# US 10,768,045 B1 · Large format three-mirror dispersive spectrometer optics > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 10,768,045 B1 - Document ID: US10768045B1 - Status: Granted - Kind: B1 - Application number: 16/552,597 - Filed: 2019-08-27 - Granted: 2020-09-08 - Earliest claimed: 2019-08-27 - Assignees: Raytheon Company - Applicants: Raytheon Company - Inventors: Lacy G. Cook - Web page: https://patentgrep.com/patent/US10768045B1 ## CPC - G01J 3/021: {using plane or convex mirrors, parallel phase plates, or particular reflectors} - G01J 3/0208: {using focussing or collimating elements, e.g. lenses or mirrors; performing aberration correction} - G01J 3/0256: {Compact construction} - G01J 3/04: Slit arrangements {slit adjustment} - G01J 3/18: using diffraction elements, e.g. grating (gratings per se G02B) - G01J 3/2823: {Imaging spectrometer} - G01J: MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY (light sources F21, H01J, H01K, H05B; investigating properties of materials by optical means G01N) ## Abstract All-reflective optics for use in imaging spectrometers. In one example, an imaging spectrometer includes an entrance slit and spectrometer optics consisting of a primary mirror, a secondary mirror having a diffraction grating formed thereon, and a tertiary mirror, each of the primary mirror, the secondary mirror, and the tertiary mirror having free-form non-rotationally symmetric surface figures defined by Zernike polynomials. The spectrometer optics are configured to receive the telecentric image at the entrance slit, to disperse the optical radiation into its spectral components to provide dispersed optical radiation, and to focus the dispersed optical radiation onto an image plane positioned in a common plane with the entrance slit. The spectrometer optics have a physical length that is twice the length of the entrance slit, a ratio of a distance of the secondary mirror from the entrance slit to a distance of the primary mirror from the entrance slit being approximately 0.32. ## Claims ### Claim 1 An imaging spectrometer comprising: an entrance slit having a length in a cross-scan direction a primary mirror having a first free-form optical surface and positioned to receive optical radiation from the entrance slit and to reflect the optical radiation, the primary mirror being spaced a first distance from the entrance slit measured along a primary optical axis of the imaging spectrometer; a secondary mirror having a second free-form optical surface and a diffraction grating formed on the second free-form optical surface, the secondary mirror positioned to receive the optical radiation from the primary mirror, and configured to disperse the optical radiation into its spectral components to produce dispersed optical radiation, and to reflect the dispersed optical radiation, the secondary mirror being spaced from the primary mirror by a second distance measured along the primary optical axis, the second distance being approximately 0.68 of the first distance; and a tertiary mirror having a third free-form optical surface and positioned and configured to receive the dispersed optical radiation reflected from the secondary mirror and to reflect and focus the dispersed optical radiation onto an image plane coincident with the entrance slit. ### Claim 2 (depends on 1) The imaging spectrometer of claim 1 wherein the primary mirror has positive optical power, the secondary mirror has negative optical power, and the tertiary mirror has positive optical power. ### Claim 3 (depends on 1) The imaging spectrometer of claim 1 wherein the length of the entrance slit is at least 11 centimeters and the first distance from the entrance slit to the primary mirror is approximately 23 cm. ### Claim 4 (depends on 1) The imaging spectrometer of claim 1 wherein an optical speed of the imaging spectrometer is F/2.6. ### Claim 5 (depends on 1) The imaging spectrometer of claim 1 wherein each of the first free-form optical surface, the second free-form optical surface, and the third free-form optical surface is defined by Zernike polynomials including a 37th Zernike term. ### Claim 6 (depends on 5) The imaging spectrometer of claim 5 wherein coefficients of the Zernike polynomials are selected to correct for optical aberrations over a field of view of the imaging spectrometer to an average RMS wavefront error of 0.08 micrometers. ### Claim 7 (depends on 6) The imaging spectrometer of claim 6 wherein a