Near Infrared Tunable Filter System for ATST Haimin Wang

1. Near Infrared Tunable Filter System for ATSTHaimin Wang Solar Research Center New Jersey Institute of Technology Today, I would like to show you some up-to-date progress about the near infrared tunable filter(NIRTF) for ATST, including design requirements of the infrared filter system, operation modes, the designs of infrared Lyot filter, infrared Fabry-Perot, and detectors. In addition, some ongoing researches and the requirements to external instruments and modules are introduced also.Today, I would like to show you some up-to-date progress about the near infrared tunable filter(NIRTF) for ATST, including design requirements of the infrared filter system, operation modes, the designs of infrared Lyot filter, infrared Fabry-Perot, and detectors. In addition, some ongoing researches and the requirements to external instruments and modules are introduced also.

2. Design Requirements Spectral coverage: 1.0~1.7?m Resolving power: 150,000 FOV: 1~3 arcmin Bandpass: ~ 0.1� Spatial resolution: ~ 0.1 arcsec Multi-operation: narrow, medium, and broad Filter aperture: Lyot filter~36mm, FP~150mm Tunable range 1.0~1.7?m High optical quality High throughput: ~40% for polarized light Stray light: ~ 10-3 Stability: ~ 0.05�/hour According to the scientific objectives of ATST, we provide the design requirements of this near infrared tunable filter for ATST. It should be mentioned that it is a preliminary version. Some design parameters are possible to be revised. Spectral coverage: 1.0~1.7 ?m, span about 7000 angstrom, and that can cover two near infrared atmosphere windows: J(1.2 ?m)? and H(1.6 ?m). For the rest two spectral bands in the near infrared K(2.2 ?m) and L(3.4 ?m), the filters need the different designs. Resolving power&Bandpass: the current design is sufficient to detect the full range of line profile features in the near infrared. The wavelength sampling (10~ 30steps)is dense enough to research radiative-transfer based Stokes inversion techniques. Multi-operation mode: for the different observation objects, we design three operation mode: narrow passband, medium passband, and broad passband. FOV: 1~3� according to different operation mode. Spatial resolution~0.1�, which is the diffraction limit of 4m telescope in NIR. Throughput: for the polarized light and 3-module of Lyot filter, ~40%According to the scientific objectives of ATST, we provide the design requirements of this near infrared tunable filter for ATST. It should be mentioned that it is a preliminary version. Some design parameters are possible to be revised. Spectral coverage: 1.0~1.7 ?m, span about 7000 angstrom, and that can cover two near infrared atmosphere windows: J(1.2 ?m)? and H(1.6 ?m). For the rest two spectral bands in the near infrared K(2.2 ?m) and L(3.4 ?m), the filters need the different designs. Resolving power&Bandpass: the current design is sufficient to detect the full range of line profile features in the near infrared. The wavelength sampling (10~ 30steps)is dense enough to research radiative-transfer based Stokes inversion techniques. Multi-operation mode: for the different observation objects, we design three operation mode: narrow passband, medium passband, and broad passband. FOV: 1~3� according to different operation mode. Spatial resolution~0.1�, which is the diffraction limit of 4m telescope in NIR. Throughput: for the polarized light and 3-module of Lyot filter, ~40%

