Visible Spectro-polarimeter (ViSP) Conceptual Design

David Elmore

HAO/NCAR

Elmore@ucar.edu

August 25-28, 2003 ViSP 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, 2003 ViSP 3

ViSP Participants

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

August 25-28, 2003 ViSP 4

ViSP Requirements from ISRD

Specification Requirement Goal Priority

Wavelength range 380nm – 900nm 296nm – 1090nm 1

Wavelength diversity 3 simultaneous lines 2

Spatial Resolution 0.05 arc sec Diffraction limit 1

Spatial field of view 3 arc min square 5 arc min dia. 1

Spectral resolution <3.5pm @600nm 2.0pm @600nm 1

Spectral sample <2.5pm @600nm 1.5pm @ 600nm 1

Polarimetric precision 10-4 Icontinuum 10-5 Icontinuum 1

Operation with NIRSP Within 5 sec. Simultaneous 2

August 25-28, 2003 ViSP 5

Spectrograph Design Drivers

• Requirements• Telescope f/#• Size of Coudé room• Detector realities

August 25-28, 2003 ViSP 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

Gratingheight

Spectralresolution

Gratingorder

August 25-28, 2003 ViSP 7

Spectrograph Specifications

Feed focal ratio f/40

Focal Length 2250mm

Slit Width 24mPixel Size 24mSlit Height 140mm

Grating Height 196mm

Grating Length 200mm

Grating Blaze Angle

57º

Based on VSP Coupling to Telescope.doc, Elmore (April 2003)

August 25-28, 2003 ViSP 8

Spectrograph Design

August 25-28, 2003 ViSP 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, 2003 ViSP 10

Spectrograph PerformanceSpectral Resolution & Sample vs. Wavelength

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

300 400 500 600 700 800 900 1000 1100

Wavelength (nm)

Sam

ple

(p

m)

f/40 telescope, 2.25 m spectrograph, 24 micron slit

Resolution

SampleRequirement

August 25-28, 2003 ViSP 11

Spectrograph Spatial SampleSpatial Sample and Resolution vs. Wavelength

0.000

0.010

0.020

0.030

0.040

0.050

0.060

300 400 500 600 700 800 900 1000 1100

Wavelength (nm)

Sp

atia

l Siz

e (a

rc s

ec)

ATST Resolution

24 Micron Slit Sample

12 Micron Slit Sample

Two samples/telescope resolution (Goal)

Two samples/0.05 arc seconds (Requirement)

August 25-28, 2003 ViSP 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

• 1.6m possible through the same slit as visible wavelengths at the expense of flux at 1.6m

August 25-28, 2003 ViSP 13

Spectrograph Modes

• Normal spectro-polarimetry: @600nm 0.062 arc sec. spatial resolution, 3.1pm spectral resolution– 24m pixel – 24m slit– 57º blaze grating

• High spectral resolution: @600nm 0.062 arc sec. spatial resolution, 1.3pm spectral resolution– 12m pixel (2x binned spatially)– 12m slit– 63.5º blaze grating

• High spatial resolution: @600nm 0.031 arc sec. spatial resolution, 3.1pm spectral resolution– 12m pixel (2x binned spectrally)– 12m slit

August 25-28, 2003 ViSP 14

Spectrograph Performance

Specification Priority Requirement GoalWavelength Range 1 380nm – 900nm 296nm – 1100nm

Wavelength Diversity 1 3 lines simultaneously

>3 lines simultaneously

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 <3.5pm @600nm* <2.0pm @600nm #

Spectral sample 1 <2.5pm @600nm * <1.5pm @600nm #

Operation with NIRSP 2 Within 5 seconds Simultaneous+ >600nm

*24m slit & pixels, 57º blaze grating, 3 arc min field

#12m slit & pixels, 63.5º blaze grating, 1.5 arc min field

August 25-28, 2003 ViSP 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, 2003 ViSP 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, 2003 ViSP 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, 2003 ViSP 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, 2003 ViSP 19

Polarimeter Performance

Specification Priority HPP FAP SAPWavelength range 380nm – 900nm 1 Yes Yes Yes

Wavelength range 295nm – 1090nm

1 Yes Yes Yes

Wavelength Diversity

3 widely separated lines

2 No Yes Yes

Polarimetric precision*

10-4 Icontinuum

1 Yes Yes No

Polarimetric precision*

10-5 Icontinuum

1 Yes No No

Simultaneous operation with NIRSP 2 No Yes Yes

*Additional study required