System Design Visible Spectro Polarimeter for the ATST

System Design: Visible Spectro. Polarimeter for the ATST Principal Investigator: Instrument Scientist: Project Manager: Lead Engineer: Electrical Engineer: Mechanical Engineer: Michael Knölker Héctor Socas Navarro Kim Streander David Elmore (SDR author) Greg Card Clarke Chambellan

Morning Contents • • Instrument science requirements Development of instrument specifications Baseline design Polarimetry Vi. SP SDR HAO 2 -3 March 2006

Vi. SP Mission • “The ATST visible spectro-polarimeter (Vi. SP) is an instrument expected to provide precision measurements of the full state of polarization (i. e. , all four Stokes parameters I, Q, U, and V; that is, intensity plus a full description of both linear and circular polarization) simultaneously at diverse wavelengths in the visible spectrum, and fully resolving (or nearly so) the spectral profiles of spectrum lines originating in the solar atmosphere. Such measurements provide quantitative diagnostics of the magnetic field vector as a function of height in the solar atmosphere, along with the associated variation of thermodynamic properties. ” – from Vi. SP ISRD Vi. SP SDR HAO 2 -3 March 2006

Vi. SP Instrument Science Requirements

Science Drivers • SRD contains the scientific goals for ATST, including many use cases • Vi. SP ISRD contains Vi. SP requirements to meet the relevant goals specified in the SRD

Flow down from ATST SRD • Flux tubes (SRD 2. 9) – Building blocks of solar magnetism – Unresolved with current instruments – Fundamental role in energy transport to upper atmosphere (? ) – Faculae and UV brightness ->Dominant source for irradiance variations?

Flow down from ATST SRD • Flux tubes (SRD 2. 9) – Building blocks of solar magnetism – Unresolved with current instruments – Fundamental role in energy transport to upper ISRD 5. 3: Spatial resolution. atmosphere • (? ) ISRD 5. 4: Image quality (AO) – Faculae and • UV brightness ->Dominant source for irradiance variations? • ISRD 5. 6: Spectral resolution • ISRD 5. 7: Spectral sample • ISRD 5. 11: Polarimetric sensitivity

Flow down from ATST SRD • Magnetic field generation and local dynamos (SRD 2. 10) – Crucial for understanding solar activity – Quiet Sun fields->Dominant contribution to magnetism – Importance for upper atmosphere?

Flow down from ATST SRD • Magnetic field generation and local dynamos (SRD 2. 10) – Crucial for understanding solar activity – Quiet Sun fields->Dominant contribution to magnetism 5. 3: atmosphere? Spatial resolution. – Importance • for. ISRD upper • ISRD 5. 4: Image quality (AO) • ISRD 5. 6: Spectral resolution • ISRD 5. 7: Spectral sample • ISRD 5. 11: Polarimetric sensitivity

Flow down from ATST SRD • Interactions of magnetic fields and mass flows (SRD 2. 11) – Fundamental (simulations) for the advection and evolution of quiet Sun and sunspot fields – Photospheric footpoints are dragged around creating complex non-potential topologies in the chromosph. – Magnetic potential energy is stored and becomes available for violent release in the corona via reconnection

Flow down from ATST SRD • Interactions of magnetic fields and mass flows (SRD 2. 11) – Fundamental (simulations) for the advection and evolution of quiet Sun and sunspot fields • footpoints ISRD 5. 3: Spatial – Photospheric are resolution. dragged around creating complex non-potential topologies in the chromosph. • ISRD 5. 4: Image quality (AO) – Magnetic potential energy is resolution stored and becomes • ISRD 5. 6: Spectral available for violent release in the corona via • ISRD 5. 7: Spectral sample reconnection • ISRD 5. 11: Polarimetric sensitivity

Flow down from ATST SRD • Inhomogeneous upper atmosphere (SRD 2. 13) – Not horizontally stratified but inhomogeneous, dominated by field lines – CO absorption shows cool clouds – Hot gas (shockwaves? ) embedded

