Visible Spectropolarimeter Vi SP Conceptual Design David Elmore

Visible Spectro-polarimeter (Vi. SP) Conceptual Design David Elmore HAO/NCAR Elmore@ucar. edu

Vi. SP 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, 2003 Vi. SP 2

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

Vi. SP Requirements from ISRD Specification Requirement Goal Priority Wavelength range 380 nm – 900 nm 296 nm – 1090 nm 1 Wavelength diversity 3 simultaneous lines Spatial Resolution 0. 05 arc sec Diffraction limit 1 Spatial field of view 3 arc min square 5 arc min dia. 1 Spectral resolution <3. 5 pm @600 nm 2. 0 pm @600 nm 1 Spectral sample <2. 5 pm @600 nm 1. 5 pm @ 600 nm 1 Polarimetric precision 10 -4 Icontinuum 10 -5 Icontinuum 1 Operation with NIRSP Within 5 sec. Simultaneous 2 August 25 -28, 2003 Vi. SP 2 4

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

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

Spectrograph Specifications Feed focal ratio f/40 Focal Length 2250 mm Slit Width 24 mm Pixel Size 24 mm Slit Height 140 mm Grating Height 196 mm Grating Length 200 mm Grating Blaze Angle 57º Based on VSP Coupling to Telescope. doc, Elmore (April 2003) August 25 -28, 2003 Vi. SP 7

Spectrograph Design August 25 -28, 2003 Vi. SP 8

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. 5 m x 2. 0 m x 0. 5 m • Mass 230 kg August 25 -28, 2003 Vi. SP 9

Spectrograph Performance August 25 -28, 2003 Vi. SP 10

Spectrograph Spatial Sample August 25 -28, 2003 Vi. SP 11

Spectrograph Overview • Design optimized for spatial sample equal to telescope spot size @600 nm • 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 mm possible through the same slit as visible wavelengths at the expense of flux at 1. 6 mm August 25 -28, 2003 Vi. SP 12

Spectrograph Modes • Normal spectro-polarimetry: @600 nm 0. 062 arc sec. spatial resolution, 3. 1 pm spectral resolution – 24 mm pixel – 24 mm slit – 57º blaze grating • High spectral resolution: @600 nm 0. 062 arc sec. spatial resolution, 1. 3 pm spectral resolution – 12 mm pixel (2 x binned spatially) – 12 mm slit – 63. 5º blaze grating • High spatial resolution: @600 nm 0. 031 arc sec. spatial resolution, 3. 1 pm spectral resolution – 12 mm pixel (2 x binned spectrally) – 12 mm slit August 25 -28, 2003 Vi. SP 13

Spectrograph Performance Specification Priority Requirement Goal Wavelength Range 1 380 nm – 900 nm 296 nm – 1100 nm Wavelength Diversity 1 Spatial Resolution 1 0. 05 arc-sec Diffraction Limit+ Spatial Field of View 1 3 arc min square 5 arc min diameter Spectral Resolution 1 <3. 5 pm @600 nm* <2. 0 pm @600 nm # Spectral sample 1 <2. 5 pm @600 nm * <1. 5 pm @600 nm # Operation with NIRSP 2 Within 5 seconds Simultaneous + 3 lines simultaneously >600 nm *24 mm slit & pixels, 57º blaze grating, 3 arc min field #12 mm slit & pixels, 63. 5º blaze grating, 1. 5 arc min field August 25 -28, 2003 Vi. SP 14

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 (k. Hz) • Seeing induced errors reduced using dual beam analyzer August 25 -28, 2003 Vi. SP 15

Polarimeter Concept (HPP) • High Precision Polarimeter – Modulator: High speed Piezo-elastic (PEM) or ferroelectric liquid crystal (Fe. LC) modulator at Gregory – Analyzer: Linear polarizer at Gregory – Advantages • k. Hz 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, 2003 Vi. SP 16

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 Fe. LC + linear polarizer for 4 -state modulation – Advantages • k. Hz 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, 2003 Vi. SP 17

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 Fe. LC 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, 2003 Vi. SP 18

Polarimeter Performance Specification Priority HPP FAP SAP Wavelength range 380 nm – 900 nm 1 Yes Yes Wavelength range 295 nm – 1090 nm 1 Yes Yes Wavelength Diversity 3 widely separated lines 2 No Yes Polarimetric precision* 10 -4 Icontinuum 1 Yes No Polarimetric precision* 10 -5 Icontinuum 1 Yes No No Simultaneous operation with NIRSP 2 No Yes *Additional study required August 25 -28, 2003 Vi. SP 19