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
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