WALOP-South: A wide-field one-shot linear optical polarimeter for PASIPHAE survey
Siddharth Maharana, John A. Kypriotakis, A. N. Ramaprakash, Pravin Khodade, Chaitanya Rajarshi, Bhushan S. Joshi, Pravin Chordia, Ramya M. Anche, Shrish Mishra, Dmitry Blinov, Hans Kristian Eriksen, Tuhin Ghosh, Eirik Gjerløw, Nikolaos Mandarakas, Georgia V. Panopoulou, Vasiliki Pavlidou, Timothy J. Pearson, Vincent Pelgrims, Stephen B. Potter, Anthony C. S. Readhead, Raphail Skalidis, Konstantinos Tassis, Ingunn K. Wehus
WWALOP-South: A wide-field one-shot linear opticalpolarimeter for PASIPHAE survey
Siddharth Maharana a* , John A. Kypriotakis b,c , A. N. Ramaprakash a,b,e , Pravin Khodade a ,Chaitanya Rajarshi a , Bhushan Joshi a , Pravin Chordia a , Ramya M. Anche a , Shrish h , DmitryBlinov b,c,i , Hans Kristian Eriksen g , Tuhin Ghosh h , Eirik Gjerløw g , Nikolaos Mandarakas b,c ,Georgia V. Panopoulou f , Vasiliki Pavlidou b,c , Timothy J. Pearson e , Vincent Pelgrims b,c ,Stephen B. Potter d,j , Anthony C. S. Readhead e , Raphael Skalidis b,c , Konstantinos Tassis b,c ,and Ingunn K. Wehus ga Inter-University Centre for Astronomy and Astrophysics, Post bag 4, Ganeshkhind, Pune,411007, India b Institute of Astrophysics, Foundation for Research and Technology-Hellas, Voutes, 70013Heraklion, Greece c Department of Physics, University of Crete, Voutes, 70013 Heraklion, Greece d South African Astronomical Observatory, PO Box 9, Observatory, 7935, Cape Town, SouthAfrica e Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena,CA, 91125, USA f Hubble Fellow, California Institute of Technology, Pasadena, CA 91125, USA g Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, NO-0315Oslo, Norway h School of Physical Sciences, National Institute of Science Education and Research, HBNI,Jatni 752050, Odisha, India i Astronomical Institute, St. Petersburg State University, 198504, St. Petersburg, Russia j Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, SouthAfrica
ABSTRACT
WALOP (Wide-Area Linear Optical Polarimeter)-South, to be mounted on the 1m SAAO telescope in SouthAfrica, is first of the two WALOP instruments currently under development for carrying out the PASIPHAEsurvey. Scheduled for commissioning in the year 2021, the WALOP instruments will be used to measure thelinear polarization of around 10 stars in the SDSS-r broadband with 0.1 % polarimetric accuracy, covering 4000square degrees in the Galactic polar regions. The combined capabilities of one-shot linear polarimetry, highpolarimetric accuracy ( < . < .