Flow down from ATST SRD • ISRD 5. 1: Wavelength range • Inhomogeneous upper atmosphere (SRD 2. 13) • ISRD 5. 2: Wavelength coverage – Not horizontally stratified but inhomogeneous, • ISRD 5. 3: Spatial resolution dominated by field lines • ISRD 5. 4: Image quality (AO) – CO absorption shows cool clouds • ISRD 5. 5: Field of view – Hot gas (shockwaves? ) embedded • ISRD 5. 6: Spectral resolution • ISRD 5. 7: Spectral sample • ISRD 5. 8: Slit scanning • ISRD 5. 11: Polarimetric sensitivity • ISRD 5. 13: Time resolution • ISRD 5. 14 through 5. 16: Simultaneous operation with NIRSP

Flow down from ATST SRD • Magnetic fields in the corona (SRD 2. 14) – Origin and heating of corona is unknown – Probably related to ~0. 1” processes in the photosphere – Observations of coronal structures on the disk (filaments) and limb (spicules, prominences) – Interesting lines: Ca. II H & K; Ca. II IRT; He. I D 3; He. I 10830 Å; Ha

ISRD 5. 1: Wavelength range Flow down • from ATST SRD • ISRD 5. 2: Wavelength coverage • Magnetic fields in the • corona (SRD 2. 14) ISRD 5. 3: Spatial resolution – Origin and heating of corona is unknown • ISRD 5. 4: Image quality (AO) – Probably related to ~0. 1” processes in the Goal: AO for limb observations photosphere • ISRD 5. 5: Field of view – Observations of coronal structures on the disk • ISRD 5. 6: prominences) Spectral resolution (filaments) and limb (spicules, – Interesting lines: Ca. II • HISRD & K; 5. 7: Ca. II IRT; sample He. I D 3; He. I Spectral 10830 Å; H • ISRD 5. 8: Slit scanning • ISRD 5. 11: Polarimetric sensitivity • ISRD 5. 13: Time resolution • ISRD 5. 14 through 5. 16: Simultaneous operation with NIRSP

Flow down from ATST SRD • Magneto-convection (SRD 3. 1) – Generation, interaction and dissipation of weak and strong fields – Are there helicity patterns? – Efficiency of convective collapse? – Penumbral filaments, umbral dots, etc may be the result of small-scale magneto-convection

Flow down from ATST SRD • Magneto-convection (SRD 3. 1) – Generation, interaction dissipation of weak and • ISRD and 5. 1: Wavelength range strong fields • ISRD 5. 2: Wavelength coverage – Are there helicity patterns? • ISRD 5. 3: Spatial resolution – Efficiency of convective collapse? • ISRD 5. 4: Image quality (AO) – Penumbral filaments, umbral dots, etc may be the • ISRD 5. 5: Field of view result of small-scale magneto-convection • ISRD 5. 6: Spectral resolution • ISRD 5. 7: Spectral sample • ISRD 5. 8: Slit scanning • ISRD 5. 11: Polarimetric sensitivity • ISRD 5. 13: Time resolution

Flow down from ATST SRD • Flux emergence and disappearance (SRD 3. 1. 2) – – – Distribution of field properties at emergence site Difference between quiet and active regions Search for dependences of quiet regions with cycle Flows associated with emergence Coalescence into flux tubes and flux cancellation

Flow down from ATST SRD • Flux emergence and disappearance (SRD 3. 1. 2) – – – Distribution of field properties at emergence site • ISRD 5. 1: Wavelength range Difference between quiet and active regions • ISRD 5. 2: Wavelength coverage Search for dependences of quiet regions with cycle • ISRD 5. 3: Spatial resolution Flows associated with emergence • ISRD 5. 4: Image quality (AO) Coalescence into flux tubes and flux cancellation • ISRD 5. 5: Field of view • ISRD 5. 6: Spectral resolution • ISRD 5. 7: Spectral sample • ISRD 5. 8: Slit scanning • ISRD 5. 11: Polarimetric sensitivity • ISRD 5. 13: Time resolution

Flow down from ATST SRD • Inner structure, dynamics and irradiance of k. G flux tubes (SRD 3. 1. 3 & 3. 1. 4) – Interaction between photospheric concentrations with turbulent motions is fundamental to estimate the amount of energy transmitted upwards – Observe flows inside and around flux tubes, MHD wave generation and propagation – Identify radiative transfer mechanism that makes them bright in UV

