Patent Description:
This disclosure relates generally to optical imaging systems, and more specifically to small form factor four-mirror based optical imaging systems for use in satellites or aerial vehicles.

Optical imaging systems are useful in many applications such as imaging planets or stars. Known optical system designs for satellite imaging include a traditional Three Mirror Anastigmat (TMA) design and a Korsch design. Existing solutions to optical imaging have drawbacks with regard to size and corresponding resolution capability. Improvements in optical imaging are therefore desirable. The European Patent Office cited various related arts documents during the prosecution. <CIT> describes a bifocal anastigmatic telescope with five aspherical mirrors. <CIT> describes methods and apparatus for reflecting, towards a sensor, an Infrared to vacuum ultra-violet (VUV) light that is reflected from a target substrate. <CIT> describes an optical system of alleged total reflection.

In one aspect, an all-reflective optical system is disclosed. The all-reflective optical system comprises a concave primary mirror having a central aperture and a radius, the primary mirror having one of a parabolic, non-parabolic conical, or aspherical surface; a convex secondary mirror facing the primary mirror, the secondary mirror having an aspherical surface, where an optical axis extends from a vertex of the primary mirror to a vertex of the secondary mirror; a concave tertiary mirror arranged behind the primary mirror, the tertiary mirror having one of a parabolic, non-parabolic conical or aspherical surface; a concave quaternary mirror arranged in the central aperture of the primary mirror or behind the primary mirror, the quaternary mirror having one of a spherical, parabolic, non-parabolic conical or aspherical surface; and at least one image plane having one or more aggregated sensors. The image plane is positioned at a distance from the optical axis that is no more than the radius of the primary mirror.

In some embodiments, the optical system may additionally comprise an entrance pupil positioned near the primary mirror or the secondary mirror, and an exit pupil or Lyot stop positioned at one of <NUM>) near the quaternary mirror, <NUM>) between the tertiary mirror and the quaternary mirror, and <NUM>) between the quaternary mirror and the image plane.

In some embodiments, the optical system may additionally comprise one or more folding mirrors arranged to deflect rays from the quaternary mirror to the image plane, wherein the one or more folding mirrors may be configured to fold a ray path. Based on using a first folding mirror, the exit pupil may be positioned between the tertiary and the quaternary mirror, or between the quaternary mirror and the first folding mirror. One of the folding mirrors may be tilted at a specific angle to an optical axis of the system. One of the folding mirrors positioned at the front of the image plane may widen the field of view with reflective and transmissive sections over a same spectral range, wherein each section may correspond to a specific sensor of the one or more sensors. One of the folding mirrors positioned at the front of the image plane may enable simultaneous multi-color imaging, wherein the one of the folding mirrors may be reflective over a first spectral range and transmissive over other spectral ranges, and may be reflective over a second spectral range and transmissive over other spectral ranges, wherein one of the aggregated sensors may be dedicated to the first spectral range and a different one of the aggregated sensors may be dedicated to the second spectral range.

A form factor, defined as a ratio of a distance between the secondary mirror and the tertiary mirror to an effective focal length of the optical system, is less than <NUM>. Vertices of the primary mirror and the secondary mirror may form an optical axis, which may be a geometric reference line extending from the vertex of the primary mirror to the vertex of the secondary mirror. The primary mirror and the secondary mirror may be symmetric or periodic about the optical axis. A diagonal of a periodic mirror may have an angle of zero degrees or <NUM> degrees from a diagonal of the image plane. The optical axis of the tertiary mirror may not coincide with a mechanical axis.

In some embodiments, a radius of the secondary mirror may be in a range of <NUM>% to <NUM>% of an effective focal length, and a radius of the tertiary mirror may be in a range of <NUM>% to <NUM>% of the effective focal length. A radius of the quaternary mirror may be in a range of <NUM>% to <NUM>% of an effective focal length.

In some embodiments, the folding mirrors may enable simultaneous multi-color imaging, wherein each of the folding mirrors may be reflective over a particular spectral range and transmissive over other spectral ranges, and wherein each added folding mirror and a corresponding one of the aggregated sensors may be associated with a different spectral range.

In some embodiments, a distance from the tertiary mirror to the image plane along the optical axis may be in a range of <NUM>% to <NUM> % of an effective focal length and the distance from the secondary mirror to the tertiary mirror along the optical axis may be in a range of <NUM>% to <NUM>% of the effective focal length. The system may have an imaging resolution better than <NUM> at a <NUM> altitude.

In some embodiments, the system may be adapted to support simultaneous multi-color imaging, including <NUM>) panchromatic and RGB and near-infrared, <NUM>) visible and infrared (near-infrared, shortwave infrared, mid-wave infrared, or longwave infrared), <NUM>) visible and visible, <NUM>) infrared and infrared, <NUM>) UV and visible, or <NUM>) UV and infrared imaging.

In some embodiments, a diameter of the primary mirror may range from <NUM>% to <NUM>% of an effective focal length. A focal point distance from the primary mirrors may be in a range of <NUM> % to <NUM> % of an effective focal length. An effective focal length may be in a range of <NUM> to <NUM>,<NUM>. The optical system may further comprise a supporting structure for one or more of the mirrors. The supporting structure may be additively manufactured.

In some embodiments, the image plane may comprise a charge coupled device (CCD)-in CMOS time delay integration (TDI) sensor. The CCD-in-CMOS TDI sensor may be a multispectral TDI, backside illumination imager. The CCD-in-CMOS TDI sensor may comprise seven CCD arrays of <NUM> × <NUM> pixels each. The CCD-in-CMOS TDI sensor may comprise four panchromatic CCD arrays of <NUM> × <NUM> pixels each and eight multispectral CCD arrays of <NUM> × <NUM> pixels.

In some embodiments, the primary mirror may have a circular or a non-circular shape, the tertiary mirror may have a segmented non-circular shape, and the quaternary mirror has a circular or non-circular shape. The non-circular shape of the primary mirror may enhance a modulation transfer function (MTF) and a signal to noise ratio (SNR).

In some embodiments, the quaternary mirror may face the tertiary mirror and may be positioned to avoid interference with rays from the secondary mirror to the tertiary mirror. The optical system may additionally comprise a supporting structure of the mirrors including a cylindrical tube or a conical baffle of the primary mirror. The four mirrors may be constructed of zero-CTE materials, low-CTE materials, or mild-CTE materials, wherein the four mirrors and a supporting structure may be made of one material. The system may be adapted to provide imaging in the modes of starring, scanning or pushbroom, video, stereo, BRDF (Bidirectional Reflectance Distribution Function), HDR (High Dynamic Range), polarimetric and low-light.

In some embodiments, the system may be adapted to be installed onboard satellites purposed for a non-imaging mission including communication satellites, or installed on imaging satellites, quasi-imaging satellites, or scientific mission satellites. The system may be adapted to be installed onboard airplanes, drones, unmanned aerial vehicles, and balloons. A back focal length between the quaternary mirror and the at least one image plane may be in a range of <NUM>% to <NUM>% of an effective focal length.

In another aspect, an all-reflective optical system is disclosed comprising a concave primary mirror having a central aperture and a radius, the primary mirror having one of a parabolic, non-parabolic, conical, or aspherical surface; a convex secondary mirror facing the primary mirror, the secondary mirror having a hyperbolic surface, where an optical axis extends from a vertex of the primary mirror to a vertex of the secondary mirror; a concave tertiary mirror arranged behind the primary mirror, the tertiary mirror having one of a parabolic, non-parabolic, conical and aspherical surface; a concave quaternary mirror arranged in front of the central aperture of the primary mirror, the quaternary mirror having one of a spherical, parabolic, non-parabolic, conical or aspherical surface; and at least one image plane having one or more aggregated sensors, wherein the image plane is positioned at a radial distance from the optical axis that is no more than the radius of the primary mirror.

Elements in the figures have not necessarily been drawn to scale in order to enhance their clarity and improve understanding of these various elements and embodiments described herein. Furthermore, elements that are known to be common and well understood to those in the industry are not depicted in order to provide a clear view of the various embodiments described herein, thus the drawings are generalized in form in the interest of clarity and conciseness.

In the following discussion that addresses a number of embodiments and applications, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the embodiments described herein may be practiced. It is to be understood that other embodiments may be utilized, and changes may be made without departing from the scope of the disclosure.

Various inventive features are described below that can each be used independently of one another or in combination with another feature or features. However, any single inventive feature may not address all of the problems discussed above or only address one of the problems discussed above. Further, one or more of the problems discussed above may not be fully addressed by the features of each embodiment described below.

Described herein are embodiments of small volume, high resolution optical imaging systems and methods that can be used in satellites and other aerial systems. An optical system <NUM> is shown in <FIG> and is one embodiment that may be used for providing high resolution imaging performance in a "micro" or small form factor (volumetric envelope). The optical systems may "piggyback" on other missions with existing high bandwidth capabilities.