05 %), and a large field of view (FOV) of35 ×
35 arcminutes make WALOP-South a unique astronomical instrument. In a single exposure, it is designedto measure the Stokes parameters I , q and u in the SDSS-r broadband and narrowband filters between 500-700 nm. During each measurement, four images of the full field corresponding to the polarization angles of 0 ◦ ,45 ◦ , 90 ◦ and 135 ◦ will be imaged on four detectors and carrying out differential photometry on these images willyield the Stokes parameters. Major challenges in designing WALOP-South instrument include- (a) in the opticaldesign, correcting for the spectral dispersion introduced by large split angle Wollaston Prisms used as polarizationanalyzers as well as aberrations from the wide field, and (b) making an optomechanical design adherent to thetolerances required to obtain good imaging and polarimetric performance under all temperature conditions as Further author information: (Send correspondence to S.M.)S.M.: E-mail: [email protected], Telephone: +91 020 2560 4213 a r X i v : . [ a s t r o - ph . I M ] F e b ell as telescope pointing positions. We present the optical and optomechanical design for WALOP-South whichovercomes these challenges. Keywords: wide-field polarimetry, four-channel polarimetry, optical polarimetry, Stokes parameters, PASIPHAE,WALOP, stellar polarimetry
1. INTRODUCTION
WALOPs (Wide-Area Linear Optical Polarimeter) are a pair of wide field linear optical polarimeters currently un-der development at the Inter-University Center for Astronomy and Astrophysics (IUCAA), Pune, India. WALOP-South will be installed at the 1 m telescope of South African Astronomical Observatory’s Sutherland Observatorywhile WALOP-North will be installed on the 1.3 m telescope of the Skinakas Observatory of the University ofCrete, Greece. Together, these will be deployed to carry out the PASIPHAE survey, which aims to cover 4000square degrees of sky in the northern and southern Galactic polar regions and measure polarization of around10 stars with polarimetric accuracy better than 0.1 %. Current optical polarization catalogues have measure-ment of 10 stars. The main scientific objective of the PASIPHAE survey is to use this high accuracy stellarpolarimetric survey data in conjugation with the GAIA survey’s stellar distance measurements to create a 3-dtomography map of the dust and magnetic field in Milky Way Galaxy’s polar regions. A detailed description ofthe scientific motivations and objectives of the PASIPHAE survey is presented in PASIPHAE program’s whitepaper. Of the two WALOP instruments, WALOP-South is scheduled first for commissioning in the year 2021.In this manuscript, we present the optical and optomechanical design of the instrument as well as its currentstatus. Section 1.1 describes the overall design goals of the instrument, while Sections 2 and 3 describe theoptical and optomechanical design of the WALOP-South instrument.
The unique science goals of the PASIPHAE survey drive the technical design goals for the WALOP instruments,which are same for both WALOP-North and WALOP-South, and are listed in Table 1. The motivation andjustification for the target values for each of the design parameters is provided in the dedicated optical designpaper of the instrument by Maharana et al. (accepted for publication in Journal for Astronomical Telescopes,Instruments and Systems), henceforth referred to as Paper I.We define polarimetric sensitivity ( s ) as the least value and change of linear polarization which the instrumentcan measure, without correction for the cross-talk and instrumental polarization of the instrument. s is a measureof the internal noise and random systematic of the instrument due to the optics. Polarimetric accuracy ( a ) isthe measure of closeness of the predicted polarization of a source to the real value after applying corrections forthe above effects using calibration techniques (described in Paper I) as well as taking into account uncertaintydue to photon noise. Sl. No . Parameter Technical Goal ×
30 arcminutes6 Detector Size 4 k × k (Pixel Size = 15 µm )7 No. of Detectors 48 Primary Filter SDSS-r9 Imaging Performance Close to seeing limited PSF9 Stray and Ghost Light Level Brightness less than sky brightness per pixel.Table 1: Design goals for WALOP-South instrument. . OPTICAL DESIGN A major challenge in the development of the WALOP-South instrument was creating a suitable optical designwhich meets the requirements listed in Table 1. Here we present the overall optical model of the instrument andit’s predicted performance. While two/four channel optical polarimeters with imaging of the two/four channelson different detector/detector-areas have been made in the past, either they have been designed for very narrowfields of view
4, 5 of around 1 × (widthof > = 100 nm ). WALOP-South is first of its kind wide-field one-shot four channel imaging polarimeter withseparate cameras for each channel. WALOP-South’s optical design has been created to work optimally for the1 m SAAO telescope’s optics prescription and Sutherland Observatory’s temperature and observing conditions,which are listed in Table 2. Parameter Value
Telescope Type Cassegrain Focus and Equatorial MountPrimary Mirror Diameter 1 mSecondary Mirror Diameter 0.33 mNominal Telescope f-Number 16.0Altitude 1800 mMedian Seeing FWHM 1.5”Extreme Site Temperatures − ◦ C to 40 ◦ C Table 2: Telescope and site details of South African Astronomical Observatory’s (SAAO) Sutherland Observatory.The optical model of WALOP-South was designed and analyzed using the Zemax ® optical design software.Figure 1 shows the optical model of the instrument. The entire instrument’s optical system consists of thefollowing assemblies: a collimator, a polarizer assembly and four cameras (one for each channel). The collimatorassembly begins from the telescope focal plane. Aligned along the z-axis, it creates a pupil image which is fedto the polarizer assembly. The polarizer assembly acts as the polarization analyzer system of the instrumentand splits the pupil beam into four channels corresponding to 0 ◦ , 45 ◦ , 90 ◦ and 135 ◦ polarization angles, whichare referred to as O1, O2, E1 and E2 beams, respectively. Additionally, this assembly folds and steers the Obeams along the +y and -y directions and the E beams along the +x and -x directions. Each channel has its owncamera to image the entire field of view on a 4 k × k CCD detector. The obtained field of view of the instrumentis 34 . × . ×
30 arcminutes. Table 3 lists the key designparameters of the instrument’s optical system. The polarizer assembly the most novel and complex aspect ofthe WALOP-South optical design, and Section 2.1 describes it’s architecture and working. As part of the opticaldesign, we also designed a guider camera for instrument as well as new baffles for the telescope to accommodatethe large field of view of WALOP-South- these are described in Paper I.
