“phase c” design of the jwst/fgs tunable filter imager

“Phase C” Design of the JWST/FGS Tunable Filter Imager

TIPS/JIM June 15, 2006

Alex Fullerton

STScI / UVic


Optical Design at PDR

Collimator

Tuneable filter Dual filter wheel

Aerial image(coronograph plane)

PickoffMirror

Shortwave cameraTMA

Longwave cameraTMA

Wavelength Coverage

SW: 1.2 to 2.1 micronsLW: 2.0 to 4.8 microns


SAT Recommendation

Collimator



PickoffMirror

Shortwave cameraTMA

Longwave cameraTMA

Wavelength Coverage



SAT Recommendation

Collimator



PickoffMirror

Shortwave cameraTMA

Longwave cameraTMA

Wavelength Coverage


No cost savings to NASA.


SAT Recommendation

Collimator



PickoffMirror

Shortwave cameraTMA

Longwave cameraTMA

Wavelength Coverage



Removes:•1 dichroic beamsplitter•1 pupil/filter wheel assembly•1 Fabry-Perot etalon•1 camera TMA•1 SCA•Half an optical bench

80 kg


SAT Recommendation

Collimator



PickoffMirror

Shortwave cameraTMA

Longwave cameraTMA

Wavelength Coverage



Removes:•1 dichroic beamsplitter•1 pupil/filter wheel assembly•1 Fabry-Perot etalon•1 camera TMA•1 SCA•Half an optical bench

80 kg

Hard to achieve science goals with this wavelength coverage!


FGS-TFI Optical Layout at RDR / Phase C

100.00 MM

Pick-off Mirror

Collimator TMA

Camera TMA

Fabry Perot

Filters

Fold

Detector


Fabry-Perot Etalons

Optical path difference = 2µl cos

Phase difference = (2/) 2µl cos + r

µ=index of refraction

m = 2µl cos + r /2

Tuning: For fixed m, on-axis (=0) l

Detuning: For fixed m, l, off-axis () cos 1 2/2


The SAT Challenge: Do More With Less

Improvements with respect to the PDR LW channel:• Significantly extended spectral waveband (1.5 to 5 μm)

• Requirements: 1.6 to 4.9 μm [PDR LW: 2.0 to 4.8 μm]

• Simplified – just 17 layers ! [PDR LW: 21 layers]

• Minimal detuning of the etalon with angle-of-incidence

An order of magnitude better than earlier versions of this coating!

Non-functional interval between 2.6 and 3 μm• Requirements: 2.5 to 3.2 μm• System operates at 2.4 μm, where NIRCam is non-functional


Reflectance and Phase

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1500 2000 2500 3000 3500 4000 4500 50001.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Re

flect

an

ce

Ph

ase

(ra

dia

n)

Wavelength (nm)

ReflectancePhase

Non-Functional Waveband


Etalon Gap and Spectral Resolution

0

20

40

60

80

100

120

140

160

1500 2000 2500 3000 3500 4000 4500 50002000

3000

4000

5000

6000

7000

8000

9000

10000

Re

solu

tion

Ga

p (

nm

)

Wavelength (nm)

ResolutionGap

Non-Functional Waveband


Etalon Detuning - I

max for TFI


Etalon Detuning - II

max for TFImax for TFI


Dual Wheel Configuration at PDR

FFCal

WCal

ND

Apo1Apo3

Apo4

Lyot

Pupil Wheel

Apo2

B1

B2B3

B4

B5

B6Open

Filter Wheel

Spare

8 position Filter Wheel allows for 6 blocking filters, an open for calibrations and one spare

8 position Pupil Wheel allows for a Lyot mask, up to 4 apodization masks,

2 calibration source positions and a neutral density position for target

acquisition.

Note: Calibration source positions on Pupil Wheel also serve as closed positions for dark calibrations


Current Dual Wheel Mechanism

Nine Position Wheels

Motors

Resolvers Bracket


“Phase C” Dual Wheel Design

At least 9 positions are required to cover the full wavelength range (8 blocking filters + 1 “open” position)• Otherwise: there is a risk that the wavelength range will be restricted

by the blocking filters (not the etalon!) A larger wheel was not considered (mass). Spare positions are available in the Pupil / Calibration Wheel

Wavelength Limit (nm) Filter #

on off

1 1473 1932

2 1505 2072

3 1718 2370

4 20 70 2609

5 3027 4020

6 3430 4531

7 4080 4978

8 4623 5181

B1

B2

B3

B4B5

B6

B7

B8

Spare

Filter Wheel

OpenFFCal

Apo1

NDWCal

Lyot

Apo2Apo3

Apo4

Spare

Pupil Wheel


Key Performance RequirementsParent* TFI Requirement Description Requirement Value Design Current Value

FGS-308 TFI Channel FOV > 4.7 sq. arcmin 4.88 sq. arcmin

FGS-638 TFI Pixel Size 65 mas 64.3 to 65.5 mas

FGS-306 TFI Wavelength Range 1.6 – 2.5 m 3.2 – 4.9 m

1.5 – 2.7 m 3.1 – 5.0 m

FGS-311 TFI Spectral Resolution > 70 & < 150 75 to 105

FGS-315 Wavelength Shift in FOV < 1.0% (50% of FOV) < 0.3% (complete FOV)

FGS-317 TFI Sensitivity at 3.5 m 126 nJy 105 nJy

FGS-355 Coronagraphic Capability 2 Pupil masks 2 Lyot masks

4 Pupil masks 4 Lyot masks

NIRCam bandpass TFI

bandpass

* CSA-JWST-RD-0002 Rev B (JWST-RQMT-002069)

- Wavelength Selection Flexibility - Good Sensitivity for Emission Line Objects

- Emission Line Diagnostic Imaging - Coronagraphy for AGN studies & Extrasolar Planet Spectra


Etalon Stability / Repeatability

Stability depends on temperature control of etalon electronics• Etalon gap changes by 0.12 nm/K with prototype ECE• Issue: will flight electronics exhibit a temperature drift?• Assume: 1.2 nm/K (10 worse)• Implement: temperature control in ECE; stable to ~1 K• Expect: Over the course of a typical observation, etalon gap may vary

by ~1.2 nm, or ~7% of a resolution element at 1.5 m. This is consistent with scientific requirements for photometric stability.

• Can use onboard calibration lamp to check gap spacing during particularly long/critical observations.

Repeatability will depend upon the interval in question.• Over ~month, expected to be the same as the stability• Over longer intervals, will depend on aging of components• Mitigate by checking/updating TFI wavelength calibration

“phase c” design of the jwst/fgs tunable filter imager

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