do not see as much thermal variation as the components at the front end <NUM> (e.g., the secondary mirror assembly <NUM>).

In one aspect of the present disclosure, the various components of the mirror system <NUM> can be selected and configured to provide suitable CTE for the thermal conditions the components will be subjected to during use. For example, the primary mirror <NUM> at the back end <NUM> can have a non-zero CTE, and the secondary mirror <NUM> at the front end <NUM> can have a near-zero CTE. As used herein, the term "near-zero CTE" includes any CTE greater than or equal to -<NUM> × <NUM>-<NUM> K-<NUM> and less than or equal to <NUM> × <NUM>-<NUM> K-<NUM>. The term "non-zero CTE" includes any CTE outside of this range. More specifically, the primary mirror <NUM> can comprise any suitable mirror material having a non-zero CTE, such as silicon carbide (SiC) (e.g., reaction bonded or sintered), aluminum, aluminum silicon metal matrix composite, aluminum silicon carbide metal matrix composite, aluminum beryllium metal matrix composite, beryllium, fused silica, borosilicate glass, magnesium, etc., in any combination. On the other hand, the secondary mirror <NUM> can comprise any suitable mirror material having a near-zero CTE, such as glass-ceramic (e.g., CLEARCERAM®, ZERØ™, ZERODUR®, lithium-aluminosilicate glass-ceramic), titania-silicate glass (e.g., ULE®), carbon composite, etc., in any combination. Thus, the mirror system <NUM> can utilize primary and secondary mirrors <NUM>, <NUM> having dissimilar CTEs, which when utilized as described herein, specifically address the different environmental variations at the back and front ends <NUM>, <NUM> of the mirror system <NUM>. For example, the near-zero CTE secondary mirror <NUM> can reduce defocus sensitivity to bulk temperature changes at the secondary mirror <NUM>, which is more susceptible to temperature variations due to its location relative to the primary mirror <NUM>.

In one aspect, any or all of the components of the support structure <NUM> can have a near-zero CTE. For example, the base <NUM>, the primary mirror struts <NUM>, the secondary mirror struts <NUM>, and/or the secondary mirror mount <NUM> can have a near-zero CTE. The components of the support structure <NUM> can comprise any suitable structural material having a near-zero CTE, such as a composite material (e.g., carbon composite such as carbon fiber, carbon nanotubes). For example, the base <NUM>, the secondary mirror struts <NUM>, and the secondary mirror mount <NUM> can be made of carbon fiber. The secondary mirror struts <NUM> and the secondary mirror mount <NUM> can be coupled to one another by INVAR® mounting hardware <NUM>. Similarly, the secondary mirror mount <NUM> and the secondary mirror <NUM> can be coupled to one another by INVAR® mounting hardware <NUM>. Although some non-zero CTE materials may be included, such as when coupling components to one another, the total or combined CTE (i.e., of a "chain" of metering structures) may be within the range specified for near-zero CTE.

High thermal conductivity "thermal straps" <NUM> can be included to thermally couple the secondary mirror struts <NUM>, the secondary mirror mount <NUM>, and/or the secondary mirror <NUM> to one another. Similarly, thermal straps <NUM> can be included to thermally couple the primary mirror struts <NUM>, the primary mirror <NUM>, and/or the base <NUM> to one another. The thermal straps <NUM>, <NUM> can thermally "tie" components to one another, such that a temperature change in one component will rapidly be experienced by another component to maintain the components at substantially the same temperature, which can minimize temperature gradients that can negatively impact image quality.

Because of where the front end <NUM> of the mirror system <NUM> is situated on a satellite, it cannot be actively temperature controlled. Accordingly, it is advantageous for the secondary assembly <NUM> to have a near-zero CTE so that it is minimally affected by temperature variations. On the other hand, the back end <NUM> of the mirror system <NUM> has a relatively large mass and is somewhat insulated by the satellite and, in some examples, may be actively temperature controlled. Thus, a relatively large potential for thermal expansion may be tolerable for components at the back end <NUM> of the mirror system <NUM>. Accordingly, the primary mirror assembly <NUM> may have a non-zero CTE. In some examples, this allows the use of a non-zero CTE material as the predominant material of the primary mirror <NUM>, which may be manufactured for lower cost and on shorter schedules than a primary mirror made predominantly of a near-zero CTE material.