Parameter Design Value/Choice
Filter SDSS-rTelescope F-number 16.0Camera F-number 6.1Collimator Length 700 mmCamera Length 340 mmNo of lenses in Collimator 6No of lenses in Each Camera 7Detector Size 4096 × µm Sky Sampling at detector 0.5”/pixelTable 3: Values of the key parameters of WALOP-South Optical Design.igure 1: Optical model of the WALOP-South instrument. Beginning at the telescope focal plane, it acceptsthe beam for the entire field of view and through the collimator assembly creates a pupil image, which is thenfed to the polarizer assembly. The polarizer assembly acts as the polarization analyzer system of the instrumentand splits the pupil beam into four channels corresponding to 0 ◦ , 45 ◦ , 90 ◦ and 135 ◦ polarization angles, whichare referred to as O1, O2, E1 and E2 beams, respectively. Additionally, this assembly folds and steers the Obeams along the +y and -y directions and the E beams along the +x and -x directions. Each channel has itsown camera to image the entire field of view on a 4 k × k CCD detector.igure 2: A cartoon illustrating the working of the polarizer assembly of the WALOP-South instrument. p and θ shown are as seen in the x-y plane when viewed along the z-axis of the cartoon and the change in the polarizationstate of the beams while passing through this system is annotated. Together, the Wollaston Prism Assemblyconsisting of the two BK7 glass wedges, Wollaston Prisms (WP) and Half-Wave Plates (HWP) and the two PBS’act as the polarization beamsplitter sub-system. The pupil is split between the two BK7 wedges which is thenfed to the twin HWP + WP system to be split into four channels with the polarization states of 0 ◦ , 45 ◦ , 90 ◦ and 135 ◦ . Afterwards, the two PBS’ direct these four beams in four directions. It consists of four sub-assemblies: (a) Wollaston Prism Assembly (WPA), (b) Wire-Grid Polarization Beam-Splitter (PBS), (c) Dispersion Corrector Prisms (DC Prisms) and (d) Fold Mirrors.The WPA consists of two identical calcite Wollaston Prisms (WP), with a half-wave plate (HWP) and a BK7glass wedge in front of each WP (Figure 2). The WPs have an aperture of 45 × mm and a wedge angle of 30 ◦ ,resulting in a split angle of 11 . ◦ at 0 . µm wavelength. The left WP has a HWP with fast-axis at 0 ◦ with respectto the instrument coordinate system to separate 0 ◦ and 90 ◦ polarizations while the right WP has a HWP withfast-axis at 22 . ◦ to split the 45 ◦ and 135 ◦ polarizations. The BK7 wedges at the beginning of the WPA, whichshare the incoming pupil beam equally, ensure that rays from the off-axis objects in the field of view entering atoblique angles of incidence do not hit the interface between the WPs, which will lead to throughput loss as wellas instrumental polarization from scattering arising at the surface. Thus the WPA, using the splitting actionof the WPs, separates the beam at the pupil into O1, O2, E1 and E2 beams- corresponding to the polarizationangles of 0 ◦ , 45 ◦ , 90 ◦ and 135 ◦ respectively. The PBS’ act as beam selectors, allowing both the O beams to passthrough while folding the E1 and E2 beams along -x and +x directions. Figure 2 shows the overall working ideaof the WPA and PBS components of the polarizer assembly. The DC Prisms are a pair of glass prisms presentin the path of each of the four beams after the PBS’ to correct for the spectral dispersion introduced by theWPA (refer to Paper I). Additionally, mirrors