Imaging,Visible, Tunable, Passband Filter System Imaging,Visible, Tunable, Narrow-Passband Filter System A Multiple Fabry-Perot Etalon Interferometer for the ATST G. Allen Gary/MSFC, K. S. Balasubramaniam/NSO, Michael Sigwarth/KIS, Thomas Kentischer/KIS, Gil Moretto/NSO, and the ATST team 27 August 2003 – ATST Conceptual Design Review

Outline of the Presentation The MFPI concept Baseline Instrument: Triple Fabry-Perot Etalons Optical Layout and Interface for the ATST Technology development Estimated Cost Science

Fabry-Perot Etalons Commensurate spectral resolution for high resolution imaging at telescope resolution 1/250,000 at 500 nm High light throughput Sufficient number of spectral samples within solar oscillation periods, solar feature changes Rapid tuning, calibrations Better compensation for atmospheric effects, And stability Provides simple spectroscopy and polarimetry of multiple lines in encompassing fashion Polarization fidelity and purity Uses commercial technology Visible range coverage (450nm-750nm)

Comparison with other alternatives ATST Visible Narrow-Band Filter Candidates Air-Gap Fabry-Perot Interferometer Solid Fabry-Perot Interferometer Lyot Birefringent Filter Solid Polarizing Michelson Interferometer FWHM(minimum)~2pm (NSO - Dual Etalon )~10pm (APL - Flare Genesis )~10pm (LMSAL - SOUP )2-10pm (NSO - GONG ) Transmittance Prefilter Factor Total Throughput ~90% ~60% ~50% ~30% ~60% ~50% ~14% ~60% ~8% ~90% ~8% - 50% ~8-50% Pre-Filter Requirement0.2nm Interference filter 0.2nm Interference FilterLyot plus 0.2nm Interf. filter FOV Variation      n 2    n e -n o )/n e n o 2    p 2 /n 2 - p 1 /n 1 )/  p 2 n 2 - p 1 n 1 ) Tuning DevicePiezoelectricLithium niobateRotating polarizersRotating waveplates Technology RequirementsLarge etalons, Ghost reflections, Mountings Thin elements, Superpolishing, Refractive index Improve Transmittance, Longer and thinner elements Multiple elements, Archomatic refractive index, Larger path differences Operational Instruments KIS TESOS, USAF ISOON, ItalianUBI, Sac Peak Dual FPI Flare Genesis, Mees UH IMaX IAC (dev) NSO UBF, LMSAL SOUP MSO GONG, SOHO MDI SWIFT WAMDII Major Disadvantages Large etalons required, Local finesse variation requires obtaining very good flat fields. Wavefront error, A minimum etalon thickness and minimum FWHM ~8pm, High voltages are required Temperature sensitivity, low transmittance, Maximum and minimum calcite elements pushing technology and availability Wavelength range is restricted and multiple wavelength elements are required. Prefilter requirements restricts transmittance Major Advantages Overall Simplicity, Known technology Working system, Universal filter with the 2pm FWHM, high transmittance Larger FOV than air-gap FPI, Universal filter Large FOV, Universal filter Largest FOV for specified, Simplicity of operation Gary, G. A., & Balasubrumanium, K. S., 2003, Additional Notes Concerning the Selection of A Multiple_Etalon for the Advanced Technology Solar Telescope.

Multiple Imaging Modes Imaging spectrograph/spectropolarimeter ( 2pm, 0.5-1’ FOV) TESOS, IBIS, NSO Dual FPs Imaging Spectro-polarimeter /Imaging Magnetograph, (5pm – 12pm, 3’ FOV) UBF/Filter Magnetographs Intermediate-band filter (20pm – 30pm, 1-3’ FOV) Dual FP System, UBF-FP combination filters Broad-band filter (0.1nm - 2nm, 3’FOV) Reflection slit-jaw spectroscopy, UBF-like spectroscopy Spectral Coverage 450 – 750 nm Imaging Spectrographic Observations