A constellation of satellites in orbit may operate in collaboration with each other for coordinated ground coverage. The orbits of the satellites in the constellation may be synchronized. For example, the orbits may be geostationary, where the satellites may have orbital periods equal to the average rotational period of Earth and in the same direction of rotation as Earth. Or the orbits may be sun-synchronous, such as a nearly polar orbit around Earth, in which the satellite passes over any given point of the Earth's surface at the same local mean solar time or the orbit processes through one complete revolution each year so it always maintains the same relationship with the Sun. Synchronous systems introduce complexity by requiring dedicated platforms and sensors, launchers, and operation stations. For remote sensing, typical examples of such synchronized constellations include the programs of PLANETS COPE (a. DOVE), SKYSAT, BLACKSKY, and CARBONITE.

The systems described herein may be used in systems in synchronous as well as asynchronous orbits. Thus, in some embodiments, the imaging systems may be used with an Asynchronous Constellation of Earth observation Camera system (ACEC). This is especially true for constellations of many small satellites, such as CUBESATs, and also with Low-Earth Orbit (LEO) broadband data relay satellite constellations, such as Space NGSO Satellite System, One Web, and Amazon's KUIPER System. Any of the optical systems or features thereof described herein may include any of the features of the micro optical and camera systems and other aspects described in "<NPL>), the entire contents of which are incorporated by reference herein in their entirety.

An asynchronous constellation may include camera systems onboard any available platforms, which have planned missions but can host additional payloads. It may be different from the nominal constellation in the sense that it will not be operated synchronously and not provide coordinated ground coverage with the sole purpose of only providing a stream of images. The most significant benefit of the asynchronous constellation is to avoid or minimize cost, time, and effort to develop a platform, require a specific launch system, and operate a dedicated ground control system, which can be a large fixed cost. An advantage of leveraging LEO broadband data relay satellites for asynchronous constellation imaging is its broad data bandwidth. CUBESATs or other platforms with dedicated imaging or other missions may suffer from decreased data bandwidth. Free from the data bandwidth issue, asynchronous constellation with the LEO data relay satellites can stream image data in dedicated channels as the satellites stream movies or other content so that users can selectively receive, record, and process image data.

To do this, much smaller or micro camera systems that have dimensional advantages and can accommodate themselves to any available space are needed. Recent developments of smaller cameras focuses on dimensional advantages only so that such development relies on optical designs that are easier to design, simpler to develop, or cheaper to build. However, such an approach may seem reasonable but may put a limit or constraint on leveraging such cameras for serious missions due to decreased performance, such as optical resolution.

The optical system <NUM> and the other embodiments of imaging systems described herein may be used for constellation operations and be micro in physical dimension as well as be advanced in performance. Systems and methods for a <NUM>-mirror telescope in a small form factor are described.

The embodiments described herein may be onboard satellite platforms that are already planned, as a secondary payload or an additional system. In some embodiments, the imaging system may have a size on the scale of a star sensor or tracker. The imaging system may be lightweight. The imaging system may minimize power consumption. The imaging system and its interface to a platform may be simple so that it can be installed and operated easily. The imaging system may be capable of proper imaging, which may be described by its specification. The imaging system may have proper MTF values. The imaging system may be designed to operate over a wide spectral range and equipped with a number of channels over the spectral range, with panchromatic, red, green, blue, and near infrared as a baseline set. The imaging system may be capable of a large field of view.

For such a camera system, an important requirement is distortion property. A camera system with a small f-number, a small aperture with a longer effective focal length for higher resolution, may need a time-delay-integration (TDI) sensor to achieve a proper signal-to-noise ratio (SNR) for further processing on the ground. Distortions induced by optical design can cause smear in the camera system. To avoid a significant degradation of image quality for TDI imaging, distortions induced by the system should be minimized over an entire field-of-view (FOV).

The optical imaging systems described herein are based on a reflective or mirror system, which may be unusual for a small form, affordable system. Usual cameras for CAN- or NANO-SAT are based on a cata-dioptric design for its design simplicity and cost reduction. The examples are PLANETSCOPE (a. DOVE), SKYSAT, BLACKSKY, and CARBONITE.

The design of the SKYSAT camera is based on a Ritchey-Cassegrain telescope, which has two mirrors (primary and secondary) and a small number of lenses. It is known for easy manufacturing, cost reduction, and simple alignment/ assembly logic. Also, it utilized COTS frame CMOS sensors. The CARBONITE camera is an example of commercially available, off-the-shelf astronomical telescope, which is modified to be accommodated to space environment, and equipped with a commercial CMOS sensor for color video imaging. Utilizing a commercial telescope seemed to be a smart move in a sense that development or manufacturing effort can be reduced, cost can be cut seriously, and operation management can be efficient. Whole processes were developed suitable for implementing constellation of Earth observation satellites.

Different from those approaches, the optical system embodiments described herein for cameras are based on a reflective design that is a four-mirror system. The optical system described herein may have no limit of spectral range to be covered. The system may have no chromatic aberration, which can be critical for multispectral imaging. The system may have high design flexibility due to degree of freedom of multi-mirror system. The system may have mass reduction deduced by mirror light-weighting. The system may have a small form factor.

<FIG> is a perspective schematic view of an optical layout of a first optical system <NUM> showing optical path lines. <FIG> is a perspective schematic view of the optical system <NUM> without the optical path lines shown for clarity. The optical lines may be indicative of multiple spectral bands. Referring to <FIG>, a perspective view of an optical layout of a second optical system <NUM> showing optical lines is illustrated. <FIG> shows the optical system <NUM> without the optical lines for clarity.

The first two mirrors of the optical systems <NUM>, <NUM>, a primary mirror <NUM> and a secondary mirror <NUM> in <FIG>, and a primary mirror <NUM> and a secondary mirror <NUM> in <FIG>, are responsible for power of the systems so that it can determine its effective focal length or resolution. "Effective focal length" as used herein has its usual and customary meaning, and includes without limitation the distance from a principal plane of an optical mirror to an imaging plane <NUM>, <NUM>. Entrance pupil <NUM> of the optical system <NUM> (shown in <FIG>), and entrance pupil <NUM> of the optical system <NUM> (shown in <FIG>), control the amount of light through the respective systems, and may be located at the respective primary mirrors. The entrance pupil may be the optical image of the physical aperture stop, as seen through the front (the object side) of the optical system. The corresponding image of the aperture as seen through the back of the optical system is called the exit pupil.

The primary mirrors <NUM>, <NUM> may be supported by a structural support <NUM> having radially extending beam <NUM> to support the mirror structure. The structure <NUM> and beams <NUM> may minimize the distortion on the primary mirror surface that may be induced by bonding and thermal environmental change. Also, it may protect the primary mirror from random vibration and shock that the camera may experience during launch.

In some embodiments, the various mirrors and supporting structures for any of the optical systems described herein may be formed of aluminum, ceramics, designed composite materials, other suitable materials, or combinations thereof. In some embodiments, the one or more structures and/or the one or more mirrors can be manufactured by 3D printing technology also known as additive manufacturing technology. For example, the mirrors and the supporting structure may all be additively manufactured as one monolithic piece.

A tertiary mirror <NUM> in <FIG>, and a tertiary mirror <NUM> in <FIG>, contribute to widening a field of view (FOV) and corrects corresponding residual optical aberrations. The tertiary mirrors <NUM>, <NUM> may not include an optical axis, for example for simpler manufacturability, and two or more tertiary mirrors may be manufactured from one base piece. A quaternary mirror <NUM> in <FIG>, and a quaternary mirror <NUM> in <FIG>, may minimize distortion and control a back focal length. "Back focal length" as used herein has it usual and customary meaning, and incudes without limitation the distance between the last surface of an optical mirror to its image plane. The fields of view of the optical systems <NUM>, <NUM> are designed so that the rays do not interfere with the respective quaternary mirror and the central aperture of the respective primary mirror. The quaternary mirrors <NUM>, <NUM> reflect the respective light along the optical path to the imaging plane <NUM>, <NUM>.

<FIG> shows the second optical system <NUM> but without showing the optical path lines for clarity. The diameter of the aperture or central hole <NUM> in <FIG> and <NUM> in <FIG> is minimized to maximize the use area of the primary mirror and in some embodiments is not larger than the corresponding secondary mirror <NUM>, <NUM>. The diameter of the central hole <NUM> in <FIG> and hole <NUM> in <FIG> may be designed large enough to not interfere with the light rays travelling through the central holes <NUM> and <NUM>.