placed at ± ◦ the y-z plane fold the O beams into +y and -ydirections to limit the length of the instrument to 1.1 m from the telescope focal plane. Figure 3 shows the spot diagram for one of the four cameras (O1 beam) at the detector for different field points.All the four beams have very similar spot diagrams. The O1 and O2 beams have identical optical paths andigure 3: Spot diagram for one of the four cameras at the detector for different field points. Different colorsrepresent different wavelengths as labeled in the image legend. RMS and GEO radius stand for the root-meansquare and geometric radius of the spot diagrams, respectively. The optical performance of E1 and E2 beamsare identical as they follow identical optical paths, and likewise for the O1 and O2 beams. Also, the O and Ebeams have similar spot diagram sizes(Table 4).thus identical spot diagrams. Same is true for E1 and E2 beams. The averaged RMS (root mean squared)radii for the O and E beams (”Nominal spot radius” parameter in Table 4) is 11.63 and 11.77 µm respectively.In comparison, the RMS radius for a 1.5 arcsecond FWHM Gaussian beam (median seeing at the SutherlandObservatory) at the detectors is 19.1 µm .A complete tolerance analysis of the optical system was done in Zemax using Monte Carlo (MC) simulationsto estimate the expected deterioration in the spot sizes for the instrument and the required tolerances for thefabrication of optical and mechanical components of the system. Two compensators were defined- (a) separationbetween the primary and secondary telescope mirror, and (b) distance between the last camera lens and thedetector of each camera. Table 4 shows the results of 20,000 MC simulations. The mean RMS spot radius forthe O and E beams based on the simulations are 17.1 and 15.37 µm respectively for the O and E beams, whichis smaller than the RMS radius for a 1.5 arcsecond FWHM Gaussian beam at the detectors (19.1 µm ). Thuswe expect to obtain near seeing limited PSF at the detectors (for a comprehensive and quantified estimate ofthe instrument’s expected imaging performance, refer to Paper I). Table 5 captures the required tolerances forthe alignment of the optical assembly based on which the MC simulation results were obtained. The tolerancevalues are common for corresponding elements in all the four beams.arameter O-Beams E-BeamsRMS Spot Radius ( µm ) RMS Spot Radius ( µm )Nominal Spot 11.63 11.77Root-Sum-Square 17.1 15.37MC Simulation Best Case 11.72 11.7MC Simulation Worst Case 37.4 25.5MC Simulation Mean 17.54 15.72MC Simulation Std Dev 0.003 0.0018Table 4: Results of Monte Carlo simulations based tolerance analysis for O and E beams. Root-Sum-Squareradius is the RMS spot radius obtained if the offset in spot radius due to all mechanical and optical tolerancesare added in quadrature.Lens Name Decentre ( µm ) Axial ( µm ) Tilt (arcminute)Collimator Lens 1 50 200 3Collimator Lens 2 50 200 3Collimator Lens 3 50 200 3Collimator Lens 4 30 200 2Collimator Lens 5 50 200 1Collimator Lens 6 20 100 2Camera Lens 1 30 50 1Camera Lens 2 30 30 1Camera Lens 3 30 50 1Camera Lens 4 30 200 1Camera Lens 5 30 200 2Camera Lens 6 50 200 3Camera Lens 7 50 200 3WPA 50 100 5PBS 50 100 5DC Prism 1 50 100 5DC Prism 2 50 100 5Fold Mirror 50 100 5Table 5: Tolerances on alignment of the optical elements of the WALOP-South instrument. The tolerance valuesare common for corresponding elements in all the four cameras.