spectral range over which the optical aberrations are corrected includes wavelengths from 0.4 micrometers to 2.5 micrometers. ### Claim 8 (depends on 1) The imaging spectrometer of claim 1 wherein the imaging spectrometer has a physical length that is twice the length of the entrance slit. ### Claim 9 An imaging spectrometer system comprising: an entrance slit having a length in a cross-scan direction; foreoptics configured to receive optical radiation from a viewed scene and to form a telecentric image at the entrance slit; and spectrometer optics consisting of a primary mirror, a secondary mirror having a diffraction grating formed thereon, and a tertiary mirror, each of the primary mirror, the secondary mirror, and the tertiary mirror having free-form non-rotationally symmetric surface figures defined by Zernike polynomials, the spectrometer optics being configured to receive the telecentric image at the entrance slit, to disperse the optical radiation into its spectral components to provide dispersed optical radiation, and to focus the dispersed optical radiation onto an image plane positioned in a common plane with the entrance slit, the spectrometer optics having a physical length that is twice the length of the entrance slit. ### Claim 10 (depends on 9) The imaging spectrometer system of claim 9 wherein the primary mirror is positioned a first distance from the entrance slit measured along a primary axis of the spectrometer optics, and the secondary mirror is positioned a second distance from the entrance slit measured along the primary axis, a ratio of the second distance to the first distance being 0.32. ### Claim 11 (depends on 10) The imaging spectrometer system of claim 10 wherein the primary mirror has positive optical power, the secondary mirror has negative optical power, and the tertiary mirror has positive optical power. ### Claim 12 (depends on 10) The imaging spectrometer system of claim 10 wherein the Zernike polynomials include a 37th Zernike term. ### Claim 13 (depends on 10) The imaging spectrometer system of claim 10 wherein the length of the entrance slit is at least 11 centimeters and the first distance from the entrance slit to the primary mirror is approximately 23 cm. ### Claim 14 (depends on 10) The imaging spectrometer system of claim 10 wherein an optical speed of the imaging spectrometer is F/2.6, and wherein coefficients of the Zernike polynomials are selected to correct for optical aberrations over a field of view of the spectrometer optics to an average RMS wavefront error of 0.08 micrometers over a wavelength range of 0.4 micrometers to 2.5 micrometers. ### Claim 15 (depends on 9) The imaging spectrometer system of claim 9 wherein the foreoptics is an all-reflective foreoptics. ### Claim 16 An imaging spectrometer optical apparatus comprising: an entrance slit having a length in a cross-scan direction; and spectrometer optics consisting of a primary mirror, a secondary mirror having a diffraction grating formed thereon, and a tertiary mirror, each of the primary mirror, the secondary mirror, and the tertiary mirror having free-form non-rotationally symmetric surface figures defined by Zernike polynomials, the spectrometer optics being configured to incident optical radiation in the form of a telecentric image at the entrance slit, to disperse, with the diffraction grating, the optical radiation into its spectral components to provide dispersed optical radiation, and to focus the dispersed optical radiation onto an image plane positioned in a common plane with the entrance slit, the spectrometer optics having a physical length that is twice the length of the entrance slit. ### Claim 17 (depends on 16) The imaging spectrometer optical apparatus of claim 16 wherein the primary mirror has positive optical power, the secondary mirror has negative optical power, and the tertiary mirror has positive optical power. ### Claim 18 (depends on 13) The imaging spectrometer optical apparatus of claim 13 wherein the Zernike polynomials include a 37th Zernike term. ### Claim 19 (depends on 16) The imaging spectrometer optical apparatus of claim 16 wherein the primary mirror is spaced a first distance from the entrance slit measured along a primary optical axis of the spectrometer optics, and wherein the secondary mirror is spaced from the primary mirror by a second distance measured along the primary optical axis, the second distance being approximately 0.68 of the first distance.