introduction to nirspec

INS NIRSpec, 12 May 2005

Introduction to NIRSpec

Michael Regan

JWST Project OrganigrammeJune 2004

JWSTProject Manager

P. Jensen

NIRSpec Pr. System Engineer

G. Bagnasco

Launcher System EngineerP. Rumler

System Group

MIRI Principal System Engineer

A. Marini

Admin. AssistantA. Plitzke

Documentation & Conf. Control

D. Green

Mech. Thermal System Engineer

J-C. Salvignol

Project Control Manager

Z. El Hamel

PA ManagerJ. van Dooren

Subsystem Expert Group

Optical System Engineer

M. Te Plate

Electr. System Engineer & AIV

P. Rumler

Ops, SW & Detector System

P. Strada

Mechanism Engineer*B. Henson * leaving

Schedule ControlJ. Molleman

Project ScientistP. Jakobsen

Deputy Project ScientistT. Boeker

Science Operations Team

NN

Science SupportNN

Contracts OfficerV. D’Hoedt

Optical SupportP. Marenaci (YGT)


JWST Architecture

Optical Telescope

Element (OTE)

Integrated Science Instrument

Module (ISIM) Element

Spacecraft Bus

Sunshield

Sun


Telescope


Telescope

FocalSurface

Prim

ary

M

irro

r

Secondary Mirror

Fine Steering

Mirror

Te

rtia

ry

Mirr

or

OTE ISIM

Secondary focus

(V1, V3) origin

V3

V1 V2

OTEf/#: 16.7Effective Focal Length: 116.6 mPM diameter = 7.0 m

FocalSurface

Prim

ary

M

irro

r

Secondary Mirror

Fine Steering

Mirror

Te

rtia

ry

Mirr

or

OTE ISIM

Secondary focus

(V1, V3) origin

V3

V1 V2

OTEf/#: 16.7Effective Focal Length: 116.6 mPM diameter = 7.0 m


ISIM & Regions


Thus Spoke ASWG


Current Mission Requirements

(the TAC will decide anyway…)


NIRSpec


NIRSpec: A Pretty Picture is Not Enough

Enter NIRSpec


NIRSpec: Thus Spoke ASWG

• 3 x 3 arcmin FOV

• 1-5 µm coverage

• R~1000, R~100 multiplexed

• >100 sources simultaneously

• Configurable slit width/length

• MEMS array preferred


NIRSpec Procurement

• Instrument built by European industry

under ESA project leadership

• Under study since 2001

• Presently entering implementation phase

• Two NASA-provided components:• 2 x 2k x 2k HgCdTe Detector Array• 4 x 384 x 185 Micro-Shutter Array


NIRSpec Modes

• R=100 (exploratory spectroscopy)• Single prism 0.6 - 5.0 µm

• Micro-shutter array or fixed Slits

• R=1000 (emission line diagnostics)• 3 gratings 1.0 - 5.0 µm

• Micro-shutter array or fixed slit(s)

• R=3000 (emission line kinematics)• 3 gratings 1.0 - 5.0 µm

• Fixed slit or integral field unit


Wavelength Coverage

• R=1000 & R=3000 modes• 1.0 - 5.0 µm• Covered by three overlapping first order gratings:

• 1.0 - 1.8 µm• 1.7 - 3.0 µm• 2.9 - 5.0 µm

• R=100 mode• 0.6 - 5.0 µm (as NIRCAM)• Covered by single dual-pass prism• Coverage below 1.0 µm is not allowed to drive anything

• Resolution to be kept within factor 2 of R=100 1.0 - 5.0 µm• Resolution below 1.0 µm to follow


You Can’t Fight Red shift

!

NIR MIR

!


Micro Shutter Array

Active MSA Area

3.6’

3.4’

Mounting Frame

Detector Array

Fixed Slits and

IFU Aperture

Direction of Dispersion

4 x 384 x 185 Shutters 9 Square Arcmin of MSA

Area

Single 200 mas x 450 mas slitssurrounded by 60 mas wide bars

>100 objects simultaneously

IFU


A Bit of MEMS History

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.





• Initially both Micro Mirrors and Shutters

• Mirrors eliminated due to excessive diffraction effects

• Initially slit to be made up of several shutters

• Later transitioned to “fat MEMS”• Huge simplification of optics• Factor ~4 reduction in MSA array• Reduced slit loss • At expense of multiplexing loss


Diffraction & Slit Loss

Telescope Exit Pupil

Spectrograph Entrance Pupil

Footprint of PSF

Diffracted OutputBeam


MSA Close-up

Key Requirements:Contrast: >2000 (!) Open Fill factor >70%(60 mas bars on 200 mas wide slits)


MSA Magnet Mechanism


MSA in Action


Integral Field Unit

Field beforeslicing

Pseudo-slit

Slicing mirror (S1)

Spectrogram

Pupil mirrors(S2)

To spectrograph

Field optics (slit mirrors S3)

From telescopeand fore-optics


Integral Field Unit

• FOV: 3” x 3”• Sampling: 0.1”• ~30 Slicers

• Entirely passive device (no moving parts)

• Shuttered by MSA magnet mechanism

• Main use R~3000 single object• But R~1000 and R~100 too....