The primary mirror struts <NUM> can have any suitable construction and can be made of any suitable material in keeping with the invention as described herein. In some examples, the primary mirror struts <NUM> can comprise a negative CTE, which can be configured to provide thermal compensation for a non-zero CTE primary mirror <NUM>. This configuration can be leveraged to match the constant conjugate position of a near-zero CTE of the secondary mirror and the various support components, as shown in <FIG> and discussed in more detail below. In other words, by selecting an appropriate negative CTE for the primary mirror struts <NUM>, the behavior of the back end <NUM> (i.e., primary mirror end) and the front end <NUM> (i.e., secondary mirror end) of the mirror system <NUM> can be matched for a given environment, such that the focal point location of each end remains constant over temperature.

As shown in <FIG>, a negative CTE strut <NUM> can include a main body portion <NUM> and first and second coupling portions <NUM>, <NUM> disposed opposite one another about the main body portion <NUM>. The coupling portions <NUM>, <NUM> can each be configured to interface with an external structure (e.g., fittings or hardware configured to facilitate coupling to the primary mirror <NUM> and the base <NUM> in <FIG>) and define a strut length <NUM>. In one aspect, the coupling portions <NUM>, <NUM> can include a spherical ball for interfacing with an external structure to facilitate relative rotation of the strut <NUM> and the external structure. In some aspects, the coupling portions <NUM>, <NUM> can be configured to facilitate length adjustment of the strut <NUM> (e.g., via threaded rods) to facilitate proper alignment of the primary mirror <NUM> at assembly.

The negative CTE strut <NUM> can also include an offsetting extension member or spacer <NUM> having a first end <NUM> coupled to the main body portion <NUM>, and a second end <NUM> coupled to the first coupling portion <NUM> by an intermediate extension member <NUM>. The first end <NUM> can be between the first coupling portion <NUM> and the second end <NUM>. The first and second ends <NUM>, <NUM> can define an offset length <NUM> parallel to the strut length <NUM>. When the negative CTE strut <NUM> increases in temperature, the offset length <NUM> can be configured to increase due to thermal expansion of the offsetting extension member <NUM> sufficient to cause the strut length <NUM> to decrease.

The main body portion <NUM>, the coupling portions <NUM>, <NUM>, and the intermediate extension member <NUM> can have any suitable CTE. In some examples, the main body portion <NUM>, the coupling portion <NUM>, the coupling portion <NUM>, and/or the intermediate extension member <NUM> can have a near-zero CTE. The main body portion <NUM>, the coupling portion <NUM>, the coupling portion <NUM>, and/or the intermediate extension member <NUM> can therefore comprise any suitable structural material having a near-zero CTE, such as a composite material (e.g., carbon fiber, carbon nanotubes, etc.). A non-zero CTE material, such as a nickel-iron alloy (e.g., 64FeNi, such as INVAR®), can be used selectively such that the total or combined CTE (i.e., of a "chain" of structures) may be within the range specified for near-zero CTE, and thereby provide a near-zero CTE for a given structure or combination of structures. In some examples, the main body portion <NUM>, the coupling portion <NUM>, the coupling portion <NUM>, and/or the intermediate extension member <NUM> can include materials with a non-zero CTE, such as nitrogen strengthened stainless steel (e.g., NITRONIC®). In some examples, the main body portion <NUM>, the coupling portion <NUM>, the coupling portion <NUM>, and/or the intermediate extension member <NUM> can have a non-zero positive CTE. The offsetting extension member <NUM> can have any suitable non-zero positive CTE (i.e., a CTE greater than a near-zero CTE). The offsetting extension member <NUM> can therefore comprise any suitable structural material having a non-zero positive CTE, such as aluminum, titanium, iron, steel, nickel, beryllium, etc..

In some embodiments, the negative CTE strut <NUM> can include an insert <NUM> coupling the main body portion <NUM> and the first end <NUM> of the offsetting extension member <NUM>. The insert <NUM> can aid in coupling a non-zero positive CTE offsetting extension member <NUM> to a near-zero CTE main body portion <NUM> by providing a structural buffer between two dissimilar CTE materials. In some examples, the insert <NUM> can comprise a material, such as nickel-iron alloy (e.g., 64FeNi, such as INVAR®), that is stronger than the material (e.g., aluminum) of the offsetting extension member <NUM> and has a similar CTE to the main body portion <NUM> (e.g., a composite material) to prevent breakage of the main body portion <NUM>.