3. Operation Mode I - Narrow passband Configuration: Interference Filters + Lyot Filter + NIR FP Passband FWHM: 0.1� FOV: ~ 1 arcmin Observation Mode: Imaging Spectrograph / Spectropolarimetry Interesting Spectral Lines: FeI 11607.6, 11783.3, 11882.8, 11884.1, 12879.8, 15207.5, 15219.6, 15245.0, FeI 15648.5, 15652.9, 1566.0, 15723.6 H 12818, Cont 16300 HeI 10830.34, 10830.25, 10829.08 CI 10683.1, 10685.4, 10691.2, 10729.5 MgII: 10952, 10914, FeXIII 10747, 10798 CN at J-band, OH at H-band: OH 15422.4, 15419.6, 15409.3, 15407.4 Peak Transmission: ~40% for polarized light As mentioned above, there are 3 observation modes for this IR tunable filter. The first mode is narrow passband operation mode. Passband FWHM ~ 0.1 angstrom. The optical configuration adopts interference filters + Lyot filter + near infrared Fabry-Perot. This mode is mainly used as the imaging spectrograph, imaging spectropolarimetry or vector magnetogram. For the 4m aperture and a full FOV of 3�, we would require an etalon aperture of >500mm. It exceeds manufacturing ability largely. So, in order to keep ~0.1angstrom passband, FOV~1� and 150mm aperture FP is a good choice. {Calculation method: For a collimated-mounted etalon, wavelength shift through an etalon is ~?2?/2(? is maximum incident angle); For a telecentric-mount etalon, effective bandpass is ~[(fwhm)2+(?2?/2)2]1/2. According to the Lagrange optical invariant, ?=Datst/Dfp*FOVatst/2.} Many interesting spectral lines can be observed in this operation mode. FeI 15648/15652 pair: plage, umbra, network and intranetwork magnetic field; HeI 10830: chromosphere magnetic field, corona hole�..; Many strong stokes Q and V atomic lines to probe weak magnetic field...; Infrared molecular lines with high temperature sensitivity�.; Cont. 1.6?m: the deepest layer of photosphere�.;As mentioned above, there are 3 observation modes for this IR tunable filter. The first mode is narrow passband operation mode. Passband FWHM ~ 0.1 angstrom. The optical configuration adopts interference filters + Lyot filter + near infrared Fabry-Perot. This mode is mainly used as the imaging spectrograph, imaging spectropolarimetry or vector magnetogram. For the 4m aperture and a full FOV of 3�, we would require an etalon aperture of >500mm. It exceeds manufacturing ability largely. So, in order to keep ~0.1angstrom passband, FOV~1� and 150mm aperture FP is a good choice. {Calculation method: For a collimated-mounted etalon, wavelength shift through an etalon is ~?2?/2(? is maximum incident angle); For a telecentric-mount etalon, effective bandpass is ~[(fwhm)2+(?2?/2)2]1/2. According to the Lagrange optical invariant, ?=Datst/Dfp*FOVatst/2.} Many interesting spectral lines can be observed in this operation mode. FeI 15648/15652 pair: plage, umbra, network and intranetwork magnetic field; HeI 10830: chromosphere magnetic field, corona hole�..; Many strong stokes Q and V atomic lines to probe weak magnetic field...; Infrared molecular lines with high temperature sensitivity�.; Cont. 1.6?m: the deepest layer of photosphere�.;

4. Configuration: Interference Filters + Lyot Filter Passband FWHM: 2~3� FOV: ~ 1-3 arcmin Observation Mode: Filtergram Interesting Spectral Lines: FeI 15648.5, 15652.4 HeI 10830.34, 10830.25, 10829.08 Continuum 1.63?m H (P?) 12818 CN at J-band, OH at H-band Peak Transmission: ~ 40% for polarized light Operation Mode II - Medium passband The second operation observation configuration is medium passband mode. Under this mode, interference filters and Lyot tunable filter are used to acquire 2~3 angstrom passband. Observation mode are the filtergrams and filter vetor magnetograms. Field of view is ~1-3�. For the polarized light, peak transmission can reach 40%. The second operation observation configuration is medium passband mode. Under this mode, interference filters and Lyot tunable filter are used to acquire 2~3 angstrom passband. Observation mode are the filtergrams and filter vetor magnetograms. Field of view is ~1-3�. For the polarized light, peak transmission can reach 40%.

