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Multiobject Spectroscopy

Multiobject Spectroscopy. Jeremy Allington-Smith University 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

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Multiobject Spectroscopy

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  1. Multiobject Spectroscopy Jeremy Allington-Smith University of Durham

  2. 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

  3. Introduction to MOS

  4. Basic principles Non-contiguous sky spectrum Object aperture Detector Sky (S1) (S1) Sky apertures A A B B (S2) C C D D Spectrum of object only Spectrum of object and contiguous sky background (S2) Non-contiguous sky spectrum

  5. 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

  6. Basic optical concepts Collimator Disperser Camera Slit mask Multislit Telescope focus From telescope (or fore-optics) Multifibre Spectrograph optics Long distance Fibres From telescope Pseudo- slit Fibre positioner Telescope focus (Dispersion shown rotated by 90 for simplicity)

  7. Multislit vs multfibres Multislit • Light goes directly from aperture into spectrograph  distribution of spectra on detector is the sameas 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

  8. 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

  9. Sky subtraction Slitsgive adjacent sky estimates, contiguous with object Fibresdo not, must build global sky model or beamswitch A = Object field B = Background field B B A slit A Object Fibres Slit

  10. Target position errors dispersion 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 Output Centroid independent of position of object within aperture of fibre  guiding/alignment errors have no effect on radial velocity measured Fibre

  11. Efficiency for surveys 100 objects in 5’x5’ 100 objects in 10’x10’ U-band dropouts Galaxies QSOs Multislitsuffers from spectrum overlaps but target spacing can be small perpendicular to dispersion direction Multifibredoes not suffer spectrum overlap, but limited by minimum closest approach of fibres Spectra/fibres overlap Max density for slits Max density for fibres Common objects (e.g normal galaxies) Rare objects (active galaxies) Log[Surface density of targets] Min density for slits Min density for fibres Sensitivity limit: Fibres Slits Magnitude Too few objects in field

  12. 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

  13. 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

  14. 2dF 4mm = 70 arcsec 400mm Positioner

  15. Positioner performance • 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

  16. 2dF data: Galaxy redshift survey 400 spectra Large scale structure of universe in a slice Each spectrograph handles 400 fibres (no overlaps)

  17. 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

  18. Flames fibre bundles Instead of 1 fibre use 20 to give image slicing or integral field capability next lecture Button deployed by positioner

  19. 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

  20. Alternative: spines From telescope • 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 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

  21. Multislit spectrographs

  22. GMOS multislit example Mask: 22 slits: 1.0” x 9” Acquisition image 300s r band 5x 1800s : B600, lc=600nm Holes for target acqusition - line fiducial stars up with hole centres dispersion 5.5 arcmin A383 observed with GMOS Note extra space required on detector to accommodate spectra

  23. Spectrum overlaps in MOS Slit mask Slit A Slit B Slit C Slit D

  24. Spectrum overlaps in MOS A and B 1st orders overlap B zero order contaminates A 1st order dispersion C 2nd order contaminates D 1st order Mask design software must correctly predict location of all orders D 1st order truncated Detector 1st order Zero order 2nd order Slit A Assuming that only a clean 1st-order spectrum is required Slit B Slit C Slit D

  25. Effect of anamorphism Extraction software must take anamorphism into account No effect on transformation between mask and direct image Detector 1st order Zero order 2nd order Slit A Slit B Images of slit in direct image Slit C Slit D

  26. Effect of distortion Lines of constant wavelength curved  "2D scrunch" Lines of constant position along slit curved  "trace" Detector Slit A Slit B Slit C Slit D

  27. Errors in centroid of VRE dispersion 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 Slit-detector error: Centroid varies due to movement between slit and detector

  28. 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

  29. Better sky subtraction? -Nod & shuffle, microslits

  30. Sky subtraction with slit B A Noise due to slit roughness Corrected photon number Signal to extract distance along slit estimated background signal uncertain slopes due non-parallel sides dispersion Do this at every wavelength!

  31. Sky subtraction near bright sky lines object Ak  background Bj  Ak - Bj object - background   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/

  32. 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

  33. Nod & shuffle in action CCD Requirements • 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

  34. 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

  35. Example object: raw object+sky I=23.8 OH line forest Courtesy: Karl Glazebrook

  36. Example object: N&S subtracted [OII]3727at 770nm I=23.8 z=1.07 Courtesy: Karl Glazebrook

  37. Microslits with N&S Galaxy cluster AC114 • AAT/LDSS++ • 586 microslitsnon-overlapping • 40nm blockingfilter @ 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)

  38. AC114 Mask

  39. Future challenges:alternatives to slit masks?

  40. MOS in space 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 of NIRSPEC 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 image from STIS (to AB=30)

  41. MOS in cooled IR spectrographs 8m telescope 0.3 arcsec/pixel system efficiency =50% emissivity =50% H-band sky (OH & continuum): Maihara et al.PASP105, 940 (1993) H-band Tspec=0C R=300 -40 mean R=3000 -80 continuum dark current • 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 with attention to thermal mismatch but difficult with lenslets  Requirements: • Focal plane must be remotely configurable

  42. Microshutter arrays and sliding slits y x slide Individual tiny elements can be swiched on or off Quantisation in both x and y Array gives finequantisation (~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

  43. Microshutter array Baseline for NGST NIRSPEC: 2kx1k (100x200mm) - Moseley et al. NASA/GSFC

  44. Sliding multislits NB: also VLT/FORS-1 has a 19-slit unit Backup for NGST/NIRSPEC (Courtesy: CSEM/Astrium)

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