Multiobject Spectroscopy

Jeremy Allington-SmithUniversity of Durham

Contents

• Introduction to MOS• Multislits and multifibres compared• Multifibre systems• Atmospheric effects • Multislit systems• Stability• Optical performance• Sky subtraction revisited• Nod & shuffle, microslits• Alternatives to slit masks

Introduction to MOS

Basic principles

AB

C

D

AB

C

D

Non-contiguoussky spectrum

Non-contiguoussky spectrum

Spectrum of object andcontiguous sky background

Detector

Sky

Object aperture

Skyapertures

Spectrum of object only

(S1)

(S2)

(S1)

(S2)

Top-level requirements

• Mandatory to obtain integrated spectrum of many objects– One spectrum per object in defined aperture – Estimate of spectrum of sky background

• preferably contiguous in same aperture• or enough non-contiguous samples to build global model of

sky

– Known mapping from sky to detector• obtained simply by (wavelength calibration)• mapping need not be simple!

• Optional to obtain spatially-resolved spectra– Spatial resolution along slit/aperture– Apertures can be tilted or curved

• to maximise throughput for extended source• radial velocity distribution within aperture

Basic optical concepts

From telescope(or fore-optics)

Multislit Slitmask

Telescopefocus

Collimator DisperserCamera

Long distanceSpectrograph optics

Multifibre

Fromtelescope

Pseudo-slit

Telescopefocus

(Dispersion shown rotatedby 90 for simplicity)

Fibres

Fibre positioner

Multislit vs multfibres

Multislit– Light goes directly from aperture into spectrograph distribution of spectra on detector is the same as that

of apertures on the sky• Overlaps between spectra are possible• Difficult to observe objects which have same position

perpendicular to the dispersion direction

Multibre– Light is conducted along flexible link (fibre) distribution of spectra on detector is independent of

that of apertures on the sky• Fibre outputs arranged as 'pseudo-slit' to avoid spectrum

overlaps• but fibre coupling may be lossy and destroys spatial info

Summary of pros and cons

Multislit– Efficient for faint sources

• fewer sources of light loss than fibres• better sky subtraction - sky estimates in same slit

– limited field (10') but fine resolution possible (~0.1")– Calibration straightforward

Multifibre– Very large fields possible ( 2)– Sky subtraction difficult - no adjacent sky estimates

– Good stability• fibres immune to target position errors or guiding errors• spectrograph can be gravity invariant: eliminate flexure

– Calibration difficult

A

B

slit

Slit

A B

Fibres

Sky subtraction

A = Object fieldB = Background field

Object

Slits give adjacent sky estimates, contiguous with object

Fibres do not, must build global sky model or beamswitch

Target position errors

Slits retain image information perpendicular to dispersion direction

Fibres scramble information on location of object within aperture Centroid varies

depending on position of object within aperture of slit guiding/alignment errors affect radial velocity measured

Input Output Centroid independent of position of object within aperture of fibre guiding/alignment errors have no effect on radial velocity measured

Slit

Fibre

dispersion

U-banddropouts

QSOs

Galaxies

100 objects in 5’x5’

100 objects in 10’x10’

Efficiency for surveys

Multislit suffers from spectrum overlaps but target spacing can be small perpendicular to dispersion direction

Multifibre does not suffer spectrum overlap, but limited by minimum closest approach of fibres

Max densityfor fibres

Max densityfor slits

Min densityfor slits

Min densityfor fibres

Log[S

urf

ace

densi

ty o

f ta

rgets

]

Magnitude

Common objects(e.g normal galaxies)

Rare objects(active galaxies)

Too few objects in field

Spectra/fibres overlap

FibresSlits

Sensitivity limit:

Multibre systems

This is a review of the capabilities of current systems . Many of the technical issues which affect these systems also apply to multislit systems and will be

discussed later

Two-degree Field (2dF, AAT)

