observational techniques workshop 19 april 2001 fred watson, anglo-australian observatory (with...

Observational Techniques Workshop

19 April 2001

Fred Watson, Anglo-Australian Observatory(with thanks to Karl Glazebrook and Terry Bridges)

Observational Techniques Workshop

19 April 2001

Fred Watson, Anglo-Australian Observatory(with thanks to Karl Glazebrook and Terry Bridges)

Overview

Basics

Slitless spectroscopy

Multislit spectroscopy

Multislit spectroscopy with LDSS++

Multifibre spectroscopy

Multifibre spectroscopy with 2dF & 6dF

Basics

Main parameters:•Telescope aperture, a; field area F•no. of discrete points that can be observed, j (can be undefined)•number densities of objects:

•differential number density, A(m,C), per unit magnitude•integrated number density, N(m,C), to limiting magnitude

…the simultaneous spectroscopic observation

of many objects in the field of a single telescope,in which the spectral content predominates over the

spatial content. Survey technique.


Drawbacks:•Large prisms required•Overlapping spectra (may need orthogonal exposures)•No comparison spectra (use red cut-off)•Resolution strongly seeing-dependent•Sky undispersed

Objective prismTrue spectroscopic survey mode: objects selected only on the

basis of position (within the field) and magnitude: j undefined.

E.g. UK Schmidt:

2480Å/mm and 830Å/mm prisms: j~105 ob jects in 40 deg2


Drawbacks:•Overlapping spectra (may need orthogonal exposures)•No comparison spectra•Resolution strongly seeing-dependent•Sky undispersed

Slitless spectrographAgain survey mode. No need for large prism can be used on

large telescopes. Focal reduction can efficiently use small-

format detector (CCD)

Little current application in multi-object spectroscopy

Multislit spectroscopy

Drawbacks:•Mask has to be prepared in advance•Mask needs to be registered with sky--careful astrometry needed•Need to avoid overlapping restricts j (or -coverage)•Field limited by collimator design

Advantages:•Overlapping can be avoided •Comparison spectra can be obtained•Spectral resolution independent of seeing•Spectra no longer lie on undispersed sky

Follow-up mode (j defined but variable).

LDSSAAT’s Low Dispersion Survey Spectrograph for faint multi-slit spectroscopy (12 arcmin FOV)

Collimator

Grism slide

Filter slide

Camera

Aperturewheel

CCD

The LDSS++ project• Nod and shuffle mode for sky-subtraction

– Up to 300 microslits (1 arcsec apertures) in a 9 arcmin FOV

– Made at MSSSO workshop using CAMM-3

• Volume-Phase Holographic Grating (red optimised)– Efficiency 57% 82%

• Deep depletion MIT/LL CCD (high red QE)– QE 66% 90%

• Performance:– Reach R=24 for S/N=3 in 3h exposure

– At R=24 80000 targets/deg2 or 600 per LDSS slit area

Charge-shuffling for sky-subtraction

• Slits - limited by irregularity of slits and flatfielding• Beamswitch approach - nod telescope object-sky

– Object/Sky observed through exactly the same slits+pixels

– But need 15-30 min exposures to beat down CCD readnoise

– Sky changes in this time A-B 0

• Sequence charge-shuffling with telescope-nodding? Rapid sampling (~secs) (Cuillandre et al. 1994 on NTT) A-B = 0

• Charge-shuffling developed at AAO for TAURUS– Joss Hawthorn, John Barton, Lew Waller, Tony Farrell

• The sky is complex+variable in the red and near-IR

`Va et Vient’

`Nod and shuffle’

CCD controllerInstrumentSequencer

TelescopeDrives

Distant cluster project

• Goal: to measure star-formation across the face of z~0.3 clusters via H line (~8500Å observed)– (Glazebrook, Bower, Couch)

• Use LDSS++ with R6 blocking filter (8500Å/400Å)– Microslits - fit ~900 in to 9 arcmin FOV

• AC114 - 4 hour exposure Aug 1999– 848 targets

– short dispersed spectra of H + [NII]

AC114

9 arcmin

AC114 Mask

Detector

Spectrograph

Slit

The answer to life, the Universe and everything...