Flow down from ATST SRD • Inner structure, • dynamics and irradiance of k. G ISRD 5. 1: Wavelength range flux tubes (SRD • 3. 1. 3 & 3. 1. 4) ISRD 5. 2: Wavelength coverage – Interaction between phot concentrations with turbulent • ISRD 5. 3: Spatial resolution motions is fundamental to estimate the amount of • ISRD 5. 4: Image quality (AO) energy transmitted upwards ISRD 5. 6: resolution – Observe flows • inside and. Spectral around flux tubes, MHD wave generation and 5. 7: propagation • ISRD Spectral sample – Identify radiative transfer mechanism that makes them • ISRD 5. 9 and 5. 10: Slit repeatability/accuracy bright in UV • ISRD 5. 11: Polarimetric sensitivity • ISRD 5. 13: Time resolution • ISRD 5. 17: Simultaneous operation with VTF

Flow down from ATST SRD • Generation of acoustic oscillations (SRD 3. 1. 8) – Excitation of p-modes? – Localized in space and time(? ). Origin? – Propagation of acoustic oscillations and heating of upper atmo(? )

Flow down from ATST SRD • Generation of acoustic oscillations (SRD 3. 1. 8) – Excitation of p-modes? • ISRD 5. 1: Wavelength range – Localized in space and Origin? • ISRD 5. 2: time(? ). Wavelength coverage – Propagation of • acoustic and heating of ISRD 5. 3: oscillations Spatial resolution upper atmo(? ) • ISRD 5. 4: Image quality (AO) • ISRD 5. 6: Spectral resolution • ISRD 5. 7: Spectral sample • ISRD 5. 9 and 5. 10: Slit repeatability/accuracy • ISRD 5. 13: Time resolution • ISRD 5. 17: Simultaneous operation with VTF

Flow down from ATST SRD • Hanle effect diagnostics (SRD 3. 1. 6) – – New window for solar upper atmosphere Few, very specific, lines of interest Signals are usually very weak Some problems require polarimetric accuracy (e. g. , continuum polarization)

Flow down from ATST SRD • Hanle effect diagnostics (SRD 3. 1. 6) – – New window for solar upper atmosphere Few, very specific, lines • of. ISRD interest 5. 1: Wavelength range Signals are usually very weak • ISRD 5. 2: Wavelength coverage Some problems require polarimetric accuracy (e. g. , • ISRD 5. 4: Image quality (AO). continuum polarization) Goal: AO for limb observations • ISRD 5. 11: Polarimteric sensitivity • ISRD 5. 12: Polarimetric accuracy • ISRD 5. 17: Simultaneous operation with VTF

Flow down from ATST SRD • New science 10 years from now. . . • Flexibility!!

Requirements flow down Vi. SP SDR HAO 2 -3 March 2006

Requirements and ATST facilities Vi. SP SDR HAO 2 -3 March 2006

Detailed design cycle Vi. SP SDR HAO 2 -3 March 2006

Spectrograph specifications Vi. SP SDR HAO 2 -3 March 2006

Reference Design • • Design latitude Options considered - playoff Baseline optical design Mechanism baseline designs Mechanism adjustment modes Covers and Tables Electrical interconnect Vi. SP SDR HAO 2 -3 March 2006

Design latitude • Keep all optics in a plane on an optical surface. – Alignment and test are simplified. – There are fewer constraints when new technology becomes available • Separate wavelengths as widely as possible on the optical surface. – This means use of separate optics for collimation of the slit image and imaging onto the cameras. If a single optic were used, multiple diverse wavelengths would be closely packed in the focal plane making them very difficult to separate • Use relatively low order gratings. – A high order grating necessitates numerous narrow band order isolation filters – Multiple diverse wavelengths would be closely packed at the focal plane Vi. SP SDR HAO 2 -3 March 2006