The primary mirrors <NUM>, <NUM> and/or the secondary mirrors <NUM>, <NUM> may be symmetric or periodic about the respective optical axis. <FIG> are schematics showing diagonals for respectively a periodic mirror and an image plane. The diagonal of a periodic mirror may have an angle of zero degrees or <NUM> degrees from a diagonal of the image plane. The optical axis of the tertiary mirror may not coincide with a mechanical axis.

<FIG> show example embodiments of optical systems <NUM>, <NUM> respectively having a periodic primary mirror <NUM>, <NUM>. The optical systems <NUM>, <NUM> further include, respectively, a secondary mirror <NUM>, <NUM>, a tertiary mirror <NUM>, <NUM>, a quaternary mirror <NUM>, <NUM> and an imaging plane <NUM>, <NUM>. The optical systems <NUM>, <NUM> may have the same or similar features and/or functions as the optical systems <NUM> or <NUM>.

The optical systems <NUM>, <NUM> may include any of the same or similar features and/or functions as the other embodiments of optical systems described herein, and vice versa. For example, the optical systems <NUM>, <NUM> may include any of the same or similar features and/or functions as optical systems <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and vice versa. For example, for any of the optical systems described herein, the primary mirror may be concave and have a central aperture.

The primary mirror may have a parabolic surface, a non-parabolic conical surface, or an aspherical surface. A "parabolic surface" as used herein has its usual and customary meaning, and includes, without limitation, a reflective surface used to collect the light energy and may have a shape that is part of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis. A "non-parabolic conical surface" as used herein has its usual and customary meaning, and includes, without limitation, a curve rotated about its axis where the curve is obtained as the intersection of the surface of a cone with a plane other than a parabola. For example, the "non-parabolic conical surface" may be hyperbolic, elliptical, or circular. An "aspherical surface" as used herein has its usual and customary meaning, and includes, without limitation, a surface that is not spherical. In some embodiments, a spherical surface may be slightly altered so as to reduce spherical aberration.

The secondary mirror may be convex and face the primary mirror. The secondary mirror may have an aspherical surface. The tertiary mirror may be concave and arranged behind the primary mirror. "Behind" may refer to a side of the primary mirror that is opposite the side of the primary mirror that reflects incoming light to the secondary mirror. The tertiary mirror may have a parabolic surface, a non-parabolic conical surface, or an aspherical surface. The quaternary mirror may be concave and arranged in the central aperture of the primary mirror, before the primary mirror or behind the primary mirror, for example as shown in <FIG>. The quaternary mirror may have one of a spherical surface, a parabolic surface, a non-parabolic conical surface, or an aspherical surface.

There may be at least one image plane having one or more aggregated sensors, where the image plane is positioned at a specific distance from an optical axis. The optical axis may be defined as a geometric reference line extending between the vertices of the primary and secondary mirrors. The vertex for a given mirror may be a point on the mirror's surface where the principal axis meets the mirror.

The optical system <NUM> may have a larger primary mirror <NUM> and thus higher resolution relative to the primary mirror <NUM> of the optical system <NUM>. The resolution of the optical system <NUM> may be better than <NUM> at <NUM> altitude. The optical system <NUM> may have a resolution of better than <NUM> at <NUM> altitude. The optical system <NUM> may have a larger field of view (FOV) than the optical system <NUM>. The optical system <NUM> may have a narrower field of view (FOV) relative to the optical system <NUM>. The optical system <NUM> may have volumetric dimensions of <NUM> (W) x <NUM> (H) x <NUM> (L). The optical system <NUM> may have volumetric dimensions of <NUM> (W) x <NUM> (H) x <NUM> (L). The optical system <NUM> may be lighter in weight than the optical system <NUM>. The optical systems <NUM>, <NUM> may both have a proper MTF for higher resolution imaging.

Both the optical systems <NUM>, <NUM> may have similar mirror types and optical paths. But their respective purposes and missions may be different. The purpose of the optical system <NUM> may be to map the surface of the Earth and acquire geospatial data. The purpose of the optical system <NUM> may be for remote sensing and environmental monitoring.

In some embodiments, the optical systems <NUM>, <NUM> may achieve various parameters for orbital systems and/or imaging systems. Example parameters achievable with the optical systems <NUM>, <NUM> are described in Table <NUM>. For example, the design orbit may be set to <NUM>, the spectral bands may be designed to be compatible with big satellites and scientific satellite imaging except the panchromatic band, etc. The panchromatic band (PAN band) may be designed to include up to red-edge, improving Modulation Transfer Function (MTF) in the band, which may be unavoidable due to its small aperture size.

<FIG> is a block diagram of an example payload system <NUM> configuration for an optical system <NUM> in a satellite. The optical system <NUM> is shown in schematic form. The optical system <NUM> includes a concave primary mirror <NUM> having a central aperture <NUM>. The primary mirror may have one of parabolic, non-parabolic conical or aspherical surface. A smaller convex secondary mirror <NUM> faces the primary mirror <NUM> and has an aspherical surface. The secondary mirror may have an aspherical surface. A concave tertiary mirror <NUM> is arranged behind the primary mirror <NUM>. The tertiary mirror may have one of parabolic, non-parabolic conical or aspherical surface. A concave quaternary mirror <NUM> is arranged slightly behind the central aperture <NUM> of the primary mirror <NUM>, where the quaternary mirror can have one of a spherical, parabolic, non-parabolic conical or aspherical surface. The primary mirror <NUM>, the tertiary mirror <NUM> and the quaternary mirror <NUM> each have positive power or focal length, and the secondary mirror <NUM> has negative power. "Behind" may be defined as described above. Behind may also refer to a direction in <FIG> as oriented that is to the right, such that "behind" the primary mirror <NUM> may mean to the right of the primary mirror <NUM> as oriented in the figure.

An image sensor <NUM> having up to 'n' aggregated sensors that convert light into electrical signals is positioned behind the primary mirror <NUM>. In certain embodiments, the image sensor <NUM> may deliver an output format of thirty-two sub-LVDS (low-voltage differential signaling) channels of digital data across an interface <NUM> to a control and processing electronics portion <NUM> of the satellite. In other embodiments, other output formats are used. The sensor <NUM> includes a readout integrated circuit (ROIC) used for infrared, visible, and other arrayed sensors. The functions supported by the ROIC include processing and shaping of an image signal and may include unit cell preamplifiers. Interface <NUM> also includes control signals from the control and processing electronics <NUM>, where the control signals may include a serial peripheral interface (SPI) and a clock signal in some embodiments.

In certain embodiments, a data formatting and distribution subsystem <NUM> receives the data across the interface <NUM> and then further sends the data to a data processing with machine learning subsystem <NUM> and to a data storage and archiving subsystem <NUM> to be stored. The stored data from the data storage and archiving subsystem <NUM> can be sent directly to the data processing subsystem <NUM> for various types of processing. The output of processed data from the data processing subsystem <NUM> can be sent directly to the data storage and archiving subsystem <NUM> for storage. The output of processed data from the data processing subsystem <NUM> and data from the data storage and archiving subsystem <NUM> can be sent to a data formatting, encryption and transmission subsystem <NUM>. The output of the data formatting, encryption and transmission subsystem <NUM>, such as image data, is then sent to a satellite Bus for further distribution, which may include transmission to an earth station, relay satellite, or other entity that receives the data. The data processing subsystem <NUM> may include one or processors and one or more memories, such as a memory for program instructions and a memory and/or a cache for data.

A payload control electronics subsystem <NUM> receives telecommands from the satellite Bus and provides housekeeping data to the satellite Bus. The payload control electronics subsystem <NUM> provides commands to portions of the payload system <NUM>, including to the image sensor <NUM> and/or to a thermal control, temperature data and optical focusing subsystem <NUM>. The thermal control, temperature data and optical focusing subsystem <NUM> provides control signals, such as thermal control and optical focusing, via an interface <NUM> to the optical system <NUM> and receives temperature data back from the optical system <NUM>.

A power conversion, distribution and telemetry subsystem <NUM> receives telecommands from the satellite Bus and provides telemetry data to the satellite Bus. The power conversion, distribution and telemetry subsystem <NUM> may also receive power, such as from solar panels or batteries of the satellite.

An important issue with imaging systems for smaller satellites, such as CUBESATs, is calibration, including absolute and inter-sensor calibrations. Most imagery from commercial CUBESATs is not calibrated in a standard way on a standard radiance or reflectance scale, for example. Thus, it may be challenging to compare the image data with big commercial satellites or scientific satellites imaging, like MODIS or LANDSAT. Even inter-sensor calibration is uncertain, which may owe mainly to temporarily unstable or inconsistent performance of commercial sensors.

On the contrary, the sensors for the optical systems described herein, such as the aggregate sensor of the sensor <NUM> in optical system <NUM>, may be developed and tailored to space applications and their consistency and stability may be validated. Importantly, the optical system <NUM> and all the optical systems described herein may be calibrated according to the standard processes and with respect to each other so that all the image data from the systems is compatible with each other and also with reference systems.