3. OPTOMECHANICAL DESIGN3.1 Technical Requirements
The requirements from the optomechanical design of the WALOP-South instrument are:1. Align and hold all the optical elements within the mechanical tolerances obtained from the toleranceanalysis (Table 5) of the instrument.2. Maintain alignment of the optics (within required tolerances) for the various possible different pointingorientations of the telescope, especially from zenith to up to 30 ◦ ) from the horizon, i.e. airmass of 2 sincemost observations will be done in this telescope pointing window.3. Optics holders should exert minimal stresses on the glasses due to the mounting method as well as dueto temperature changes at the telescope site during observations. Stress on glass leads to stress birefrin-gence which will modify the polarization state of the light ray passing through the glass and will lead toinstrumental polarization and cross-talk between the Stokes parameters.. The maximum mass of instrument that can be mounted on the SAAO 1 m telescope is 150 Kgs; so theinstrument mass should be under 150 Kg.5. The instrument requires controlled motions for many subsystems. A list of all motion systems are capturedin Table 6 and elaborated in Section 3.4. The optomechanical system should provide provisions for all suchmovements to the required accuracy.6. The instrument model should have provisions for mounting of all electrical connectors and control boxessuch as CCD control boxes as well motion motion control boxes, taking into consideration locations wherethe connections are needed. Figure 4 shows the overall optomechanical model of the instrument, without electrical connectors and controlboxes mounted. The different subsystems of the instrument are annotated in the image. The instrument beginswith an instrument window to mechanically seal the instrument from the outside environment to prevent dustsettling on optics surfaces that can lead to spurious polarization signals in polarimeters (refer to Figure 4 inRoboPol instrument paper ). Before the main instrument, the auto-guider camera and calibration polarizersub-assemblies are present. The optical design of the auto-guider camera described in Paper I. The calibrationpolarizer is a linear polarizer sheet which is provided for creating the polarization calibration model of theinstrument. The main optomechanical model of the instrument, like the optical design, can be divided intofollowing subsystems- (a) collimator barrel, (b) polarizer box and (c) four camera barrels. As the name suggests,the collimator and camera assemblies are in form of barrels (Section 3.3) while the polarizer assembly consistingof the Wollaston Prism Assembly, wire-grid Polarization Beam-Splitters, Dispersion Corrector Prisms and theFold Mirrors are all enclosed in a box from which the four camera barrels project in four directions. .Figure 4: The overall optomechanical model of the WALOP-South instrument, without electrical connectors andcontrol boxes mounted. The various major subsystems in the model have been marked. .3 Barrel Design The lenses are held in their individual holders using flexure based lens mounts which are the widely used toachieve and maintain high accuracy alignments. Figure 5 shows a lens holder with a lens mounted on it. Thelens is attached to the holder using the multiple flexures which are glued to the lens around its rim (cylindricalface). The collimator and the camera barrels are made by placing the lens holders in sequence with cylindricalspacers. Figure 6 show the images of the one of the four camera barrels of WALOP-South instrument. Followingdesign decisions were made in the barrel design:1. Most lens mounts, except for the largest lenses (collimator lens 1 and 2), the lens mount material is madeof Aluminium-6061 alloy; for the larger lenses, Titanium 6Al-4V alloy is used. The CTE (coefficient ofthermal expansion) of Titanium 6Al-4V alloy is 8 . × − , which is close to the CTE of most glasses(7 − × − ), while the CTE of Aluminium-6061 alloy is 24 × − and is farther away from that ofmost lens materials. While Titanium 6Al-4V is apt for reducing mechanical stresses on the glasses due totemperature changes, it is a heavier material 4 . g/cc than Aluminium-6061 alloy (2 . g/cc ). Additionally,Aluminium-6061 is cheaper and easier to procure, and more importantly easy to machine to the tighttolerances required by us. So barring the first two lenses where we expect large thermal stresses to arisedue to the larger