• Field: 2 diameter via corrector at f/3 prime focus

• 400 object fibres/field plate + 4 guide fibre bundles,

• Fibre aperture: 140m (2 arcsec) diameter• Fibre positioned by pick & place robot • Double-buffered: observe with one plate while

the other is configured• Atmopheric dispersion compensator

2dF

4mm = 70 arcsec

Positioner

400mm

Positioner performance

• Speed: 6-7 seconds/fibre ~1 hour/field double buffering

• Relibility: one failure in every four fields configured• Local positioning accuracy ~15 m (~0.25 arcsec). • Atmopheric refraction limits to Hour Angle +/- 2.5• Active position control : image back-illuminated fibres• Fibre cross-overs must be dealt with carefully by s/w

2dF data: Galaxy redshift survey

400 spectra

Large scale structure of universe in a slice

Each spectrograph handles 400 fibres (no overlaps)

Flames (ESO VLT)OzPoz (AAO)double-buffered fibre positioner at VLT Nasmyth• 0.1" accuracy• 10" minimum dist.

Fibre input (single fibres)

Pseudoslit

Gravity-stableGiraffe spectrograph

Flames fibre bundles

Instead of 1 fibre use 20 to give image slicing or integral field capability next lecture

Button deployed by positioner

Issues for multifibre system

• Can't get fibres close together• Limits on configuration flexibility due to cross-overs• Reconfiguration time - longer for more fibres• Atmospheric refraction update fibre positions but

can't do this during observation

• Calibration of fibre throughput for each plate?• Sky subtraction strategies: global sky/beam-switch• Stability:

– fibres move but spectrograph stable (not 2DF)– guiding error immunity for fibres

Alternative: spines

• Mount fibres on spines, tilt to access small patrol field• Natural match to studies of LSS (less good for clusters)• Good for fast focii (PF of 8/10m) where inter-object

distance is small (f/1.2, 8m = 50m/arcsec) esp. ELTs

Echidna (AAO) in progress for F/2 prime focus of 8m Subaru as part of UK-Aus-Japan FMOS instrument • 400 fibres/spines• 7mm pitch (90")Possible for GSMT

From

te

lesc

ope

Multislit spectrographs

GMOS multislit exampleAcquisition image

300s r band

5.5

arc

min

A383 observedwith GMOS

Mask: 22 slits: 1.0” x 9”

Holes for targetacqusition - line fiducial stars up with hole centres

dis

pers

ion

5x 1800s : B600,c=600nm

Note extra space required on detector to accommodate spectra

Spectrum overlaps in MOS

Slit A

Slit B

Slit C

Slit D

Slit mask

Spectrum overlaps in MOS

1st order2nd orderZero

order

Slit A

Slit B

Slit C

Slit D

Detector

Assuming that only a clean 1st-order spectrum is required

D 1st order truncated

B zero order contaminates A 1st order

A and B 1st orders overlap

C 2nd order contaminates D 1st order

dis

pers

ion

• Mask design software must correctly predict location of all orders

Effect of anamorphism

1st order2nd orderZero

order

Slit A

Slit B

Slit C

Slit D

Detector

Images of slit in direct image

• Extraction software must take anamorphism into account• No effect on transformation between mask and direct image

Effect of distortion

Slit A

Slit B

Slit C

Slit D

Detector

• Lines of constant wavelength curved "2D scrunch"• Lines of constant position along slit curved "trace"

Target-slit error: Centroid varies depending on position of object with respect to slit due to guiding error or movement between telescope and slit

Slit-detector error: Centroid varies due to movement between slit and detector

Errors in centroid of VRE

VRE = velocity resolution element,the monochromatic image of the slit as recorded by the detector

dispersion

Centroid errors

• Errors in slit position cause– loss of throughput– error in measured radial velocity

• Two nasty sources of astrophysical error– plate scale error spurious radial dependence of RV or

intensity and overestimate of velocity dispersion– Mask rotated with respect to targets errors as above

• Some causes of error:– Errors in position of target (celestial or from image)– Error in assumed plate scale (error depends on radius)– Inaccuracy in mask maker (random or systematic)– Error in guiding and aligning mask with sky during acquistion– Atmospheric refraction varying through observation– Instability in spectrograph between slit and detector