Referring to <FIG>, an example embodiment of a sensor circuit for the image sensor <NUM> is shown. The image sensor <NUM> may include a readout integrated circuit (ROIC) <NUM> and a charged coupling device (CCD) array <NUM>. Photons incident on the surface of the CCD array <NUM> (top surface as oriented in the figure) generate a charge that can be read by electronics and turned into a digital copy of the light patterns falling on the device. In certain embodiments, a charge coupled device in complementary metal-oxide- semiconductor (CCD-in-CMOS) time delay integration (TDI) sensor from IMEC International may be used for the optical system <NUM>, even though a pixel size of <NUM> micrometers (µm) is preferred. In some embodiments, using a format of <NUM> columns and <NUM> stages per multiband CCD array <NUM>, a backside illumination sensor combines a TDI CCD array with CMOS drivers and readout pixels on a pitch of <NUM>. An on-chip control and sequencer circuit may be included. In certain embodiments, a <NUM> clock <NUM> may be an input to the image sensor along with a serial peripheral interface (SPI) for control. The imager may interface through the SPI and may integrate on-chip PLLs to deliver an output format of <NUM> sub-LVDS (low-voltage differential signaling) channels as part of the ROIC <NUM>. A seven-band version of the circuit can contain seven CCD arrays of <NUM> × <NUM> pixels each.

In other embodiments, other sensor circuits may be used for the image sensor <NUM>, which may have differing sizes of the array <NUM> and a different ROIC <NUM> for the output of data. For example, the image sensor <NUM> may include four panchromatic CCD arrays of <NUM> × <NUM> pixels each and eight multispectral CCD arrays of <NUM> × <NUM> pixels.

To maximize the area that is exposed to light, backside illumination technology can be used. This consists of bonding the sensor wafer to a carrier wafer and thinning it from the backside. This directly exposes the CCD gates to the light, without obstruction of metal lines. An effective fill factor thus reaches <NUM>% percent. Backside illuminated CMOS imagers feature a very high intrinsic light sensitivity and are very efficient in detecting (near) ultraviolet and blue light. Several antireflective coatings (ARCs) are available to reach a high quantum efficiency in selected regions of the spectrum, e.g. more than <NUM>% in the UV range or more than <NUM>% in the visible range.

With a TDI sensor, image quality is sensitive to platform motion, which can be represented by image smear MTF. The image smear MTF of the optical system <NUM> may be <NUM> with smearing of <NUM> pixels, the number of TDI steps at <NUM>, and a clocking phase of <NUM>. This may impose requirements of attitude stability of the platform, which may be twenty-two micro-radians per second (µrad/sec) or <NUM> arcseconds per second (arcsec/sec). When the attitude stability requirement is relaxed to smearing of one pixel, then the smear MTF becomes <NUM> and the stability may be <NUM> arcsec/sec.

Referring to <FIG>, a schematic of an embodiment of an all-reflective optical system <NUM> is shown. The optical design of the optical system <NUM> may be different from a traditional Three Mirror Anastigmat TMA or three mirror Korsch design. The Korsch design may have an ellipsoid surface for the primary mirror, a hyperbola surface for the secondary mirror, and an ellipsoid surface for the tertiary mirror.

The optical system <NUM> includes a concave primary mirror <NUM> having a central aperture <NUM>, where the primary mirror can have one of a parabolic, non-parabolic conical, or aspherical surface. A smaller convex secondary mirror <NUM> faces the primary mirror <NUM> and has an aspherical surface. A concave tertiary mirror <NUM> is arranged behind the primary mirror <NUM>, where the tertiary mirror can have one of a parabolic, non-parabolic conical or aspherical surface. A concave quaternary mirror <NUM> is arranged in front of the central aperture <NUM> of the primary mirror <NUM>, where the quaternary mirror can have one of a spherical, parabolic, non-parabolic conical or aspherical surface. The primary mirror <NUM>, the tertiary mirror <NUM> and the quaternary mirror <NUM> each have positive power or focal length, and the secondary mirror <NUM> has negative power.

An image plane <NUM> having one or more aggregated sensors that convert light into electrical signals is positioned behind the primary mirror <NUM>. In certain embodiments, the image plane <NUM> is positioned at a specific distance from an optical axis that is defined by a mechanical symmetry around a line through the vertices of the primary and the secondary mirrors, which may define the "optical axis. " The specific distance is within the physical radius (from the optical axis) of the primary mirror. Thus, the image plane will not exceed a cylindrical envelope that is defined by the radius from the optical axis of the primary mirror. The radius of the primary mirror may extend perpendicularly from the principal axis of the mirror to an outermost edge of the mirror. The principal axis may be a geometric reference line going through the center of the mirror that is exactly perpendicular to the surface of the mirror.

The optical system <NUM> uses the secondary mirror <NUM> that is symmetric around the optical axis. The tertiary mirror <NUM> can have a segmented non-circular shape. The quaternary mirror <NUM> can have a circular or non-circular shape. The primary mirror <NUM> can have a circular or a non-circular shape, where the latter is to enhance modulation transfer function (MTF) and signal to noise ratio (SNR). A circular shape is inscribed to a non-circular shape, which may be periodic about the optical axis.

For an example of a square and its incircle, the incircle may be the shape of a primary mirror for traditional optical system design. If the radius of the incircle is "r," then the area of the square will be larger by <NUM>/pi. This is not usually an issue for a larger camera for which a large volume is allocated. However, for a small satellite, which is usually a cuboid, a primary mirror in a square shape can have a larger area by <NUM>/pi and boost MTF and SNR.

Both Korsch and other four-mirror optical designs do not use a parabolic surface for the primary and/or the tertiary mirrors. With the primary and/or the tertiary mirrors of the optical system <NUM> having a parabolic surface, the optical system <NUM> can provide a unique, affordable solution to a mission with budget constraints. For a parabolic surface, a general test setup can be used for manufacturing, or a stitching measurement is possible. Also, a commercial product line can be used for parabolic mirror manufacturing, especially when mirrors are smaller than <NUM>. In contrast, non-parabolic conic or aspherical surface may require a dedicated test tool, including computer generated hologram (CGH) or nulling optics.

For a parabolic surface, a general test setup can be used for manufacturing, or stitching measurement is possible. Also, a commercial product line can be used for parabolic mirror manufacturing, especially, when mirrors are smaller than <NUM>.

The primary and the secondary mirrors <NUM>, <NUM> forming the optical axis are symmetric or periodic about this axis. The primary and secondary mirrors face each other. The tertiary mirror <NUM> faces the back of the primary mirror <NUM> and may be a segmented mirror. As used herein, "segmented mirror" includes its usual and customary meaning and includes, without limitation, an array of smaller mirrors designed to act as segments of a single, larger curved mirror. The optical axis of the tertiary mirror <NUM> may not coincide with a mechanical axis. As used herein, the "mechanical axis" has its usual and customary meaning, and may includes, without limitation, a normal vector at the center or at the edge of the mirror. In certain embodiments, the tertiary mirror <NUM> is a segment of a larger mirror. In such embodiments, the optical axis for the tertiary mirror <NUM> may refer to the optical axis of the larger mirror and the mechanical axis may refer to the axis of the segmented mirror. The quaternary mirror <NUM> faces the tertiary mirror <NUM> and is positioned to avoid interference with rays from the secondary mirror <NUM> to the tertiary mirror <NUM>.

The metering and supporting structure of the mirrors can be a cylindrical tube or a conical baffle of the primary mirror <NUM>, such as those shown and described with respect to <FIG>. The cylindrical envelope may be coextensive with a cylindrical structure to limit the specific distance at which the imaging plane is located relative to the optical axis between the primary and secondary mirrors. For example, the location of the imaging plane may be radially limited by the radius of the cylindrical structure.

Light rays impinge upon and are reflected by the primary mirror <NUM> first, the secondary mirror <NUM> next, the tertiary mirror <NUM> thirdly, and finally the quaternary mirror <NUM>, so that the rays reach the image plane <NUM>. The image plane <NUM> includes one or more sensors, which may be aggregated in an orderly manner. An entrance pupil of the optical system <NUM> can be positioned near the primary or the secondary mirrors <NUM>, <NUM>. An intermediate focus is formed around a vertex of the primary mirror <NUM>, positioned between the primary and the secondary mirrors <NUM>, <NUM>, or between the primary mirror <NUM> and the tertiary mirror <NUM>. An exit pupil or Lyot stop can be positioned near the quaternary mirror <NUM>, between the tertiary and the quaternary mirrors <NUM>, <NUM>, or between the quaternary mirror <NUM> and the image plane <NUM>. As used herein, a "Lyot stop" has its usual and customary meaning and includes, without limitation, an optical stop that reduces the amount of flare which may be caused by diffraction of other stops and baffles in the optical system. The Lyot stops may be located at images of the system's entrance pupil and have a diameter slightly smaller than the pupil's image.

