Visible Spectro-polarimeter (ViSP) Conceptual Design David Elmore HAO/NCAR

August 25-28, 2003ViSP 2 ViSP Mission* Precision measurements of full state of polarization –Simultaneously at diverse wavelengths –Visible spectrum range –Fully resolved line profiles Provides quantitative diagnostics of –Magnetic field vector as a function of solar height –Variation in thermodynamic properties *From Instrument Science Requirements 2001, Sept. 17, B. Lites, C. Keller

August 25-28, 2003ViSP 3 ViSP Participants Hector Socas-Navarro (PI) Kim Streander (Program Manager) David Elmore (Lead Engineer) Paul Seagraves (Telescope modeling)

August 25-28, 2003ViSP 4 ViSP Requirements from ISRD SpecificationRequirementGoalPriority Wavelength range380nm – 900nm296nm – 1090nm1 Wavelength diversity3 simultaneous lines2 Spatial Resolution0.05 arc secDiffraction limit1 Spatial field of view3 arc min square5 arc min dia nm1 Polarimetric precision10 -4 I continuum I continuum 1 Operation with NIRSPWithin 5 sec.Simultaneous2

August 25-28, 2003ViSP 5 Spectrograph Design Drivers Requirements Telescope f/# Size of Coudé room Detector realities

August 25-28, 2003ViSP 6 Spectrograph Design Flow Telescope f/# Pixel size Slit width Littrow configuration Coudé room dimensions Wavelength Spatial resolution SP focal length Dispersion Detector realities Grating blaze angle Grating length Grating height Spectral resolution Grating order

August 25-28, 2003ViSP 7 Spectrograph Specifications Feed focal ratiof/40 Focal Length2250mm Slit Width 24 m m Pixel Size 24 m m Slit Height140mm Grating Height196mm Grating Length200mm Grating Blaze Angle57º Based on VSP Coupling to Telescope.doc, Elmore (April 2003)

August 25-28, 2003ViSP 8 Spectrograph Design

August 25-28, 2003ViSP 9 Spectrograph Features Accessible: all in one plane Adjustable slit width Selectable gratings mounted on a turntable Spectrum scanned by translating the entire spectrograph, or optionally by preceding spectrograph with beam scanning mirrors Dimensions 2.5m x 2.0m x 0.5m Mass 230 kg

August 25-28, 2003ViSP 10 Spectrograph Performance

August 25-28, 2003ViSP 11 Spectrograph Spatial Sample

August 25-28, 2003ViSP 12 Spectrograph Overview Design optimized for spatial sample equal to telescope spot Dispersion optimized for spectro-polarimetry Finer spatial resolution possible using narrow slit and smaller pixels at the expense of lower flux Higher spectral resolution possible using narrow slit, smaller pixels, and higher blaze angle grating at the expense of lower flux 1.6 m m possible through the same slit as visible wavelengths at the expense of flux at 1.6 m m

August 25-28, 2003ViSP 13 Spectrograph Modes Normal arc sec. spatial resolution, 3.1pm spectral resolution –24  m pixel –24  m slit –57º blaze grating High spectral arc sec. spatial resolution, 1.3pm spectral resolution –12  m pixel (2x binned spatially) –12  m slit –63.5º blaze grating High spatial arc sec. spatial resolution, 3.1pm spectral resolution –12  m pixel (2x binned spectrally) –12  m slit

August 25-28, 2003ViSP 14 Spectrograph Performance SpecificationPriorityRequirementGoal Wavelength Range1380nm – 900nm296nm – 1100nm Wavelength Diversity13 lines simultaneously>3 lines simultaneously Spatial Resolution10.05 arc-secDiffraction Limit + Spatial Field of View13 arc min square5 arc min diameter Spectral * # Spectral * # Operation with NIRSP2Within 5 secondsSimultaneous + >600nm * 24 m m slit & pixels, 57º blaze grating, 3 arc min field # 12 m m slit & pixels, 63.5º blaze grating, 1.5 arc min field

August 25-28, 2003ViSP 15 Polarimeter Guidelines Time multiplexed polarization modulation and analysis used –Versatile –Issues are understood Calibration optics precede highly polarizing reflections (Calibration station at Gregory focus) Polarization modulators precede highly polarizing reflections (Modulator turret at Gregory focus) Seeing induced errors reduced at high modulation frequency (kHz) Seeing induced errors reduced using dual beam analyzer

August 25-28, 2003ViSP 16 Polarimeter Concept (HPP) High Precision Polarimeter –Modulator: High speed Piezo-elastic (PEM) or ferroelectric liquid crystal (FeLC) modulator at Gregory –Analyzer: Linear polarizer at Gregory –Advantages kHz modulation frequency rejects residual seeing No highly polarizing optics between modulator and analyzer –Disadvantages Narrow wavelength range, but tunable 4 State Charge caching photo-detector required

August 25-28, 2003ViSP 17 Polarimeter Concept (FAP) Fast Achromatic Polarimeter –Modulator: Rapidly rotating achromatic retarder at Gregory –Analyzer at detector: Linear polarizer for 8-state modulation or high frequency FeLC + linear polarizer for 4-state modulation –Advantages kHz modulation frequency rejects residual seeing Wavelength diversity possible –Disadvantages Highly polarizing optics between modulator and analyzer Time varying polarimeter response matrix Calibration intensive data collection and reduction 4 State Charge caching photo-detector required

August 25-28, 2003ViSP 18 Polarimeter Concept (SAP) Slow Achromatic Polarimeter –Modulator: Achromatic slowly rotating retarder at Gregory –Analyzer: Polarizing beam splitter at detectors –Advantages Wavelength diversity possible Conventional CCD or IR detectors can be mixed with charge caching photo-detectors (that use rapidly chopped FeLC before analyzer) –Disadvantages Highly polarizing optics between modulator and analyzer Time varying polarimeter response matrix Calibration intensive data collection and reduction 8 State Charge caching detector needed for mixed scheme

August 25-28, 2003ViSP 19 Polarimeter Performance SpecificationPriorityHPPFAPSAP Wavelength range 380nm – 900nm1Yes Wavelength range 295nm – 1090nm1Yes Wavelength Diversity 3 widely separated lines 2NoYes Polarimetric precision* I continuum 1Yes No Polarimetric precision* I continuum 1YesNo Simultaneous operation with NIRSP2NoYes *Additional study required