Design latitude cont. • Automatic positioning of camera imaging optics is not practical – One cannot anticipate the exact combination of spectral regions of interest. – Often diverse wavelengths can be imaged using different optics – Sometimes two wavelengths will be closely spaced at the focal plane and can be separated using beam folding – Sometimes diverse wavelengths will overlap in the focal plane and will require a dichroic beam splitter for separation • Lenses are easier to use than mirrors when imaging onto the cameras – The exact geometry of placement of a parabola is critical – With a lens one needs only to determine a straight line from grating through the lens to the camera. Even if fold mirrors must be placed in the path, precise mirror placement is not required to maintain image quality Vi. SP SDR HAO 2 -3 March 2006

Design latitude cont. • For solar scanning, move as few optical components as possible – A scan mirror before the slit qualifies as few optics, but could cause a change in illumination of optics inside the spectrograph – a problem when performing polarimetry through a polarizing telescope. Also a scanner complicates spectrograph feed optics and compromises use of generalized feed optics for various ATST instruments – Moving the entire spectrograph has huge mechanical design impacts – A clean solution is to translate the slit and collimator maintaining constant distance from slit to collimator • Establish one ‘fixed’ configuration of camera and camera optic making it possible to perform spectro-polarimetry at a selectable wavelength with the click of a mouse • Provide for rapid selection of a grating with appropriate efficiency for wavelengths of interest Vi. SP SDR HAO 2 -3 March 2006

Configuration options • Is the collimator reflective or refractive? • Are paths folded between the grating and camera lenses? • Is the path folded between the slit and collimator? Vi. SP SDR HAO 2 -3 March 2006

Vi. SP SDR HAO 2 -3 March 2006

Baseline optical design Cameras Camera lenses Filter wheel Fixed beam PBS Slit (scans) Collimator (scans) Vi. SP SDR HAO 2 -3 March 2006 Grating

Fold Mirrors Collimator Camera Lenses Grating Vi. SP SDR HAO 2 -3 March 2006

Mechanism baseline designs • Performance requirements from ISRD • Typical designs for existing spectrographs • Motions are automated and provide absolute encoding of position • Motion attributes – Continuous or discrete – Range – Accuracy • The number of different actuators is limited to reduce mechanical and software design and fabrication efforts. – Linear stage – Servo-motor – Encoder micrometer Vi. SP SDR HAO 2 -3 March 2006

Mechanism baseline designs • HAO has a heritage of using Newport actuators and networked controllers on other projects (Co. MP, Pro. Mag, HMI PCU, FPI) Vi. SP SDR HAO 2 -3 March 2006

Mechanism baseline designs cont. • CAD renderings…. . other projector Vi. SP SDR HAO 2 -3 March 2006

Mechanism baseline designs cont. • Slit scan stage: – Type: Continuous – Range: 2. 8 arc minutes on the Sun. @f/40 this is ± 50 mm. – Accuracy: 0. 001 arc seconds or 3. 5 mm. – Mechanism: Linear stage • Slit width – – Type: Continuous Range: 0 to 1 mm x 100 mm high Accuracy: 1 mm Mechanism: Encoder micrometer Vi. SP SDR HAO 2 -3 March 2006

Mechanism baseline designs cont. • Slit rotation – Type: Continuous – Range: ± 2º – Accuracy: ± 0. 003 arc seconds over 3 arc min on the Sun or ± 30 arc seconds along the slit – Mechanism: Encoder micrometer • Slit decker – – Type: continuous Range: closed to 100 mm centered in height Accuracy: 100 mm Mechanism: Encoder micrometer Vi. SP SDR HAO 2 -3 March 2006

Mechanism baseline designs cont. • Spatial polarizing beam splitter – Type: fixed – Size: 75 mm diagonal • Spectral polarizing beam splitter – Type: fixed – Size: 140 mm x 6 mm • Fold mirrors – Size: 150 mm x 50 mm – Type: fixed Vi. SP SDR HAO 2 -3 March 2006

Mechanism baseline designs cont. • Grating selector – – Type: Discrete Range: 3 positions Accuracy: ± 1 mm with angular tolerances set by grating (below) Mechanism: Servo-motor driven linear stage • Grating a (selects wavelength) – – Type: Continuous Range: ± 90º Accuracy: ± 8 arc seconds (≈10 pm at 630 nm) Mechanism: Servo-motor Vi. SP SDR HAO 2 -3 March 2006