The optical system <NUM> has a small form factor. The form factor is defined as the ratio of <NUM>) a distance between the secondary and tertiary mirrors <NUM>, <NUM> to <NUM>) an effective focal length of the optical system <NUM>. The optical system <NUM> has a form factor of less than <NUM>. The form factor may be from about <NUM> to less than <NUM>.

The form factor may have the following values or about the following values: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. The form factor may be less than <NUM>, less than <NUM>, less than <NUM>, less than <NUM>, less than <NUM>, less than <NUM>.

In addition to the small form factor, the optical system <NUM> has a benefit over the prior art in having a much shorter physical distance from the tertiary mirror <NUM> to the image plane <NUM>. The prior art has quite a long distance between the tertiary mirror and image plane and mandates one or more folding mirrors to fit into a limited dimension. This configuration may lead to difficulty in optical alignment, thermal instability during operation, which may end with performance degradation. The optical system <NUM>, because of the small form factor and the short distance between the tertiary mirror <NUM> and the image plane <NUM>, eliminates unnecessary folding mirrors and simplifies the alignment and assembly and stability of operation.

The optical system can be designed to have mirrors of zero coefficient of thermal expansion (CTE) materials (such as Zerodur, Fused Silica, Suprasil, Astrostiall, etc.), low-CTE materials (such as Borosilicate glass, like BOROFLOAT, Pyrex, etc.), and mild-CTE materials (such as Crown glass, like NBK7).

For CTE matching, a specific combination of mirror and structure materials are used for the optical system. Super-invar, invar, or designed composite material can be used for zero-CTE mirror materials. Invar, Kovar, ceramics, or designed composite material can be used for low-CTE mirror materials. Titanium, ceramics, or design composite materials can be used for mild-CTE mirror materials.

A monolithic structure can be used for the optical system as an ultimate solution. Mirrors and structures can be made of one material, including aluminum, ceramics, designed composite materials, and is not limited to this list.

Referring to <FIG>, a schematic of another embodiment of an all-reflective optical system <NUM> is shown. The optical system <NUM> includes a primary mirror <NUM>, a secondary mirror <NUM>, a tertiary mirror <NUM>, a quaternary mirror <NUM>, and image plane <NUM>. The primary mirror <NUM>, secondary mirror <NUM>, tertiary mirror <NUM>, the quaternary mirror <NUM> and image plane <NUM> may have the same or similar features and/or functions as, respectively, the primary mirror <NUM>, the secondary mirror <NUM>, the tertiary mirror <NUM>, the quaternary mirror <NUM> and image plane <NUM> of the optical system <NUM>, and vice versa.

However, in the optical system <NUM>, the quaternary mirror <NUM> is located behind the primary mirror <NUM>, but close to an aperture <NUM> in the primary mirror <NUM>. The tertiary mirror <NUM> is positioned further behind the primary mirror <NUM> in the optical system <NUM> than in the optical system <NUM>. In certain embodiments, the tertiary mirror <NUM> may be positioned a distance behind the primary mirror <NUM> that is in a range of <NUM>% to <NUM>%, of <NUM>% to <NUM>%, or of <NUM>% to <NUM>%, of the diameter of the primary mirror <NUM>. The primary mirror <NUM> of the optical system <NUM> in <FIG> can be manufactured in one body with the tertiary mirror <NUM> so that the center of mass is closer to the primary mirror <NUM> and the crosstalk moment of inertia (MOI) of the system can be reduced. The optical system <NUM> of <FIG> is different from the optical system <NUM> with regard to effective focal length and field-of-view. An advantage of the configuration of the optical system <NUM> is that placing the quaternary mirror <NUM> closer to the primary mirror <NUM> may make it easier to set a Lyot stop on the quaternary mirror <NUM> and the central hole size or aperture <NUM> of the primary mirror can be minimized.

Referring to <FIG>, a schematic of another embodiment of an all-reflective optical system <NUM> is shown. The optical system <NUM> includes a primary mirror <NUM>, a secondary mirror <NUM>, a tertiary mirror <NUM>, a quaternary mirror <NUM>, and an image plane <NUM>. The primary mirror <NUM>, secondary mirror <NUM>, tertiary mirror <NUM>, quaternary mirror <NUM> and image plane <NUM> may have the same or similar features and/or functions as, respectively, the primary mirror <NUM>, the secondary mirror <NUM>, the tertiary mirror <NUM>, the quaternary mirror <NUM> and the image plane <NUM> of the optical system <NUM>, and vice versa.

However, in the optical system <NUM>, the quaternary mirror <NUM> is located a distance behind the primary mirror <NUM> that is greater than a distance between the quaternary mirror <NUM> and the primary mirror <NUM> of the optical system <NUM> (see <FIG>). Further, in the optical system <NUM>, the tertiary mirror <NUM> is positioned a distance behind the primary mirror <NUM> that is greater than a distance of the respective corresponding mirrors of the optical system <NUM>. In certain embodiments, the tertiary mirror <NUM> may be positioned a distance behind the primary mirror <NUM> that is in a range of <NUM>% to <NUM>% of the diameter of the primary mirror <NUM>. The optical system <NUM> may be designed for much smaller pixel sensors, such as having a pixel size of less than <NUM> micrometers in certain embodiments. The optical system <NUM> may be different from the optical system <NUM> with respect to effective focal length and field-of-view. In some embodiments, the optical system <NUM> may have have a shorter effective focal length and a wider field-of-view relative to the optical system <NUM>, which may allow the system <NUM> to include sensors with smaller pixel size. ut it may be relatively closer to an aperture <NUM> in the primary mirror <NUM>. The tertiary mirror <NUM> is similarly positioned behind the primary mirror <NUM> as in the optical system <NUM>. An added folding mirror <NUM> receives rays from the quaternary mirror <NUM> and reflects them to the image plane <NUM>, which is positioned above the folding mirror <NUM>. In certain embodiments, the image plane <NUM> is positioned to be above and parallel to the optical axis.

Some embodiments of the optical systems may have a longer system optical path length between the quaternary mirror <NUM> and the image plane <NUM> using the folding mirror <NUM>. If the image plane <NUM> is behind the tertiary mirror <NUM>, the system optical path length is the distance between the secondary mirror <NUM> and the image plane <NUM>. With using the folding mirror <NUM>, the system optical path length is the distance between the secondary mirror <NUM> and the tertiary mirror <NUM>. The image plane <NUM> may be positioned to satisfy the requirement of focal length and field-of-view. The configuration of the optical system <NUM> may provide a compact design. Another advantage is that the system <NUM> may allow for easier installation of a sensor cooler and a radiating plate for the cooler. Furthermore, in the optical system <NUM>, a sensor for the image plane can be positioned closer to the primary mirror supporting structure and the sensor can be held in a more stable way.

Referring to <FIG>, another embodiment of an all-reflective optical system <NUM> having a folding mirror <NUM> is shown. The optical system <NUM> may have the same or similar features and/or functions as the optical system <NUM>, and vice versa. The optical system <NUM> includes a primary mirror <NUM>, a secondary mirror <NUM>, a tertiary mirror <NUM>, a quaternary mirror <NUM>, and an image plane <NUM>, which may have the same or similar features and/or functions as, respectively, the primary mirror <NUM>, the secondary mirror <NUM>, the tertiary mirror <NUM>, the quaternary mirror <NUM>, and the image plane <NUM> of the optical system <NUM>, and vice versa. The quaternary mirror <NUM> is behind the primary mirror <NUM>, as in the optical system <NUM>, but is close to an aperture <NUM> in the primary mirror <NUM>. The tertiary mirror <NUM> is similarly positioned behind the primary mirror <NUM> as in the optical system <NUM>. The folding mirror <NUM> receives rays from the quaternary mirror <NUM> and reflects them to the image plane <NUM>, which is positioned below the folding mirror <NUM>. In certain embodiments, the image plane <NUM> is positioned to be below and parallel to the optical axis. An advantage of the configuration of the optical system <NUM> is that the configuration of the mirrors including the folding mirror <NUM> leads to a more compact design. Another advantage is that the optical system <NUM> can use a sensor for the image plane in a larger package. A CMOS sensor or a sensor with a ROIC tends to have larger package so that it can embrace more circuits or components that may help minimize readout noise, crosstalk, and blooming.