The main body portion <NUM>, the insert <NUM>, the offsetting extension member <NUM>, and the intermediate extension member <NUM> can be coupled in any suitable manner, such as utilizing an adhesive, threaded interface surfaces (e.g., mating threaded surfaces or portions between the joined components), rivets, welds, etc. In one example, the insert <NUM> can be bonded at <NUM> to the main body portion <NUM>, the first end <NUM> of the offsetting extension member <NUM> can be coupled to the insert <NUM> at <NUM> with threaded interface surfaces, and the intermediate extension member <NUM> can be coupled to the second end <NUM> of the offsetting extension member <NUM> at <NUM> with threaded interface surfaces. Flanges <NUM>-<NUM> of the respective main body portion <NUM>, insert <NUM>, and offsetting extension member <NUM> can facilitate coupling of the components and can provide proper relative positioning of the components at the first end <NUM> of the offsetting extension member <NUM>. At the second end <NUM> of the offsetting extension member <NUM>, the intermediate extension member <NUM> can be placed in full contact with a bottom surface <NUM> of the offsetting extension member <NUM>. The flange <NUM> and the bottom surface <NUM> of the offsetting extension member <NUM> can establish the offset length <NUM>.

The main body portion <NUM>, the offsetting extension member <NUM>, and/or the intermediate extension member <NUM> can each comprise a cylindrical configuration. In some examples, the main body portion <NUM>, the offsetting extension member <NUM>, and/or the intermediate extension member <NUM> can be coaxially aligned along a longitudinal axis <NUM>. The main body portion <NUM>, the offsetting extension member <NUM>, and/or the intermediate extension member <NUM> can therefore be in-line with one another. The offsetting extension member <NUM> can be configured to fit at least partially within the main body portion <NUM>, and the intermediate extension member <NUM> can be configured to fit within the offsetting extension member <NUM>. Thus, an outer diameter <NUM> of the offsetting extension member <NUM> can be less than an inner diameter <NUM> of the main body portion <NUM>, and an outer diameter <NUM> of the intermediate extension member <NUM> can be less than an inner diameter <NUM> of the offsetting extension member <NUM>. This can enable free movement of these components relative to one another due to thermal expansion/contraction without generating undue internal stress while the offset length <NUM> and the strut length <NUM> vary due to temperature. A thermal strap (<NUM> in <FIG>) can be utilized to closely link the temperatures of the primary mirror <NUM> and the offsetting extension member <NUM> to maintain the primary mirror <NUM> and the offsetting extension member <NUM> at the same temperature for proper operation and thermal compensation, as described herein.

The CTE and axial expansion lengths of each of the various components of the strut <NUM> can be configured to provide a desired composite CTE of the strut <NUM> that is tuned for a given operating temperature range to provide passive thermal compensation for the non-zero CTE primary mirror <NUM>. In examples where the main body portion <NUM>, the coupling portion <NUM>, the coupling portion <NUM>, and the intermediate extension member <NUM> are constructed primarily of near-zero CTE materials, the offset length <NUM> of the offsetting extension member <NUM> is the primary variable in establishing the CTE of the strut <NUM>.

In operation, as the temperature of the offsetting extension member <NUM> increases, the offsetting extension member <NUM> expands and increases the offset length <NUM>. Due to the connection of the intermediate extension member <NUM> to the second end <NUM> of the offsetting extension member <NUM>, the coupling portion <NUM> is caused to move toward the opposite coupling portion <NUM>, thus effectively shrinking or contracting the strut length <NUM>. The opposite occurs with a decrease in temperature. The result is a net negative CTE for the strut <NUM>, which decreases in strut length <NUM> as the temperature increases, and increases in strut length <NUM> as the temperature decreases.

In one aspect, as schematically illustrated in <FIG>, the negative CTE strut <NUM> can be configured to have a negative CTE to provide a suitable contraction/expansion over a given temperature range that offsets the expansion/contraction of a given positive CTE primary mirror <NUM>. In general, for a positive CTE primary mirror <NUM> as shown in <FIG>, as the primary mirror <NUM> contracts with decreased temperature, a focus point <NUM> of the primary mirror <NUM> tends to shift toward the primary mirror, in front of the secondary mirror (not shown). On the other hand, as shown in <FIG>, as the primary mirror <NUM> expands with increased temperature, the focus point <NUM> tends to shift away from the primary mirror <NUM>, behind the secondary mirror (not shown). Such tendencies of the focus point <NUM> to shift with temperature variation can make it difficult to maintain focus.