aperture of the lenses, all other lens mounts have been made of Aluminium-6061 alloy.2. In optical lenses requiring high accuracy alignment, the spacer and lens mount have been combined so tobe made a single mechanical component, reducing additional mechanical misalignment.Figure 5: A lens holder of the instrument with lens mounted on it. The lens is attached to the holder using themultiple flexures which are then glued to the lens around its rim (cylindrical face) at multiple places. Table 6 lists all the control systems in the WALOP-South instrument. The calibration linear polarizer sheet atthe beginning of the instrument and the calibration Half-Wave Plate at the pupil need to have rotation motionswhen in the optical path with the provision of being moved out of the optical path when not in use. The filterwheel, placed at the pupil, has 4 filters mounted on a linear stage, and any of the four can be placed in theoptical path by the linear motion of the stage. The guider camera (refer to Paper I for guider camera opticaldesign) has two linear stages on which the entire camera optics is mounted. The x-y motion of these two stagesis used to patrol an effective field of view of 540 square arcminutes. There is a provision for placing a filter inthe auto-guider camera’s optical path by in-out motion. A common shutter for all the four cameras is placedinside the collimator barrel which is controlled electronically to open and close.igure 6: Cross Section of one of the four camera barrels. Every lens is held in it’s holder, which is then connectedto the succeeding lens holder through a cylindrical spacer. For additional protection, each lens has retainer madefrom a soft material (Teflon) attached to it. In this barrel, to obtain better alignment accuracy, the lens holderand spacer for all the individual lenses have been integrated into one component.The Wollaston Prism Assembly is the most delicate and temperature sensitive optical component in theinstrument (refer to Paper I for details). While it has been cemented with the flexible Norland-65 cement, chosensuch that it can withstand all temperature conditions at SAAO without mechanical fracture, we will temperaturecontrol the Wollaston Prism Assembly at 23 ◦ C , the temperature at which the assembly has been cemented. Thisprovides an additional measure for safety against thermal stresses in the Wollaston Prism Assembly to preventdamage as well as reduce stress birefringence which can affect it’s polarimetric performance.Each dewar houses a 4 k × k E2V CCD which is maintained at − C through thermo-electric coolingsystem. The CCDs are read-out using controllers developed in-house in the IUCAA lab. Each dewar has linearmotion drive to allow motion along the optical axis for focusing individual cameras (used as compensator intolerance analysis).
4. CURRENT STATUS
We are finalizing the optomechanical design of the instrument after which we will proceed towards assembly andtesting of the instrument in the lab. The instrument is scheduled for commissioning in the year 2021.
ACKNOWLEDGMENTS
The PASIPHAE program is supported by grants from the European Research Council (ERC) under grantagreement No 771282 and No 772253, from the National Science Foundation, under grant number AST-1611547and the National Research Foundation of South Africa under the National Equipment Programme. This projectis also funded by an infrastructure development grant from the Stavros Niarchos Foundation and from the InfosysFoundation.erial No. Subsystem ControlRequired1 ControlRequired2 ControlRequired3 No. ofMotions Location1 CalibrationPolarizer In-out Rotation - 2 Guider + Cal.Polarizer Box2 Auto-GuiderCameraPatrolling X-directionmotion Y-directionmotion - 2 Guider + Cal.Polarizer Box3 Auto-GuiderCamera Filter in-out ExposureControl - 1 Guider + Cal.Polarizer Box4 Half WavePlate In-out Rotation - 2 HWP + FilterWheel Box5 Filter Wheel Rotation - - 1 HWP + FilterWheel Box6 Shutter Open/Close - - - CollimatorBarrel7 WollastonPrismAssembly TemperatureControl - - - Polarizer Box8 Dewar 1 Linear FocusMechanism TemperatureControl CCDReadout 1 Camera 1Dewar9 Dewar 2 Linear FocusMechanism TemperatureControl CCDReadout 1 Camera 2Dewar10 Dewar 3 Linear FocusMechanism TemperatureControl CCDReadout 1 Camera 3Dewar11 Dewar 4 Linear FocusMechanism TemperatureControl CCDReadout 1 Camera 4DewarTable 6: Details of the various control systems used in WALOP-South instrument.
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