Mechanism baseline designs cont. • Grating b (tip) – – Type: Continuous Range: ± 70 mm at 2. 25 m or ± 1. 8º Accuracy: Ten 12 mm pixels at 2. 25 m or 10 arc seconds Mechanism: Encoder micrometer • Grating g (end to end) – Type: Continuous – Range: ± 2 mm – Accuracy: 10 pixels across 10º range of a angles = 12 arc seconds. For a 200 mm long grating this is 6 mm – Mechanism: Encoder micrometer Vi. SP SDR HAO 2 -3 March 2006

Mechanism baseline designs cont. • Collimator lens Z motion (focus) – – – Size: 150 mm wide center section of 200 mm diameter triplet Type: Continuous Range: ± 50 mm Accuracy: 3. 5 mm Mechanism: Linear stage • Fold mirror tilt (spectral) – – – Size: 175 mm x 100 mm Type: Continuous Range: ± 70 mm at 2. 25 m or ± 1. 8º Accuracy: Ten 12 mm pixels at 2. 25 m or 10 arc seconds Mechanism: Encoder micrometer Vi. SP SDR HAO 2 -3 March 2006

Mechanism baseline designs cont. • Camera lenses – Size: 150 mm wide center section of 200 mm diameter triplet – Type: Fixed • Filter wheel for fixed beam – – – Size: 50 mm diameter filters (baseline) Type: Discrete Range: 8 positions Accuracy: 1 º Mechanism: Servo-motor Vi. SP SDR HAO 2 -3 March 2006

Mechanism baseline designs cont. • Camera X (Plane of the table) and Y (Up/Down) – – Type: Continuous Range: ± 12 mm Accuracy: 1 pixel or 12 mm Mechanism: Encoder micrometer • Camera Z (focus) – – Type: Continuous Range: ± 12 mm Accuracy: 6 mm Mechanism: Encoder micrometer Vi. SP SDR HAO 2 -3 March 2006

Mechanism baseline designs cont. • Camera rotation – – Type: Continuous Range: ± 5º Accuracy: 1 pixel out of 1000 or 20 arc seconds Mechanism: Encoder micrometer Vi. SP SDR HAO 2 -3 March 2006

Adjustment modes Vi. SP SDR HAO 2 -3 March 2006

Optical tables • Optical tables span 5. 5 m x 2 m • Grating linear stage is stepped down • Optical surface is 800 mm above the floor • Light path is 1 m above the floor • Vendor TMC Vi. SP SDR HAO 2 -3 March 2006

Covers • Cover frame elements are spaced on 1 m centers • Side panels are removable and most are interchangeable • Cover panels are center hinged to avoid the risk of dropping one • Vendor 80/20 Vi. SP SDR HAO 2 -3 March 2006

Electrical Interconnections • There are 9 controllers, five assigned to cameras, two for grating motions, and one for slit, collimator and filter wheel • All controllers are on the network with the Instrument Control System somewhere else on the network Vi. SP SDR HAO 2 -3 March 2006

Mechanism Tests • No control computer or software required for testing • Use network to access web based interface on mechanism controllers to perform motions Vi. SP SDR HAO 2 -3 March 2006

Polarimetry Issues • • Measurement technique Polarimeter efficiency Wavelength range Calibration – Requirements – Polarimeter response matrix – Telescope matrix • Seeing-induced cross talk • Telescope polarization Vi. SP SDR HAO 2 -3 March 2006

Polarimetry technique • Polarimetry Analysis & Calibration facilities and Camera Systems produce and detect time multiplexed polarization images • Polarimetry problem: Ssun is input Stokes vector, Smeasured is detected vector, X is the polarimeter response matrix, T is the telescope Mueller matrix. Vi. SP SDR HAO 2 -3 March 2006

Measurement technique: Efficiency • The magnitude intensity changes corresponding to each Stokes parameter compared to the total intensity over the time of the measurement is the efficiency of the polarimeter • Examples – “Stokes definition polarimeter” – Rotating retarder – Liquid crystal Vi. SP SDR HAO 2 -3 March 2006