Utilizing the negative CTE struts <NUM> in conjunction with the positive CTE primary mirror <NUM>, however, can passively maintain the focus point <NUM> of the primary mirror <NUM> in substantially the same location relative to the secondary mirror over a given temperature range. For example, as the primary mirror <NUM> contracts with decreased temperature and shifts the focus point <NUM> toward the primary mirror <NUM>, the negative CTE struts <NUM> correspondingly expand or lengthen and push the primary mirror <NUM> toward the secondary mirror to passively maintain the focus point <NUM> in substantially the same location relative to the secondary mirror. As the primary mirror <NUM> expands with increased temperature and shifts the focus point <NUM> away from the primary mirror <NUM>, the negative CTE struts <NUM> correspondingly contract or shorten and pull the primary mirror <NUM> away from the secondary mirror to passively maintain the focus point <NUM> in substantially the same location relative to the secondary mirror. Thus, the negative CTE struts <NUM> can be tuned or configured to accommodate varying temperature gradients and reduce primary mirror defocus sensitivity. In other words, the passive thermal compensation provided by the negative CTE struts <NUM> can provide a mirror system that is effectively insensitive to thermal variation. The result is a mirror system that can maintain the primary mirror focal point over a wide range of temperatures. In addition, the negative CTE struts <NUM> can enable use of a relatively inexpensive, high CTE primary mirror at a low cost while eliminating or minimizing the need for active thermal controls.

In some examples, a negative CTE strut may not be utilized depending on the operating environment of the mirror system or if the primary mirror (i.e., the back end of the mirror system) can be maintained at a given temperature, such as by an active temperature control.

The principles disclosed herein can maintain image quality at an acceptable level while the mirror system <NUM> experiences temperature variations. For example, <FIG> graphically represents component temperature and image quality (quantified as wavefront error). The temperature of the exposed front end components (e.g., the secondary mirror and struts) can vary greatly in response to changing environmental conditions, while the temperature of the relatively massive, insulated back end components (e.g., the primary mirror) remain relatively constant. The back end components will typically see temperature variation over a longer period of time. As shown in the graph, the wavefront error (i.e., image quality) remains at an acceptable level while the front end and back end components experience different types of temperature variation characteristics. The mirror system <NUM> can therefore benefit from material choices specific to the operating environment, so that the mirror system is thermally insensitive where it needs to be (i.e., at the exposed front end utilizing a near-zero CTE mirror and structural components) and allows for some thermal sensitivity where it is tolerable (i.e., at the relatively massive, insulated back end utilizing a non-zero CTE mirror and, optionally, negative CTE struts). This strategic use of materials (i.e., CTE characteristics) in the mirror system <NUM> can save manufacturing time and money, particularly with regard to the primary mirror, which is often the most difficult and expensive component to manufacture. As a result, the image quality of the mirror system <NUM> may be comparable to that of a telescope made entirely of near-zero CTE materials, while being much less expensive to make in a much shorter time frame. In addition, the cost and manufacturing time of the mirror system <NUM> may be close to that of a telescope made entirely of the same high CTE material (e.g., SiC), while producing a superior image quality. Thus, the mirror system <NUM> disclosed herein can produce high spatial resolution imaging with low wavefront error at reduced cost compared to other imaging systems.

Reference was made to the examples illustrated in the drawings and specific language was used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the technology is thereby intended. Alterations and further modifications of the features illustrated herein and additional applications of the examples as illustrated herein are to be considered within the scope of the description.

Although the disclosure may not expressly disclose that some embodiments or features described herein may be combined with other embodiments or features described herein, this disclosure should be read to describe any such combinations that would be practicable by one of ordinary skill in the art. The user of "or" in this disclosure should be understood to mean non-exclusive or, i.e., "and/or," unless otherwise indicated herein.

Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more examples. In the preceding description, numerous specific details were provided, such as examples of various configurations to provide a thorough understanding of examples of the described technology. It will be recognized, however, that the technology may be practiced without one or more of the specific details, or with other methods, components, devices, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of the technology.

Claim 1:
A mirror system (<NUM>), comprising:
a base (<NUM>) having a near-zero coefficient of thermal expansion (CTE);
a primary mirror strut (<NUM>) having a negative CTE;
a primary mirror (<NUM>, <NUM>) coupled to, and supported by, the base (<NUM>) via the primary mirror strut (<NUM>), the primary mirror (<NUM>, <NUM>) having a non-zero CTE; and
a secondary mirror (<NUM>, <NUM>) supported relative to the primary mirror (<NUM>, <NUM>), wherein the primary mirror (<NUM>, <NUM>) and the secondary mirror (<NUM>, <NUM>) have different coefficients of thermal expansion.