Transcript No Slide Title

Multiobject Spectroscopy
Jeremy Allington-Smith
University of Durham
Contents
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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
Object
aperture
Sky
A
B
C
Non-contiguous
sky spectrum
Detector
(S1)
(S1)
Sky
apertures
A
B
(S2)
C
D
D
Spectrum of object and
contiguous sky background
Spectrum of
object only
(S2)
Non-contiguous
sky spectrum
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
Telescope
focus
Multislit
Slit
mask
Collimator Disperser Camera
From telescope
(or fore-optics)
Multifibre
Spectrograph optics
Long distance
Fibres
From
telescope
Telescope
focus
Fibre
positioner
Pseudoslit
(Dispersion shown rotated
by 90 for simplicity)
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
Sky subtraction
Slits give adjacent sky estimates, contiguous with object
Fibres do not, must build global sky model or beamswitch
A = Object field
B = Background field
B
A
slit
A
Object
Slit
Fibres
B
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
Slit
Input
Fibre
Output
dispersion
Centroid independent of
position of object within
aperture of fibre
 guiding/alignment
errors have no effect on
radial velocity measured
Efficiency for surveys
Log[Surface density of targets]
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
Spectra/fibres overlap
Max density
for slits
100 objects in 5’x5’
100 objects in 10’x10’
Common objects
Galaxies
(e.g normal
galaxies)
U-band
dropouts
Max density
for fibres
Rare objects
(active galaxies) Min density
QSOs
for slits
Sensitivity Fibres
limit: Slits
Magnitude
Min density
for fibres
Too few objects in field
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: 140mm (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
400mm
Positioner
Positioner performance
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Speed: 6-7 seconds/fibre ~1 hour/field  double buffering
Relibility: one failure in every four fields configured
Local positioning accuracy ~15 mm (~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
Each spectrograph handles 400 fibres (no overlaps)
400 spectra
Large scale structure
of universe in a slice
Flames (ESO VLT)
OzPoz (AAO)
double-buffered
fibre positioner at
VLT Nasmyth
• 0.1" accuracy
• 10" minimum dist.
Gravity-stable
Giraffe spectrograph
Fibre input (single fibres)
Pseudoslit
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
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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 = 50mm/arcsec) esp. ELTs
Possible for GSMT
From
progress for F/2
prime focus of 8m
Subaru as part of
UK-Aus-Japan
FMOS instrument
• 400 fibres/spines
• 7mm pitch (90")
telescope
Echidna (AAO) in
Multislit spectrographs
GMOS multislit example
Holes for target
acqusition
- line
A383 observed
fiducial
stars up
with GMOS
with hole centres
dispersion
5.5 arcmin
Mask:
22
1.0”
x 9” lc=600nm
5xslits:
1800s
: B600,
Acquisition
image
300s r band
Note extra space required on detector to accommodate spectra
Spectrum overlaps in MOS
Slit mask
Slit A
Slit B
Slit C
Slit D
Zero
order
1st order
Detector
2nd order
Slit A
Slit B
A and B 1st orders overlap
dispersion
Spectrum overlaps in MOS
Assuming that
only a clean
1st-order
spectrum is
required
B zero order contaminates A 1st order
C 2nd order contaminates D 1st order
Slit C
Slit D
• Mask design software must correctly
predict location of all orders
D 1st order truncated
Effect of anamorphism
Zero
order
1st order
2nd order
Detector
Slit A
Slit B
Images of slit in
direct image
Slit C
Slit D
• Extraction software must take anamorphism into account
• No effect on transformation between mask and direct image
Effect of distortion
Detector
Slit A
Slit B
Slit C
Slit D
• Lines of constant wavelength curved  "2D scrunch"
• Lines of constant position along slit curved  "trace"
Errors in centroid of VRE
VRE = velocity resolution element,
the monochromatic image of the slit as recorded
by the detector
Target-slit error:
Centroid varies depending
on position of object with
respect to slit due to
guiding error or movement
between telescope and slit
dispersion
Slit-detector error:
Centroid varies due to
movement between slit and
detector
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:
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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
Better sky subtraction? Nod & shuffle, microslits
Sky subtraction with slit
B
A
Corrected
photon
number
Noise due
to slit
roughness
Signal to
extract
distance
along slit
Do this at every wavelength!
estimated background
signal uncertain
slopes due non-parallel sides
dispersion
Sky subtraction
near bright sky
lines
Poor cancellation of sky
line due to:
object
Ak
background
Bj
l
– Difference in line
profile due to:
• uneven slit width
• IQ varies over field
– Difference in line
location due to:
l
Ak - B j
object background
• tilt of slit
• poor wavelength
calibration/ solution/
l
l
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
CCD
Requirements
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ability to move telescope with
good repeatability
ability to move charge on CCD
(controller upgrade)
Glazebrook & Bland-Hawthorn PASP 113, 197 (2001)
Courtesy: Karl Glazebrook
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 px
Slit
length
After subtracting bottom half from top half
Anti-object
Object
Example object: raw object+sky
I=23.8
Courtesy: Karl Glazebrook
OH line forest
Example object: N&S subtracted
I=23.8 z=1.07
Courtesy: Karl Glazebrook
[OII]3727
at 770nm
Microslits
with N&S
Galaxy cluster AC114
• AAT/LDSS++
• 586 microslits
non-overlapping
• 40nm blocking
filter @ Ha
• I < 22
Mask design
software predicts
layout of spectra
must have microslit
landing on clean sky
after telescope nod
Couch et al. ApJ 549, 820 (2001)
AC114
Mask
Future challenges:
alternatives to slit masks?
MOS in space
Key goal of NGST: explore the epoch of initial galaxy formation
The faintest galaxies are small and far apart.
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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 of
NIRSPEC will access most
interesting galaxies in a large
field simultaneously.
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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 image
from STIS
(to AB=30)
MOS in cooled IR spectrographs
• Need to operate in temperatures depending on red
cutoff and spectral resolution: 240K80K 30K
• Slit masks must pre-cooled before installation in
instrument cryostat equipped with gate valves
• Fibres can work in cold
H-band
with attention to
Tspec=0C
thermal mismatch but
R=300
-40
difficult with lenslets
•8m telescope
•0.3 arcsec/pixel
•system efficiency =50%
•emissivity =50%
•H-band sky (OH & continuum):
Maihara et al. PASP 105, 940 (1993)
mean
 Requirements:
• Focal plane must be
remotely configurable
R=3000
continuum
dark current
-80
Microshutter arrays and sliding slits
slide
y
x
• 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
• 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
Microshutter array
Baseline for NGST NIRSPEC: 2kx1k (100x200mm) - Moseley et al. NASA/GSFC
Sliding multislits
NB: also VLT/FORS-1
has a 19-slit unit
Backup for NGST/NIRSPEC
(Courtesy: CSEM/Astrium)