Stokes definition polarimeter Put a polarizer in the beam and orient it to two angles for ± Q, and two angles for ± U Add a retarder in front and position the linear polarizer to two angles for ±V With perfect optics efficiency is 1/3 since 1/3 of the time is spent measuring each of the polarization parameters. Vi. SP SDR HAO 2 -3 March 2006

Modulator/Analyzer • Modulator is some sort of retarder or combination of retarders that change orientation or magnitude of retardation from one modulation ‘state’ to the next. • Analyzer is a linear polarizer or a polarizing beam splitter (dual beam) • The combination of these results in a sequence of intensity measurements, one for each state, a linear combination of which is the measured vector Vi. SP SDR HAO 2 -3 March 2006

Rotating retarder • Uses a rotating retarder as the modulator and a polarizer or polarizing beam splitter as the analyzer • Circular polarization produces an intensity modulation twice per rotation. • Linear polarization produces an intensity modulation 4 times per rotation. The phase indicates orientation of the linear polarization vector. • 8 samples span one modulation cycle and are recorded in a half rotation (6 works but with less efficiency) Vi. SP SDR HAO 2 -3 March 2006

Rotating retarder • Demodulation is creation of the measured vector from the modulation states • In the table Signs indicate adding or subtracting samples to determine I, Q, U, and V • Efficiency for a stepped 123. 1º retarder sampled continuously is 0. 547 Vi. SP SDR HAO 2 -3 March 2006

Liquid crystal • Two liquid crystal variable retarders (LCVR) one with fast axis oriented at 0º and the other at 45º with retardation values set to the following for the four modulation states – 45º, 135º, and 45º for the first LCVR – 234. 73º, 305. 27º, 54. 73º, and 125. 27º for the second* • Two ferroelectric liquid crystals. The first has a retardation of 180º and switches between 0º and 45º. The second has a retardation of 102. 22º and switches between -18º and +18º *Jorge Sánchez-Almeida Vi. SP SDR HAO 2 -3 March 2006

Liquid crystal Efficiency achieves maximum possible of 0. 577 for each of the polarization parameters. Vi. SP SDR HAO 2 -3 March 2006

Wavelength range • An ‘achromatic’ rotating retarder is possible capable of significant modulation efficiency from 380 nm to 1600 nm. (SPINOR) • Shown is polarimeter response, X, for an bicrystalline achromat (blue) and for a Pancharatnam plate of three bi-cyrstalline achromats (red). Note large values on the diagonal and small ones elsewhere. Vi. SP SDR HAO 2 -3 March 2006

LCVR • When optimized for a particular wavelength or perhaps a pair, the X matrix elements vs. wavelength look crazy. Demodulation signs will vary depending upon the wavelength of observation. Vi. SP SDR HAO 2 -3 March 2006

LCVR • When total efficiency, a Stokes parameter detected in any of the modulation states) is computed, a large portion of the spectrum has significant efficiency, but cannot be optimized arbitrary set of diverse wavelengths. Vi. SP SDR HAO 2 -3 March 2006

Wavelength range • An ‘achromatic’ retarder is possible capable of significant modulation efficiency from 380 nm to 1600 nm. (SPINOR) • Nematic liquid crystal variable retarders (LCVR) can be tuned to optimize any wavelength of interest but not an arbitrary set of diverse wavelengths • Ferroelectric liquid crystals (Fe. LC) are built for a particular wavelength Vi. SP SDR HAO 2 -3 March 2006

Other modulator issues • Mechanical motions add cost and effort – Rotating retarder must go ‘fast’ – LCVRs can rotate diurnally • Rotating retarders can introduce beam wobble – Ok if kept with range of the AO • Some modulators are inherently faster than others – Vi. SP requirements are not that tight – LCVRs are relatively slow though new designs may overcome this • Spectral fringing is always an issue and must be considered for any design Vi. SP SDR HAO 2 -3 March 2006

Other modulators • Pockel’s cells are tunable like LCVRs and fast but require high voltage • Piezo-elastic modulators (ZIMPOL) are tunable, but so fast as to require custom cameras and are tricky to synchronize • Achromatic designs for LCVRs and Fe. LCs have been explored (Gandorfer, Sankar) and will continue to be examined as possibilities for ATST. Vi. SP SDR HAO 2 -3 March 2006