Better sky subtraction? -Nod & shuffle, microslits

estimated background signal uncertain

slopes due non-parallel sides

distancealong slit

Correctedphoton

number

B

Sky subtraction with slit

Noise due to slit

roughness

A

Signal to extract

Do this at every wavelength!dispersion

Ak

Bj

Ak - Bj

object

background

object -background

Sky subtraction near bright sky

lines

Poor cancellation of sky line due to:– Difference in line

profile due to:• uneven slit width• IQ varies over field

– Difference in line location due to:

• tilt of slit• poor wavelength

calibration/ solution/

Nod & shuffle (Va & vient)

• Errors in sky subtraction– Sky is spatially structured on scale of slit width– Errors in slit fabrication lead to extra noise– problems with flatfielding since calibration spectrum needs

to match sky's spectrum– fringing in CCDs

• Solution: Use same detector pixels and optical path to alternately sample object and sky (beam-switch)?– Advantages

• improved background subtraction• can use shorter slits (microslits) to increase multiplex

– Potential drawbacks• must alternate fast enough to cancel out temporal variations• detector readnoise is increased due to multiple readouts

Nod & shuffle in action

Requirements• ability to move telescope with

good repeatability• ability to move charge on CCD

(controller upgrade)

Courtesy: Karl Glazebrook

CCD

Glazebrook & Bland-Hawthorn PASP 113, 197 (2001)

Nod & shuffle on GMOS

•Example from engineering tests:– Shift object along normal slit– 2 cycles of 60s in each position: nod +/- 1.5”, shuffle

70 pxSlit length

Object

Anti-object

After subtracting bottom half from top half

Example object: raw object+sky

I=23.8 OH line forest

Courtesy: Karl Glazebrook

Example object: N&S subtracted

I=23.8 z=1.07 [OII]3727at 770nm

Courtesy: Karl Glazebrook

Microslits with N&SGalaxy cluster

AC114• AAT/LDSS++• 586 microslits

non-overlapping• 40nm blocking

filter @ H• I < 22

Couch et al. ApJ 549, 820 (2001)

Mask design software predicts layout of spectramust have microslit landing on clean sky after telescope nod

AC114 Mask

Future challenges:alternatives to slit masks?

Key goal of NGST: explore the epoch of initial galaxy formation

The faintest galaxies are small and far apart.• At AB=29 half light diameter ~ 0.2’’ • At AB=30 galaxy density is 3 x 106 deg-2

17000 in 7.5 x 3.75 arcmin

The multiobject capability ofNIRSPEC will access most interesting galaxies in a large field simultaneously.

• 6000 galaxies at R~40, 30 < KAB < 32 or z>1.6

• 1600 galaxies at R~1500, 28 < KAB < 29 or z>2

• 600 galaxies at R~5000, KAB < 23.1

Requirements:• Focal plane must be remotely configurable

with no consumables and be reliable• Address high surface density of targets

HDS-S imagefrom STIS (to AB=30)

MOS in space

Tspec=0C

-80

-40R=300

R=3000

mean

continuum

dark current

H-band

•8m telescope•0.3 arcsec/pixel•system efficiency =50%•emissivity =50%•H-band sky (OH & continuum): Maihara et al. PASP 105, 940 (1993)

MOS in cooled IR spectrographs

• Need to operate in temperatures depending on red cutoff and spectral resolution: 240K80K 30K

• Slit masks must pre-cooled before installation in instrument cryostat equipped with gate valves

• Fibres can work in cold with attention to thermal mismatch but difficult with lenslets

Requirements:• Focal plane must be

remotely configurable

Microshutter arrays and sliding slits

• Each half of slitlet slides individually to give precise slit width and location in y

• Inflexibility in matching object locations in x

• Only 20-40 slits possible• Multiple banks impossible• Contrast ratio high

• Individual tiny elements can be swiched on or off

• Quantisation in both x and y • Array gives fine quantisation

(~1k x 1k via mosaicing)• Multiple banks OK• Filling factor limited (support

grid)• Contrast ratio limited

slide

y

x

Microshutter arrayBaseline for NGST NIRSPEC: 2kx1k (100x200m) - Moseley et al. NASA/GSFC

Sliding multislits

Backup for NGST/NIRSPEC(Courtesy: CSEM/Astrium)

NB: also VLT/FORS-1 has a 19-slit unit