Multiple Observational Modes

Resolution & Spectral Purity Single etalon system – Airy Function, very narrow blocking filter (~0.2nm) – for R=0.94, minimum transmittance is of the maximum. Multiple etalon system –Reflectance of coatings, of combined etalons –Narrow blocking filters (~1nm) –Optimize spectral purity with Ratio of Finesse Free spectral range Design of prefilters Triple Etalons have superior out-of-band rejection by placement of etalon minima –Darvaan and Owner-Peterson (1994) Performance based on analysis of maximum ghost and SNR Reference: Gary, G. A., Balasubramaniam, K. S., and Sigwarth, M.: 2003,’ Multiple Etalon Systems for the Advanced Technology Solar Telescope’, SPIE proceeding: “Innovative Telescopes and Instruments for Solar Physics”, eds. Stephen L. Keil and Serge V. Avakyan, SPIE , p

Single vs. Multiple Etalons Objective: Spectral resolution =  ~ 0.5x10 -5 or 2 pm Single etalon system – Spectral resolution:  ~  F 2 d  –F is the finesse and d is the etalon gap distance. –For d~1mm, =500nm,  ~10 -3 /F; the FSR=0.1nm, and narrow FWHM blocking filters are required. Spectral resolution of requires high finesse (F>100!). Multiple etalon system – Spectral resolution is given by  ~ FSR / F – FSR is the free spectral range of the multi-etalons in combination. – For =500nm, then  ~  FSR, the Spectral resolution of requires only a FSR~10nm, hence need low finesse of 20 and wide FWHM for the blocking filters

TESOS Heritage

Required Etalon Aperture The wavelength variation versus aperture, for 4m ATST primary with FOV's of 1 and 3’. The solid lines are for  5250Å and the dashed curve is for  6302Å. For 3’ FOV the shift is 120mA for 250mm-aperture (A). For a 1’ FOV a 100mm etalon would allow a reasonable shift of ~100mA across the field of view (B). The narrow band filter system in a ~100mA mode could do filter magnetograms. For a spectro-polarimeter with (~20mA) it seems that mm aperture is required for 1’ FOV ( C ). Grey Bar: F~25 realistic apertures n=1 (Air/He gap) n=2.1 (liquid gap) x 0.48 FOV=3’ FOV=1’  200mm

Requirements Minimum aperture: 200mm diameter Wavelength range: nm Bi-modal operation - dual and triple system Spectral resolution –1/250,000 for triple etalon - 50,000 for dual etalon Minimum Peak Transmission –50% (with blocking filters) Minimum Peak Transmission –10 -4 Maximum Stray-light –10 -3 Drift Stability –1mÅ/hr

TESOS/KIS Optics

TESOS/KIS Optical Design 3 or 2 Etalons Laser Source Monitors Cameras Ghost Suppressors Focal Plane Reticules, Pinholes Targets, & Stops White Light Imaging, radiance, scaling Alignment Mask Variable Motorized

ATST Telecentric Optical Design

MFPI 36”FOV MODE Optical Design Specifications: Input Beam: ATST Coudé F/20 200MM Etalons on F/300 Tel. Beam 100MM Collimated Beam (Filters) WVL Band: 450 to 750nm 36” FOV Camera: Ps=1.25”/mm All Spherical Lenses All Spherical Mirrors Gil Moretto/NSO

ATST COUDÉ F/20 + MFPI 36”FOV MODE

MFPI 36”FOV MODE LAYOUT

TELECENTRICITY

MFPI 36”FOV CAMERA OPTICAL PERFORMANCE POLYCHROMATIC

Issues Determine etalon parameters (d,R,F) Detail Darvann-Owner-Peterson- Analysis: Minimize light from parasitic orders Emphasizing compatible with operation: actual parameters, electronic control, optical setup, & drifts The final finesse and tunability of the 20cm etalons Early purchase and test of first (or a) 20cm etalon Flat fielding problems due to drift Atmospheric monitoring and correction Polarimeter design Refine dual camera polarimeter Compatibility with multiple mirror and nonpolarizing beam splitters and ATST Mueller matrix

Issues Spatial reflection ghost Tilt, wedge angles, calibration, optical testing at TESOS Determine building strategy Full or partial construction, prefilter set Refine estimated cost Updated pricing and cost analysis Telecentric beam/ Collimated Option F/250 at FPIs Detail pupil apodization analysis Complexities of off-axis optical systems Detail optical ray tracing and analysis Polarization study and Coude focus