Vi. SP Baseline Modulators • Achromatic rotating retarder – Maximum wavelength diversity – Proven technology – Works for Vi. SP and NIRSP (<1. 6 mm) • Liquid crystal – Optimized modulation for a particular wavelength range – Rapidly developing technology Vi. SP SDR HAO 2 -3 March 2006

Calibration Accuracy • The polarimetric accuracy requirement is 5۰ 10 -4 • Each element of the polarimeter response telescope matrix product (XT) does not need to be determined to the polarimetric accuracy – Flat fielding to high accuracy is not required – Q, U, and V are generally much smaller than I, especially when maximum accuracy is required. • A general form for specifying the “Error matrix” has been worked out (Ichimoto) and is included in the supplemental documents. Vi. SP SDR HAO 2 -3 March 2006

Error Matrix • Polarization calibration must determine X and T to within the bounds of the error matrix. Vi. SP SDR HAO 2 -3 March 2006

Calibration by element • 1, 1 Flat fielding – Method 1 is to raster the telescope near disc centre pointing while recording numerous images – Method 2 is to insert an artificial source at the Gregorian focus, use a wide slit width and record numerous images • Top row – Requirements are not tight so use a model for contributions from X and T Vi. SP SDR HAO 2 -3 March 2006

Calibration by element cont. • First column – Use continuum polarization as is done with ASP. This solves for contributions from X and T. • Diagonal elements and linear phase – Use calibration optics at the Gregorian Optical System (GOS) – Linear polarizer at various orientations for center four elements. – Linear polarizer at 45° to a following quarter wave retarder for circular efficiency (4, 4). Vi. SP SDR HAO 2 -3 March 2006

Calibration by element cont. • Linear to and from circular – Use calibration linear polarizer and polarizer plus retarder for X – Use the sub aperture method to determine T. Vi. SP SDR HAO 2 -3 March 2006

Calibration Solution Sun Sin Telescope (M 1+M 2) GOS (T) T Sin Sub-aperture method (Determine T) Instrument (X) Smeasured X T Sin Calibration optics at GOS (Determine X) Vi. SP SDR HAO 2 -3 March 2006

Polarimeter response, X, calibration • Polarimeter response calibration optics are located at the GOS ahead of the polarization modulator • A calibration linear polarizer and retarder will be used to create linearly and circularly polarized ‘known’ polarization input to the modulator – Inserted into the beam simultaneously – Each rotates independently to any angle • Elements of the X matrix are inferred from the measured vectors for these known input Stokes vectors Vi. SP SDR HAO 2 -3 March 2006

Polarimeter response calibration cont. Linear polarizer (Versalight) followed by a zero order quartz retarder. Reasonable calibration efficiency is possible from 380 nm to 1600 nm Achromatic retarder designs will also be considered such a Pancharatnam plate or bi -crystalline achromat. Vi. SP SDR HAO 2 -3 March 2006

Telescope calibration: Sub-aperture method • Baseline ATST telescope calibration method • Empirical measurement of T • Tests with ASP data show it meets ATST requirements (Socas-Navarro 2005, JOSA-A, 22, 907)

Telescope calibration – Achromatic polarizer and retarder (20 cm) – Mask (sub-aperture) at a pupil image

Telescope calibration – Achromatic polarizer and retarder (20 cm) – Mask (sub-aperture) at a pupil image

Telescope calibration – Achromatic polarizer and retarder (20 cm) – Mask (sub-aperture) at a pupil image

The Sub-Aperture Method • Basic procedure: – The mask turns the ATST into a small-aperture system – The sub-aperture system is fully calibrated using optics in front of M 1 – Observe a solar target (e. g. , sunspot) with sub-aperture and then with full aperture – Sub-aperture observations are calibrated -> Sin is known! – With known Sin and the full aperture X matrix known (using GOS calibration optics) infer the full aperture T matrix