• Superb backup in case of MSA failure• Point and shoot operations


Integral Field Unit

Slicer Stack(30 slices)

IFU aperturePupil mirror Line(30 elements)

Slit mirror Line(30 elements)

Relay optics




Detector Array• 2K4K FPA comprised of two 2K2K

sensor chip assemblies (SCAs)

• =0.6–5.0 µm HgCdTe detectors (Rockwell)

• FPA passively cooled to T=34–37 K

• Key Performance Parameters:• Total noise =6 electrons rms per t=1000 seconds exposure)• QE = >80%

• NIRSpec is detector background limited in nearly all

modes (!)

• Non-stop (“up the ramp”) read and telemetry• 12 s frame time, 1 frame downlink each 50 s


Why R=1000?


Why R=1000?

• Science requirement• Clean emission line separation

• Main challenge:• [NII] 6548.1 :1• H 6562.8 metallicity diagnostic• [NII] 6583.4 :3

• and also:• [SII] 6716.4 density diagnostic• [SII] 6730.8

R=446

R=319

R=467

[NII]

H

[NII]


Why 200 mas Slit?

1:2 MSA aspect ratio fixed

1.4 µm - Band I2.4 µm - Band II4.0 µm - Band III


Why 200 mas Slit?

200 mas optimal?

2 pixels across slit


Optical Schematic

MIcro-shutterArray Collimator

Prism/Grating CameraDetector

Array

Telescope Focus

Foreoptics FilterWheel

200 mas per 79.5 µm wide shutter

2.52 “/mm, f/12.5

100 mas per 18 µm pixel

5.56 “/mm, f/5.671.58 “/mm, f/20


Optical Layout

Buzzwords: • TMA’s• Scheimflug


Focal Plane Layout & Scheimflug

NIRCam NIRSpec


Physical Layout


SiCThe wonder of modern ceramics


WFE at MSA

• Requirement:• Diffraction-limited at 2.4 µm• WFE = 180 nm rms or better• 131 nm rms OTE input specified

• Fore optics challenging

• Relaxation requires degrading sensitivity• Increased slit losses• Reduced photometric accuracy


WFE at FPA

• Requirement:• Diffraction-limited at 3.0 µm• WFE = 225 nm rms or better

• Camera optics challenging• Relaxation requires degrading

sensitivity• Larger resolution element• Decreased sensitivity• Jeopardize spectral resolution [NII]

H

[NII]


NIRSpec Image Quality

PSF at MSA

PSF at FPA

FPA 100 mas pixels

PSF at MSA

FPA with Cross-talk


Grating Wheel8 Positions:

1. R=100 Prism 2. Band I R=1000 Grating 3. Band II R=1000 Grating 4. Band III R=1000 Grating 5. Band I R=3000 Grating 6. Band II R=3000 Grating 7. Band III R=3000 Grating 8. Provision for imaging mode


Filter Wheel

8 Positions:

1. Clear Aperture 2. Closed 3. Band I Long Pass >1.0 µm4. Band II Long Pass >1.7 µm5. Band III Long Pass >2.9 µm6. Narrow Band TA =1.1 µm7. Broad Band TA 0.9 < 1.1 µm8. Broad Prism 0.9 < 5 µm

Current Issue: Filters in pupil – should they second as pupil stops?


Why Long Pass?One Object per Row


Calibration Unit

QuickTime™ and aTIFF (LZW) decompressor


Carries both continuum and (FP-filtered) line sources


NIRSpec Sensitivity

Formal Level I Specs


Mission Requirements


Target Acquisition

Partially documented in gory detail… • NIRSpec Operations Concept• Two dedicated tech notes (more to come)

Accuracy Goal: 12.5 mas (1)


Image Mode Simulation

2.2 µm

Centroid shift!


Target Acquisition will be interesting


STScI is currently working on several NIRSpec related studies

• MSA planning tool (J. Valenti)• NIRSpec Target Acquisition Alternatives

(M. Regan)• What’s in phase 1 and phase 2 with

JWST (J. Valenti)


STScI plays a crucial role in NIRSpec

• MSA planning tool (J. Valenti)• NIRSpec Target Acquisition Alternatives (M.

Regan)• What’s in phase 1 and phase 2 with JWST (J.

Valenti)• NIRSpec Calibration Plan ( T. Keyes)• Guide Star Availability and planning (J. Valenti)• Do we use Observing templates? (J. Valenti)

introduction to nirspec

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