5. Operation Mode III - Broad passband Configuration: Interference Filters Passband FWHM: 20~50� FOV: ~ 1-3 arcmin Observation Mode: Active Region Evolution and Morphology Analysis Peak Transmission: >80% The last operation configuration is broad passband mode. Infrared interference filters are used to provide 20~50 angstrom passband as the research and observation of active region evolution and morphology analysis. Under this mode, peak transmission can reach >80%.The last operation configuration is broad passband mode. Infrared interference filters are used to provide 20~50 angstrom passband as the research and observation of active region evolution and morphology analysis. Under this mode, peak transmission can reach >80%.

6. NIR Tunable Filter System Here is the schematic optical path of near infrared tunable filter system for the narrow passband operation mode. The light from ATST telescope is split into two beams by a light splitting. One beam is fed into CCD1 as reference white-light images. Another beam is fed into the infrared tunable filter system, polarization analyzer. The monochromatic or polarization images are captured by an infrared camera(CCD2). Every component in optical path can be moved out easily to fulfill the different observation operation mode. I will discuss every instrument in detail.Here is the schematic optical path of near infrared tunable filter system for the narrow passband operation mode. The light from ATST telescope is split into two beams by a light splitting. One beam is fed into CCD1 as reference white-light images. Another beam is fed into the infrared tunable filter system, polarization analyzer. The monochromatic or polarization images are captured by an infrared camera(CCD2). Every component in optical path can be moved out easily to fulfill the different observation operation mode. I will discuss every instrument in detail.

7. Instrument I � NIR Lyot Filter Tunable ability: 1.0~1.7 ?m Clear Aperture: ~ 36 mm Bandpass FWHM: 2.5 ~ 3.0 � Peak Transmission: ~ 40% for polarized light Internal Structure: 3-module or 4-module Temperature Controller: 45 � 0.05�C Achromatic Components: waveplates, polarizers Wavelength Tunable Methods: I. Calcite + 1/4 waveplate + Rotating 1/2 waveplate II. Calcite + LC variable retarder As the prefilter of Fabry-Perot, Loyt filter should possess wide tunable ability. Here, we provide that its tunable range is from 1.0 ?m to 1.7 ?m. By the reason of size limit of natural calcite, the aperture of Lyot filter is impossible to be very big, ~36 mm is a reasonable size for manufacturing. Bandpass FWHM of Lyot filter should match the transmission profile of Fabry-Perot so as to restrain sidelobes contamination. Simulation for the interesting near infrared spectral lines is carrying out to determine the last bandpass FWHM and the thickness of calcite. In the design of BBSO�s IRIM (infrared imaging magnetograph), we used a 4 calcite module in Lyot filter system. The peak transmission is about 37% for the polarized light. In order to increase the poor transmission, we are considering to change it into 3-module configuration. The corresponding peak transmission can reach ~40%. To keep the tunable ability in 1.0~1.7 ?m, achromatic waveplates and polarizers are necessary. Some results have been obtained in the design of the achromatic waveplate and will be given later. Usually, there are two wavelength tunable method for Lyot birefringent filter: one is calcite+1/4 waveplate + rotating � waveplate; another is calcite+liquid crystal variable retarder. Here, we are considering the first tunable method. As the prefilter of Fabry-Perot, Loyt filter should possess wide tunable ability. Here, we provide that its tunable range is from 1.0 ?m to 1.7 ?m. By the reason of size limit of natural calcite, the aperture of Lyot filter is impossible to be very big, ~36 mm is a reasonable size for manufacturing. Bandpass FWHM of Lyot filter should match the transmission profile of Fabry-Perot so as to restrain sidelobes contamination. Simulation for the interesting near infrared spectral lines is carrying out to determine the last bandpass FWHM and the thickness of calcite. In the design of BBSO�s IRIM (infrared imaging magnetograph), we used a 4 calcite module in Lyot filter system. The peak transmission is about 37% for the polarized light. In order to increase the poor transmission, we are considering to change it into 3-module configuration. The corresponding peak transmission can reach ~40%. To keep the tunable ability in 1.0~1.7 ?m, achromatic waveplates and polarizers are necessary. Some results have been obtained in the design of the achromatic waveplate and will be given later. Usually, there are two wavelength tunable method for Lyot birefringent filter: one is calcite+1/4 waveplate + rotating � waveplate; another is calcite+liquid crystal variable retarder. Here, we are considering the first tunable method.