Seeing-induced cross talk • Cross talk among Stokes parameters due to seeing often limits polarimetric accuracy. • Cross talk models have been created by Lites for ASP and Judge for ATST – Stable image helps: Tip-tilt stabilized seeing model is used – A fast modulation rate helps – Dual beam polarimetry • Helps to the extent the beams are balanced • Cannot correct crosstalk that occurs in a polarizing telescope in front of the polarimeter • With the prospect of MCAO, dual beam analysis must be performed at the coudé. Optics between modulator and analyzer can significantly change the X matrix over a day – If the modulation rate is high enough then even a single beam polarimeter can have low seeing-induced crosstalk Vi. SP SDR HAO 2 -3 March 2006

Seeing-induced cross talk cont. • • Total seeing induced cross talk from I, Q, U, & V into Q, U, and V Model: – Tip tilt corrected DST seeing – slow=100 Hz samples – fast=1600 Hz samples – 100% imbalance = single beam – small imbalance = dual beam – solar gradient of I, Q, U, V of 100% per 1/8 arc second assumed – Green=requirement, Blue=goal Vi. SP SDR HAO 2 -3 March 2006

Seeing-induced cross talk conclusions • Within uncertainties of the model, the polarization accuracy requirement is met with dual beam analysis and 100 Hz sample rate – a fast CCD • To meet the polarization accuracy goal, a dual beam Demodulating Imaging Detector (DID) could be used when and if one becomes available • In the modulation frequency range studied, single beam polarimetry will not meet the accuracy requirement – much higher frequency is needed Vi. SP SDR HAO 2 -3 March 2006

Telescope polarization • Optics between the polarization modulator and the Sun constitute the ‘telescope’ • The more optics there are, the higher the polarization and the correction that must be applied through calibration • Polarimetry through a polarizing telescope suffers from increased seeing-induced cross talk • Time varying optical coatings make frequent calibrations a requirement (yes there are models) Vi. SP SDR HAO 2 -3 March 2006

Telescope Geometry • Haleakala • +10° declination Vi. SP SDR HAO 2 -3 March 2006

Telescope Mueller matrix • Telescope Mueller matrix vs. hour angle for the modulator at the GOS(dark) and for the modulator at the coudé station (light). Modulator and coudé – azimuth (analyzer) are fixed relative to solar image • Note a variation of 1. 4 in the (2, 3) and (3, 2) elements over the span of a few minutes near noon! Vi. SP SDR HAO 2 -3 March 2006

Telescope polarization • Due to telescope polarization and its variation over a day, the modulator should be located as close to the Sun as possible – practically that means at the GOS Vi. SP SDR HAO 2 -3 March 2006

Modulator at GOS analyzer at coudé • Dark lines protected Ag, light lines Al • Measure X at a range of geometries and model for all geometries • Protected Ag should be stable Vi. SP SDR HAO 2 -3 March 2006

Dual Beam Polarimetry • Vastly reduces seeing induced cross talk • Impractical at the GOS – Differential aberrations all the way to the coudé room – Not compatible with AO • Polarizing optics between modulator and polarizing beam splitter though a problem can be handled Vi. SP SDR HAO 2 -3 March 2006

Dual Beam Polarimetry: Baseline • Fast CCD polarimetry – PBS behind slit of spectrograph like ASP – Good for wavelength diversity – With small field of view imager optics can handle both beams • Large CCD polarimetry – PBS is behind slit of spectrograph that separates beams spectrally – Narrow band pass filter is needed so that the two spectra do not overlap Vi. SP SDR HAO 2 -3 March 2006

Polarimetry Summary • Position the modulator at the GOS • Locate calibration optics at the GOS in front of the modulator to determine the X matrix • Use the sub-aperture method to determine the telescope T matrix • Use an achromatic rotating retarder for wavelength diverse polarimetry • Use a liquid crystal modulator for polarimetry at specific wavelengths • Use a fast CCD to meet the polarization accuracy requirement • Use a large CCD to meet the field of view requirement • It is expected that large fast CCDs will be available in the future perhaps by the time ATST has to decide upon cameras. • Place polarizing beam splitter behind spectrograph slit Vi. SP SDR HAO 2 -3 March 2006

Lunch • Did this run over into the afternoon? Vi. SP SDR HAO 2 -3 March 2006