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Visible Spectro-polarimeter (ViSP) Conceptual Design

Visible Spectro-polarimeter (ViSP) Conceptual Design. David Elmore HAO/NCAR Elmore@ucar.edu. ViSP Mission*. Precision measurements of full state of polarization Simultaneously at diverse wavelengths Visible spectrum range Fully resolved line profiles Provides quantitative diagnostics of

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Visible Spectro-polarimeter (ViSP) Conceptual Design

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

  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 ViSP 2

  3. ViSP Participants • Hector Socas-Navarro (PI) • Kim Streander (Program Manager) • David Elmore (Lead Engineer) • Paul Seagraves (Telescope modeling) ViSP 3

  4. ViSP Requirements from ISRD ViSP 4

  5. Spectrograph Design Drivers • Requirements • Telescope f/# • Size of Coudé room • Detector realities ViSP 5

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

  7. Spectrograph Specifications Based on VSP Coupling to Telescope.doc, Elmore (April 2003) ViSP 7

  8. Spectrograph Design ViSP 8

  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 ViSP 9

  10. Spectrograph Performance ViSP 10

  11. Spectrograph Spatial Sample ViSP 11

  12. Spectrograph Overview • Design optimized for spatial sample equal to telescope spot size @600nm • 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 • 1.6mm possible through the same slit as visible wavelengths at the expense of flux at 1.6mm ViSP 12

  13. Spectrograph Modes • Normal spectro-polarimetry: @600nm 0.062 arc sec. spatial resolution, 3.1pm spectral resolution • 24mm pixel • 24mm slit • 57º blaze grating • High spectral resolution: @600nm 0.062 arc sec. spatial resolution, 1.3pm spectral resolution • 12mm pixel (2x binned spatially) • 12mm slit • 63.5º blaze grating • High spatial resolution: @600nm 0.031 arc sec. spatial resolution, 3.1pm spectral resolution • 12mm pixel (2x binned spectrally) • 12mm slit ViSP 13

  14. Spectrograph Performance + >600nm *24mm slit & pixels, 57º blaze grating, 3 arc min field #12mm slit & pixels, 63.5º blaze grating, 1.5 arc min field ViSP 14

  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 ViSP 15

  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 ViSP 16

  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 ViSP 17

  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 ViSP 18

  19. Polarimeter Performance *Additional study required ViSP 19

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