8. Sketch of the NIR Tunable Birefringent Filter This figure is an optical sketch of NIR tunable birefringent filter. Here, �P� represents polarizer, C is calcite, � is half waveplate, and � is quarter waveplate. The lines beneath the figure indicate the direction of optical axis. This is 4-module configuration. Each module consists of same structures. It should be noticed that there are two advantages in the design: wide-field configuration For either collimating-mount or telecentric-mount optical configuration, incident light beam is not normal to surface of birefringent crystal plates, there will be an error in the retardation of the crystal. In order to reduce the retardation errors, we adopt wide-configuration: the calcite plate of an element is split into two equally thick parts and a half waveplate is sandwiched between them. wavelength tuning In every module, there is a rotatable half waveplate. The equation gives the transmission of the birefringent filter. Rotating these � waveplates can shift the wavelength of passband within a period in every module.This figure is an optical sketch of NIR tunable birefringent filter. Here, �P� represents polarizer, C is calcite, � is half waveplate, and � is quarter waveplate. The lines beneath the figure indicate the direction of optical axis. This is 4-module configuration. Each module consists of same structures. It should be noticed that there are two advantages in the design: wide-field configuration For either collimating-mount or telecentric-mount optical configuration, incident light beam is not normal to surface of birefringent crystal plates, there will be an error in the retardation of the crystal. In order to reduce the retardation errors, we adopt wide-configuration: the calcite plate of an element is split into two equally thick parts and a half waveplate is sandwiched between them. wavelength tuning In every module, there is a rotatable half waveplate. The equation gives the transmission of the birefringent filter. Rotating these � waveplates can shift the wavelength of passband within a period in every module.

9. Instrument II � NIR Fabry-Perot Tunable ability: 1.0~1.7 ?m Clear Aperture: ~ 150 mm Bandpass FWHM: ~ 0.1 � @ 15648 � Peak Transmission: > 90% Effective Finesse: ~ 60 FOV: ~ 1 arcmin for spectropolarimeter Prof. Goode, here I just give you a figure to show how to determine the aperture of Fabry-Perot and FOV. The following figure is relation between aperture of FP and wavelength shift. All calculation method is introduced in previous page. So, FOV~1 arcmin and aperture~150mm should be a good choice. Prof. Goode, here I just give you a figure to show how to determine the aperture of Fabry-Perot and FOV. The following figure is relation between aperture of FP and wavelength shift. All calculation method is introduced in previous page. So, FOV~1 arcmin and aperture~150mm should be a good choice.

10. Instrument III � CCD Cameras NIR camera(CCD2) Candidate: HgCdTe & InGaAs Format: 1024?1024 A/D: 14 bit or 16 bit Readout: 4 quadrant output Fill factor: 100% FOV: 60? diffraction limit 0.1?@1.56?m sampling 0.058? QE: > 60% LN2 Cooling Visible camera(CCD1) for image alignment Si CCD Format: 2048?2048 A/D: 14 bit or 16 bit Readout: frame transfer Fill factor: 100% FOV: 60? diffraction limit 0.03?@5000� sampling 0.029? QE: >1% In the data acquiring system, we consider to use two set of CCD cameras. One is visible camera, which is used as images alignment. Another one is near infrared focal plane array camera. The detailed requirements are shown the above. �������������� The above is the main design of near infrared tunable filter system. Now, I would like to spend some time to introduce the process and some results in the design.In the data acquiring system, we consider to use two set of CCD cameras. One is visible camera, which is used as images alignment. Another one is near infrared focal plane array camera. The detailed requirements are shown the above. �������������� The above is the main design of near infrared tunable filter system. Now, I would like to spend some time to introduce the process and some results in the design.