Technology Studies Laura Allaire (Ph.D. student) in Optics at the University of Rochester is centering her thesis work on multiple Fabry-Perot interferometry and will assist in the ATST MFPI design. She started this summer (2003). Gil Moretto/NSO will continue to improve the breadboard design of the ATST MFPI. Ghost, apodization,and general concerns listed above will continue to be studied. (Allaire/UR) A second observational run at TESOS will hopefully provide a more through understanding of their instrument (e.g. ghost, drift, and spectral resolution). Thomas Kentischer/KIS is active member of the team. An improved cost estimate will be developed (lens vs mirrror) Alternate concepts will be explored, e.g. dual etalons, as first light configuration Flexible optical design to be considered to allow for advancement in technology.

Cost estimate (preliminary) Engineering design…………………....$ 1,200,000 [Optical, Mechanical, Electronic–Engineers and Scientist-Project Manager for 2 years each] Optical elements……………………….$ 21,000 Mechanical elements………………….$ 12,000 Electronic elements……………………$ 140,000 Three 20cm etalons……………………$ 834,000 Commercial software ………….……...$ 4,000 Electronic and computer Interface…....$ 175,000 Assembly, test, & integration…………$ 300,000 [Optical, Mechanical, Electronic-Engineers and Scientist-Project Manager for 1/2 years each] Total…… $2,686,000

ATST Science The imaging filter system for the ATST will provide the observational opportunities to spectrally probe the magnetic and hydrodynamic fine structures of the photosphere and the chromosphere at ultra-high spatial resolution. This filter will possess high-transmittance, and allow instantaneous, narrow- band spectral observations over an extended area of the Sun. The observations will allow rapid 3D-imaging spectrometry, Stokes spectropolarimetry, accurate surface photometry, and provide spectroheliograms that will measure Doppler velocity, transverse flows, and allow feature tracking, and the study of evolutionary changes of solar activity. When incorporated with the adaptive optics (AO) system, (with added optical correction techniques such as speckle interferometry), focal- volume and other post-focal techniques will allow finer spatio-spectral analysis. Narrow-band spectral imagery offers the advantage of avoiding spectrograph rasterization, with a distinct disadvantage of sequential tuning; but its high throughput and resultant cadence, coupled with active and adaptive optics corrections provide a good mitigation for this disadvantage. Provide one of the core instruments in multiple instrument mode of observing solar phenomena.

Summary Multiple-Filter Modes Narrow Passband Spectral Power: 250,000 (2pm) Throughput: 50% (goal) Field of View: 1-3 arcmin (mode dependent) Wavelength Coverage: nm Dual Camera Polarimetry Spectral Purity: Parasitic peaks < Existing Technology

References Gary, G. A., Balasubramaniam, K. S., & Sigwarth, M.: 2003, “Multiple-etalon Systems for the Advanced Technology Solar Telescope, Innovative Telescopes and Instrumentation for Solar Astrophysics”, eds. S. L. Keil and S. V. Avakyan, SPIE Proceedings 4853, 252. Gary, G. A., Balasubramaniam, K. S., & Sigwarth, M.: 2003, “Additional Notes Concerning the Selection of a Multiple-Etalon System for the Advanced Technology Solar Telescope”, Internal ATST document (currently) Kentischer, T., Sigwarth, M., Schmidt, W., and v. Uexkull, M.: 1998, “TESOS- Telecentric Etalon Solar Spectrometer”, TB v1.0, Kiepenheuer Institut fur Sonnenphysik, Freiburg, Germany. Kentischer, T., Sigwarth, M., Schmidt, W., and v. Uexkull, M.: 1998, "TESOS, a double Fabry-Perot instrument for solar spectroscopy", A&A, 340, 569. Langhans, K.; Schmidt, W.; Tritschler, A., 2002,“2D-spectroscopic observations of G-band bright structures in the solar photosphere”, Astronomy and Astrophysics, 394, Tritschler, A.; Schmidt, W.; Langhans, K.; Kentischer, T., 2002,“High-resolution solar spectroscopy with TESOS - Upgrade from a double to a triple system”, Solar Physics, 211, 17. von der Lühe, O. and Kentischer, Th. J.: 2000, “High Spatial Resolution of a Triple Fabry-Perot Filtergraph”,Astron. Astrophys. Suppl. Ser., 146, 499.