Referring to <FIG>, another embodiment of an all-reflective optical system <NUM> having a folding mirror <NUM> is shown. The optical system <NUM> may have the same or similar features and/or functions as the optical system <NUM>, and vice versa. The optical system <NUM> includes a primary mirror <NUM>, a secondary mirror <NUM>, a tertiary mirror <NUM>, a quaternary mirror <NUM>, and an image plane <NUM>, which may have the same or similar features and/or functions as, respectively, the primary mirror <NUM>, the secondary mirror <NUM>, the tertiary mirror <NUM>, the quaternary mirror <NUM>, and the image plane <NUM> of the optical system <NUM>, and vice versa. However, the image plane <NUM> is closer to the optical axis than the image plane <NUM> is close to its respective optical axis. The quaternary mirror <NUM> is behind the primary mirror <NUM> but is further behind it than in the respective corresponding components of the optical system <NUM>. The tertiary mirror <NUM> is positioned further behind the primary mirror <NUM> than in the optical system <NUM>. The folding mirror <NUM> receives rays from the quaternary mirror <NUM> and reflects them to the image plane <NUM>, which is positioned below the folding mirror <NUM>. In certain embodiments, the image plane <NUM> is positioned to be below and parallel to the optical axis. The optical system <NUM> is designed for smaller pixel sensors, which are usually commercial or MIL-STD. An advantage of the optical system <NUM> is that it can utilize up-to-date sensors, including commercial or MIL-STD sensors.

Referring to <FIG>, another embodiment of an all-reflective optical system <NUM> having a folding mirror <NUM> is shown. The optical system <NUM> may have the same or similar features and/or functions as the optical system <NUM>. The optical system <NUM> includes a primary mirror <NUM>, a secondary mirror <NUM>, a tertiary mirror <NUM>, a quaternary mirror <NUM>, and an image plane <NUM>, which may have the same or similar features and/or functions as, respectively, the primary mirror <NUM>, the secondary mirror <NUM>, the tertiary mirror <NUM>, the quaternary mirror <NUM>, and the image plane <NUM> of the optical system <NUM>, and vice versa.

However, in the optical system <NUM>, the image plane <NUM> is closer to the optical axis than the image plane <NUM> is close to its respective optical axis. The quaternary mirror <NUM> is behind the primary mirror <NUM>, in a similar distance to that of the optical system <NUM>. The tertiary mirror <NUM> is positioned behind the primary mirror <NUM> in a similar distance to that of the optical system <NUM>. The folding mirror <NUM> receives rays from the quaternary mirror <NUM> and reflects them to the image plane <NUM>, which is positioned above the folding mirror <NUM>. In certain embodiments, the image plane <NUM> is positioned to be above and parallel to the optical axis. Advantages of the optical system <NUM> are that a sensor of the image plane can be more stable against vibration and that a cooler with a radiator can be installed in an easier way than in other optical system configurations.

Referring to <FIG>, an embodiment of an all-reflective optical system <NUM> having multiple image planes and a folding mirror <NUM> is shown. The optical system <NUM> may have the same or similar features and/or functions as the optical system <NUM>. The optical system <NUM> includes a primary mirror <NUM>, a secondary mirror <NUM>, a tertiary mirror <NUM>, a quaternary mirror <NUM>, and a first image plane <NUM>, which may have the same or similar features and/or functions as, respectively, the primary mirror <NUM>, the secondary mirror <NUM>, the tertiary mirror <NUM>, the quaternary mirror <NUM> and the image plane <NUM> of the optical system <NUM>, and vice-versa. The first image plane <NUM> is a similar distance to the optical axis as the image plane <NUM> is to its respective optical axis. However, optical system <NUM> has a second image plane <NUM>' similar to the first image plane <NUM>. The first image plane <NUM> can be dedicated to a first spectral range and the second image plane <NUM>' can be dedicated to a second spectral range.

The quaternary mirror <NUM> is behind the primary mirror <NUM> and is close to an aperture <NUM> in the primary mirror <NUM> at a distance similar to that in the respective corresponding components of the optical system <NUM>. The tertiary mirror <NUM> is positioned behind the primary mirror <NUM> in a similar distance to that in the respective corresponding components of the optical system <NUM>. The folding mirror <NUM> receives rays from the quaternary mirror <NUM> and reflects some of the rays within a certain spectral range to the first image plane <NUM>, which is positioned above the folding mirror <NUM>. The folding mirror <NUM> may be transmissive to rays within a second different range from that which is reflected. The optical system <NUM> enables simultaneous multi-color imaging by having the folding mirror <NUM> be reflective over the first spectral range and transmissive over the second spectral range. In certain embodiments, the first image plane <NUM> is positioned to be above and parallel to the optical axis, and the second image plane <NUM>' is positioned to be below and perpendicular to the optical axis on an opposite side of the optical axis as the first image plane <NUM>. An advantage of the optical system <NUM> is that it can perform multicolor imaging due to the properties of the folding mirror and the multiple imaging planes.

Referring to <FIG>, another embodiment of an all-reflective optical system <NUM> having multiple image planes and a folding mirror <NUM> is shown. The optical system <NUM> may have the same or similar features and/or functions as the optical system <NUM>. The optical system <NUM> includes a primary mirror <NUM>, a secondary mirror <NUM>, a tertiary mirror <NUM>, a quaternary mirror <NUM>, a folding mirror <NUM> and a first image plane <NUM>, which may have the same or similar features and/or functions as, respectively, the primary mirror <NUM>, the secondary mirror <NUM>, the tertiary mirror <NUM>, the quaternary mirror <NUM>, the folding mirror <NUM> and the first image plane <NUM> of the optical system <NUM>, and vice versa. However, in the optical system <NUM>, the first image plane <NUM> is positioned at a greater distance to the optical axis than the first image plane <NUM> is to its respective optical axis. The optical system <NUM> has a second image plane <NUM>' similar to the first image plane <NUM>. The first image plane <NUM> can be dedicated to a first spectral range and the second image plane <NUM>' can be dedicated to a second spectral range.

The quaternary mirror <NUM> is behind the primary mirror <NUM> and is close to an aperture <NUM> in the primary mirror <NUM> at a distance similar to that in the respective corresponding components of the optical system <NUM>. The tertiary mirror <NUM> is positioned behind the primary mirror <NUM> in a similar distance to that in the respective corresponding components of the optical system <NUM>. The folding mirror <NUM> receives rays from the quaternary mirror <NUM> and reflects some of them to the first image plane <NUM>, which is positioned below the folding mirror <NUM>. The optical system <NUM> enables simultaneous multi-color imaging by having the folding mirror <NUM> be reflective over a first spectral range and transmissive over a second spectral range. In certain embodiments, the first image plane <NUM> is positioned to be below and parallel to the optical axis, and the second image plane <NUM>' is positioned to be below and perpendicular to the optical axis. An advantage is that the optical system <NUM> can use a sensor for the image plane in a larger package. A CMOS sensor or a sensor with a ROIC tends to have larger package so that it can embrace more circuits or components that may help minimize readout noise, crosstalk, and blooming.

Referring to <FIG>, another embodiment of an all-reflective optical system <NUM> having multiple image planes and a folding mirror <NUM> is shown. The optical system <NUM> may have the same or similar features and/or functions as the optical system <NUM>. The optical system <NUM> includes a primary mirror <NUM>, a secondary mirror <NUM>, a tertiary mirror <NUM>, a quaternary mirror <NUM>, a folding mirror <NUM> and a first image plane <NUM>, which may have the same or similar features and/or functions as, respectively, the primary mirror <NUM>, the secondary mirror <NUM>, the tertiary mirror <NUM>, the quaternary mirror <NUM>, the folding mirror <NUM> and the first image plane <NUM> of the optical system <NUM>, and vice versa. However, in the optical system <NUM>, the first image plane <NUM> is positioned at a shorter distance to the optical axis than the first image plane <NUM> is to its respective optical axis. The optical system <NUM> has a second image plane <NUM>' similar to the first image plane <NUM>. The first image plane <NUM> can be dedicated to a first spectral range and the second image plane <NUM>' can be dedicated to a second spectral range.

The quaternary mirror <NUM> is behind the primary mirror <NUM> at a distance greater than in the respective corresponding components of the optical system <NUM>. The tertiary mirror <NUM> is positioned behind the primary mirror <NUM> at a greater distance than in the respective corresponding components of the optical system <NUM>. The folding mirror <NUM> receives rays from the quaternary mirror <NUM> and reflects them to the first image plane <NUM>, which is positioned below the folding mirror <NUM>. The optical system <NUM> enables simultaneous multi-color imaging by having the folding mirror <NUM> be reflective over the first spectral range and transmissive over the second spectral range. In certain embodiments, the first image plane <NUM> is positioned to be below and parallel to the optical axis, and the second image plane <NUM>' is positioned to be below and perpendicular to the optical axis. The second image plane <NUM>' is positioned to be closer to the optical axis than the second image plane <NUM>' is close to its respective optical axis. The optical system <NUM> is designed to utilize smaller pixel sensors for the image plane. An advantage of the optical system <NUM> is that it can utilize up-to-date sensors, including commercial or MIL-STD sensors.