11. Process Design of achromatic waveplates from 1.0 to 1.7?m The above figure gives an example of achromatic quarter waveplate design in NIR. From 1.0 to 1.7?m, new quarter waveplate shows satisfying retardance.The above figure gives an example of achromatic quarter waveplate design in NIR. From 1.0 to 1.7?m, new quarter waveplate shows satisfying retardance.

12. Process Measurement of birefringent index of calcite from 1.0 to 1.7?m The birefringent index of a crystal is an important paramenter for the design and fabrication of a birefringent filter. In the visible light range, the values of the Birefringent index of calcite and quartz are quite reliable. However, in the IR, we could extropolate the value up to 1.6?m. Obviously, these �known� values of the birefringent index of calcite is unreliable in the NIR. Therefore, the birefringent index of calcite and its temperature coefficient in the NIR are measured.The birefringent index of a crystal is an important paramenter for the design and fabrication of a birefringent filter. In the visible light range, the values of the Birefringent index of calcite and quartz are quite reliable. However, in the IR, we could extropolate the value up to 1.6?m. Obviously, these �known� values of the birefringent index of calcite is unreliable in the NIR. Therefore, the birefringent index of calcite and its temperature coefficient in the NIR are measured.

13. Process 3-module design of Lyot filter to increase transmission In order to improve the poor transmission problem in Lyot filter design, we are considering to use a 3-module design to replace the old 4-module design. The above left figure shows the combined transmission of 3-module birefrigent filter and Fabry-Perot. In order to take a close look of sidelobe contamination, we present right figure by rescaling the transmission. From the result, the sidelobe contamination ratio for 3-module system is about 0.54%, compared with 0.40% for 4-module system. However, the transmission increases from 37% to 45%.In order to improve the poor transmission problem in Lyot filter design, we are considering to use a 3-module design to replace the old 4-module design. The above left figure shows the combined transmission of 3-module birefrigent filter and Fabry-Perot. In order to take a close look of sidelobe contamination, we present right figure by rescaling the transmission. From the result, the sidelobe contamination ratio for 3-module system is about 0.54%, compared with 0.40% for 4-module system. However, the transmission increases from 37% to 45%.

14. On going�... Simulation of match between Lyot filter and Fabry-Perot Experiment of liquid crystal variable retarder Design of near infrared polarization analyzer Calculation of ghost image of near infrared filter system Consideration of dual near infrared Fabry-Perot system Study of stability and repeatability of NIR Fabry-Perot Optical design for NIR tunable filter system Optimization of NIR tunable filter system In addition, there are several ongoing works. 1.2.3.4 ���In addition, there are several ongoing works. 1.2.3.4 ���

15. External Requirement Room temperature control Pressure and vibration monitoring and controlling Frequency stabilized infrared laser source Polarization modulator system handshake Telescope control system handshake CCD cameras capture system control AO system on-off control ���.. The external requirements of near infrared tunable filter system are summarized as: 1.2.3.4.5�����.The external requirements of near infrared tunable filter system are summarized as: 1.2.3.4.5�����.

16. The End Thanks a lot Prof. Goode, Here is our preliminary design and research progress of the infrared tunable filter system for ATST. In every page of ppt, I added some footnote which can help you to understand what we want to express. Obviously, some parameters are still at the stage of concept design and need time to be determined. If you have question, please contact with me. Thank you very much and have a good trip. WendaProf. Goode, Here is our preliminary design and research progress of the infrared tunable filter system for ATST. In every page of ppt, I added some footnote which can help you to understand what we want to express. Obviously, some parameters are still at the stage of concept design and need time to be determined. If you have question, please contact with me. Thank you very much and have a good trip. Wenda