The quaternary mirror <NUM> is behind the primary mirror <NUM> at a distance greater than in the respective corresponding components of the optical system <NUM><NUM>. The tertiary mirror <NUM> is positioned behind the primary mirror <NUM> at a greater distance than in the respective corresponding components of the optical system <NUM>. The folding mirror <NUM> receives rays from the quaternary mirror <NUM> and reflects them to the first image plane <NUM>, which is positioned above the folding mirror <NUM>. The optical system <NUM> enables simultaneous multi-color imaging by having the folding mirror <NUM> be reflective over the first spectral range and transmissive over the second spectral range. In certain embodiments, the first image plane <NUM> is positioned to be above and parallel to the optical axis, and the second image plane <NUM>' is positioned to be below and perpendicular to the optical axis. The second image plane <NUM>' is positioned to be closer to the optical axis than the second image plane <NUM>' is close to its respective optical axis. An advantage of the optical system <NUM> is that a cooler with a radiator for the sensor can be installed in an easier way than in other optical system configurations.

Referring to <FIG>, a cross-sectional perspective view of a camera system <NUM> having an optical system is illustrated. A box <NUM> illustrates enclosing of the camera and can be a mechanical interface to a satellite BUS. A metering structure <NUM>, shown as a cone shaped structure, maintains a distance between a primary mirror <NUM> and a secondary mirror <NUM>. The metering structure <NUM> may maintain this distance within one micrometer when a temperature changes by <NUM> degree. A supporting structure <NUM>, best shown as a cylindrical tube in <FIG>, supports the primary mirror <NUM>. In certain embodiments, the radius of the cylindrical structure <NUM> can be defined by a radius from the portion of the optical axis extending between the primary mirror <NUM> and the secondary mirror <NUM>. The inner surface of the curved sidewall of the cylindrical structure <NUM> can be a limit of the specific distance from the optical axis for the image plane <NUM> described above.

In certain embodiments, dimensions of the camera are <NUM> x <NUM> x <NUM>. Depending on the focal length of an optical system, the dimensions may range from <NUM> x <NUM> x <NUM>, designed for <NUM> resolution at <NUM>, to dimensions of <NUM> x <NUM> x <NUM>, designed for <NUM> resolution at <NUM>. The overall volumetric envelope of the camera system may be less than <NUM>. 01m3, less than <NUM>. 008m3, less than <NUM>. 006m3, less than <NUM>. 004m3, less than <NUM>. 003m3, less than <NUM>. 001m3, or from. <NUM> m3 to <NUM>.

The form factor is defined as the ratio of a distance between the secondary and tertiary mirror to a focal length of the optical system. The distance between the secondary and tertiary mirror may be measured along the optical path. The optical system can be implemented in a form factor having the values described above, for example of less than <NUM>. For the prior art, the form factor is known to be more than <NUM>. With the relatively smaller form factor of the optical systems described herein, the optical system can provide imaging resolution better than <NUM>, <NUM>, or <NUM> at <NUM> altitude. The optical system can also be capable of imaging resolution better than <NUM> in an elliptical orbit. In other embodiments, the form factor can be in a range between <NUM> and <NUM>. Examples of focal lengths, distances between the secondary mirror and the tertiary mirror for each focal length and a corresponding form factor of the system are provided in Table <NUM>.

Referring to <FIG>, an embodiment of an optical system <NUM> for cameras is illustrated. A metering structure <NUM>, shown as a cone shaped structure, keeps a distance between a primary mirror <NUM> and a secondary mirror <NUM> to the design within +/- one micrometer when a temperature changes by one Celsius degree.

For thermal controlling of the metering structure, temperature sensors and heaters (wire or patch type) can be installed on the metering structure. Payload control electronics reads the data from the temperature sensors and control the heaters to keep the metering structure <NUM> within a specified range so that the focus of the camera system is on aggregated sensors.

A ring structure <NUM> is a supporting structure for the primary mirror <NUM> and supports the primary mirror kinematically so as to minimize structural distortion that may be induced during assembly. Also, the ring structure <NUM> can be an interface to a satellite BUS, which can eliminate the need of a box-type enclosure, such as the enclosure <NUM> shown in <FIG>.

Referring to <FIG>, a partial cross-sectional perspective view of an optical system <NUM> for cameras is illustrated. A metering structure <NUM>, shown cone-shaped, maintains a distance between a primary mirror <NUM> and a secondary mirror <NUM>, which may be maintained in some embodiments within ± one micrometer when a temperature of the metering structure <NUM> changes by one Celsius degree. The supporting structure <NUM>, shown as a ring-shaped structure, for the primary mirror <NUM> supports a primary mirror kinematic mounting structure <NUM> so as to minimize structural distortion that may be induced during assembly. Also, the supporting structure <NUM> can be an interface to a satellite BUS. In certain embodiments, the radius of the supporting structure <NUM> can be defined by a physical radius from the optical axis of the primary mirror <NUM>. The inner surface of the ring structure <NUM> can be a limit of the specific distance from the optical axis for the image plane <NUM> described above. _ A diameter of the primary mirror <NUM>, which is about <NUM> % of the focal length of the optical system, determines a width and height of the camera system. The length of the camera system is determined by the distance between the secondary mirror <NUM> and a tertiary mirror, which is about <NUM> to <NUM> % of the focal length of the optical system.

<FIG> is a partial cross-sectional perspective view of an optical system <NUM> showing the cylindrical housing <NUM> having a radius <NUM> equal to the radius of the primary mirror <NUM>. The imaging plane may be located a radial distance from the optical axis that is no more the radius <NUM>. The housing <NUM> may thus also have the same or nearly the same radius as the primary mirror for space savings. The optical axis extends between vertices of the primary and secondary mirrors.

The performance of the optical system <NUM> and the optical system <NUM> was analyzed to assess its design Modulation Transfer Function (MTF), tolerance MTF, and its distortion. Even though MTF and distortion is a way to evaluate optical performance of the system, they also indicate how the quality of resulting images will be. The MTF of panchromatic band is lower than other big camera systems, which cannot be avoided due to its smaller aperture size. Despite the lower MTF values, image quality can be enhanced by post processing on ground and also benefits by having a smaller anti-aliasing effect.

Referring to <FIG>, the graphs present the optical design MTF and tolerance MTF, respectively, of the panchromatic band for the optical system <NUM>. The Nyquist frequencies at which the MTF values are estimated are <NUM> / cycle for the panchromatic band and <NUM> / cycle for the multispectral bands. For tolerancing, sensitivity of each component is studied with assembly and alignment logics considered.

Referring to <FIG>, the graphs present the optical design MTF and tolerance MTF, respectively, of the near-infrared (NIR) band for the optical system <NUM>. Referring to <FIG>, the graphs present the optical design MTF and tolerance MTF, respectively, of the blue band for the optical system <NUM>.

The estimated MTF values of optical system <NUM> are summarized in Table <NUM>. For panchromatic band, the design MTF is higher than <NUM> % and tolerance value is slightly above <NUM> %. For multispectral bands, the design values are greater than <NUM> % and tolerance values are better than <NUM> %. With tolerancing, MTF drop is higher in multispectral bands because those are located away from optical axis with their lower sampling frequency reflected.

Referring to <FIG>, the graphs present the analysis results of optical design MTF and tolerance MTF, respectively, of the panchromatic band for the optical system <NUM>. In a similar manner as for the optical system <NUM>, the Nyquist frequencies are <NUM> / cycle for the panchromatic band and <NUM>/ cycle for the multispectral bands. Sensitivity of each component was studied and fed into the analysis with assembly and alignment logics considered.

Referring to <FIG>, the graphs present the optical design MTF and tolerance MTF, respectively, of the NIR band for the optical system <NUM>. Referring to <FIG>, the graphs present the optical design MTF and tolerance MTF, respectively, of the blue band for the optical system <NUM>.

The estimated MTF values of the optical system <NUM> are summarized in Table <NUM>. The design MTF of the panchromatic band is higher than <NUM> % and the tolerance value is greater than <NUM> %. For the multispectral bands, the results are different from the optical system <NUM>. Due to the wide field-of-view (FOV) and their location in the FOV, the MTF drops are strange and get much harsher than for the optical system <NUM>. The lowest multispectral MTF value is just above <NUM> % at the outer field and surprisingly at the near-infrared band, which is located closer to optical axis. The tolerance values are managed to be higher than <NUM> %.

Referring to <FIG>, the distortion performance of the optical system <NUM> and the optical system <NUM>, respectively, is illustrated. The distortion magnitude of the optical system <NUM> is <NUM> micrometer, higher than that of the optical system <NUM>, <NUM> micrometer at the edge, due to its larger field of view. But it should be noted that the distortion magnitudes of both camera systems are still much lower than <NUM>/<NUM> pixels, which leads to enough margin for TDI imaging and indicates much less probability of image quality degradation.

Despite having a small form factor, the optical system <NUM> has performance better than other camera systems in constellation operation as shown in Table <NUM>. The optical system <NUM> is designed to have a ground sample distance of <NUM>-meter and a swath-width of <NUM> kilometer at <NUM> kilometer altitude, which are comparable to or better than those of SKYSAT. It should be also highlighted that the optical system <NUM> can operate a panchromatic band and a near-infrared band simultaneously on the fly, which are optimized compatible with other remote sensing missions and the other cameras identified in Table <NUM> are lacking.

The benefits of the optical system <NUM> over DOVE cameras are better resolution, diverse spectral bands and shorter in axial direction as shown in Table <NUM>. At <NUM> kilometer altitude, the optical system <NUM> has a ground sample distance of <NUM> meters, which is half resolution of DOVE or PLANETSCOPE. The optical system <NUM> can be equipped with the customized spectral bands, which are essential to extract meaningful spectral information.

The optical system is based on the <NUM>-mirror all-reflective optical design and is free from chromatic aberration and distortion. Being free from chromatic aberration helps the optical system go beyond the visible spectral range so it can support imaging in the infrared and UV spectral range. Being distortion-free helps the optical system to support TDI imaging in orbit and precision metrics in post-processing.

Some of the prior art, especially the less expensive solutions, still relies heavily on a combination of lenses and mirrors in a catadioptric design, so that limits its application, or its optical design needs be revised from the beginning to adapt to a different spectral range. Furthermore, the catadioptric design does not easily embrace TDI imaging, especially for a wider field of view imaging because of inherent or residual aberrations.

The form factor is smaller compared to bigger, more massive systems of the prior systems. The optical system described herein has a much smaller form factor compared to the prior art. It is quite small so that it can be installed on a small flying object, including CUBESAT, minisatellite, airplane, UAV, drone, or balloon. It can also be onboard flying objects as a secondary or tertiary payload, which helps provide diverse missions or more opportunities for missions. The optical system is small and lightweight so that it helps reduce launch cost and increase the opportunity of a launch compared to the prior art. The benefit stands out when it comes to constellation operation, where launch cost is a driving factor. The optical system can be developed at a lower cost so that it is more affordable than the prior art. In developing the optical system, smaller test equipment and facilities can be used due to the smaller aperture size. Also, the optical system is lightweight, and it can be transported with lower logistics costs.

The developing process of the optical system can be automated more efficiently compared to the prior art. Developing the prior art, which is quite bigger, always mandates labor resources, leading to an increasing budget. For the optical system with its smaller aperture size and being lightweight, iterative or repetitive processes or procedures can be automated, even with affordable equipment. The processes may include optical alignment, optical measurements (such as wavefront error, modulation transfer function, a focal length, a field of view, instantaneous field of view, distortion, signal to noise ratio), and those under various conditions. In addition to the financial benefit, the optical system can sustain stability in operation because of shorter physical distance among mirrors.

The optical system is based on the <NUM>-mirror optical design and provides design flexibility that is backed by a degree of freedom of the optical design. With a minimal modification of the optical design, it can be adapted to provide imaging in the modes of starring, scanning or push broom, video, stereo, BRDF (Bidirectional Reflectance Distribution Function), HDR (High Dynamic Range), Polarimetric, or low-light. The optical system, based on the <NUM>-mirror optical design, can support panchromatic, multispectral, hyperspectral, infrared, and UV imaging with minimal design modification, mainly due to different pixel sizes. The optical system has a degree of freedom of optical design and can support super-resolution, high dynamic range, polarimetric, and other remote sensing or scientific imaging.

The optical system can support planetary or deep space missions, which mandates a small form factor for payload selection. The optical system can embrace diverse missions because of its affordability and launch opportunity, which may include At-based imaging. The optical system can be used for a precision star sensor and a stellar sensor.

The optical system, based on the <NUM>-mirror optical design, can support simultaneous multi-color imaging. It can include, for example, but is not limited to, panchromatic + RGB + near-infrared, visible + infrared (near-infrared, shortwave infrared, mid-wave infrared, or longwave infrared), visible + visible, infrared + infrared, UV + visible, or UV + infrared imaging.

The optical system, being of the small form factor, can be onboard the satellites of a non-imaging mission, like communication satellites (for example, Starlink of SpaceX). The optical system can also be installed on other imaging satellites, quasi-imaging satellites, like SAR mission, or scientific mission satellites. This functionality potentially leads to synchronous or asynchronous constellation operation of the optical system, which enhances the temporal resolution of imaging or increases imaging opportunity. Constellation operation of the prior art tends to mandate substantial fixed cost of expensive satellite and camera system, <NUM>/<NUM> operation of a dedicated control station, and non-automated image-receiving centers. The optical system enables synchronous or asynchronous constellation operations so that the resources for control and data receiving can be distributed, leading to significantly reduced fixed cost.

While there has been illustrated and described what are presently considered to be example embodiments, it will be understood by those skilled in the art that various other modifications may be made, and equivalents may be substituted, without departing from claimed subject matter. Additionally, many modifications may be made to adapt a particular situation to the teachings of claimed subject matter without departing from the central concept described herein. Therefore, it is intended that claimed subject matter not be limited to the particular embodiments disclosed, but that such claimed subject matter may also include all embodiments falling within the scope of the appended claims, and equivalents thereof.

It is contemplated that various combinations or subcombinations of the specific features and aspects of the embodiments disclosed above may be made and still fall within one or more of the inventions. Further, the disclosure herein of any particular feature, aspect, method, property, characteristic, quality, attribute, element, or the like in connection with an embodiment may be used in all other embodiments set forth herein. Accordingly, it should be understood that various features and aspects of the disclosed embodiments can be combined with or substituted for one another in order to form varying modes of the disclosed inventions. Thus, it is intended that the scope of the present inventions herein disclosed should not be limited by the particular disclosed embodiments described above. Moreover, while the inventions are susceptible to various modifications, and alternative forms, specific examples thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the inventions are not to be limited to the particular forms or methods disclosed, but to the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the scope of the appended claims. Any methods disclosed herein need not be performed in the order recited.

The ranges disclosed herein also encompass any and all overlap, sub-ranges, and combinations thereof. Language such as "up to," "at least," "greater than," "less than," "between," and the like includes the number recited. Numbers preceded by a term such as "approximately", "about", "up to about," and "substantially" as used herein include the recited numbers, and also represent an amount or characteristic close to the stated amount or characteristic that still performs a desired function or achieves a desired result. For example, the terms "approximately", "about", and "substantially" may refer to an amount that is within less than <NUM>% of, within less than <NUM>% of, within less than <NUM>% of, within less than <NUM>% of, and within less than <NUM>% of the stated amount or characteristic. Features of embodiments disclosed herein preceded by a term such as "approximately", "about", and "substantially" as used herein represent the feature with some variability that still performs a desired function or achieves a desired result for that feature.

It will be understood by those within the art that, in general, terms used herein, are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but is not limited to," etc.). It will be further understood by those within the art that if a specific number of an introduced embodiment recitation is intended, such an intent will be explicitly recited in the embodiment, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the disclosure may contain usage of the introductory phrases "at least one" and "one or more" to introduce embodiment recitations. However, the use of such phrases should not be construed to imply that the introduction of an embodiment recitation by the indefinite articles "a" or "an" limits any particular embodiment containing such introduced embodiment recitation to embodiments containing only one such recitation, even when the same embodiment includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should typically be interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite articles used to introduce embodiment recitations. In addition, even if a specific number of an introduced embodiment recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to "at least one of A, B, and C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, and C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to "at least one of A, B, or C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, or C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, embodiments, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms.

Claim 1:
An all-reflective optical system (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>), comprising:
a concave primary mirror (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) having a central aperture (<NUM>, <NUM>) and a radius, the primary mirror having one of a parabolic, non-parabolic conical, or aspherical surface;
a convex secondary mirror (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) facing the primary mirror, the secondary mirror having an aspherical surface, wherein an optical axis extends from a vertex of the primary mirror to a vertex of the secondary mirror;
a concave tertiary mirror (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) arranged behind the primary mirror, the tertiary mirror having one of a parabolic, non-parabolic conical, or aspherical surface;
a concave quaternary mirror (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) arranged in the central aperture of the primary mirror or behind the primary mirror, the quaternary mirror having one of a spherical, parabolic, non-parabolic conical, or aspherical surface; and
at least one image plane (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) having one or more aggregated sensors, wherein the image plane (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) is positioned at a radial distance from the optical axis that is no more than the radius of the primary mirror, characterized by
a form factor, defined as a ratio of a distance between the secondary mirror and the tertiary mirror to an effective focal length of the optical system, is less than <NUM>.