Drawbacks:•Fibres have to be configured in advance•Fibres have to be registered with sky--careful astrometry needed•Sky-subtraction less effective than multi-slit•Thus lower ultimate sensitivity

Advantages:•Maximises use of detector area (because of reformatting)•Greater freedom in placing fibres in telescope field

•no overlapping spectra•not limited by collimator field

•Can easily feed more than one spectrograph

Follow-up mode (j fixed--sort of).


•Bulk transmission losses low from 0.5m to 2.0m•Low-OH (“dry”) fibres excellent for > 0.55m•High-OH (“wet”) fibres good for > 0.35m•But OH absorption features at 0.73, 0.95 and 1.37m•New Heraeus STU fibre combines best of both

•Focal ratio degradation (FRD)•Result of modal coupling in the fibre•Can cause overfilling of the collimator

•Image scrambling•Complete for beams slower than f/5

Fibre properties


•Multifibre spectroscopy started in Dec 1979 with Medusa•20-fibre plug-plate system on Steward 2.3-m Cass focus•AAT’s FOCAP followed in 1981--up to 64 fibres•ESO OPTOPUS, UKST FLAIR followed•Plug-plates still in use at SLOAN (660 fibres)

•Individual actuator systems•Steward’s 32-fibre MX-- “fishermen-round-the-pond” (1987)•HET ~10-fibre (x,y) actuator system•Echidna 400-fibre spine system (Subaru)

•Pick-place systems•Autofib (AAT), Autofib-2 (WHT), 2dF, 6dF, OzPos etc.

Fibre positioners

2dF on the AAT

Light from telescope

2dF corrector2 FOV

ADC

Plate 0 - setting up

Plate 1 - observing

Robot gripper

Spectrographs

4 metres

2dF on the AAT

Two spectrographs

Robot positioner400 fibres

Robot positioning fibres

Fibres on Plate

Configure

2dF spectra

Extracted 2dF spectra

Nod and Shuffle for 2dF

(Glazebrook, Hawthorn, Farrell, Waller, Barton, Lewis)

• Nod telescope and charge-shuffle simultaneously– Object and sky observed through exactly the same fibres

and CCD pixels– Excellent sky subtraction

•

First demonstration for 2dF in July 1999

(see AAO Newsletter #90)

More detailed recent investigation by Cannon et al. (see latest AAO Newsletter)

Limitation: only use 1 CCD, 1/4 fibres, and spend 1/2 time on sky

2dF Nod and Shuffle

(July 1999)

Raw arc spectra Shuffled exposure

Results on cloudy sky

Raw

Normalsky

subtract

N&S

sky

subtract

6dF local history—Multi-fibre spectroscopy at UKST

1982: Dawe & Watson suggest ‘radical’ change

1985: FLAIR prototype in operation

1988: PANACHE upgrade

1992: FLAIR II in operation—but still manual

1998: AAT Board approves 6dF at ~$A0.6M

The 6dF instrument

Specification:•6-deg field-of-view pick-place fibre system•Off-telescope robot•150×7” science fibres •FLAIR spectrograph•Marconi 10242 CCD•4 acquisition fibres•Two field plate units •Turn-round ~20 min•Reconfig. time ~40m•Up to ~6 fields/night

S/g6dF

6dF Robot

(r, ) robot

positions on

spherical

field-plate.

Air-bearings

throughout;

pneumatic

gripper.

Gripper uses air-bearings for z-motion, jaws open/shut and frictionless rotation.

Robot assembly—gripper

Fibre buttons have extended cylindrical upper section so gripper collet can grasp them

Fibre buttons

Prisms are SF2, cylindrical (2mm dia.× 5.7mm long) with flat to allow fibre to be cemented on. Fibre/prism unit then cemented into button. (Accepts full f/2.5 beam.)

Field-plate unit

40 cm

Field-plate unit

Local History—Multi-fibre spectroscopy at UKST

1982: Dawe & Watson suggest ‘radical’ change

1985: FLAIR prototype in operation

1988: PANACHE upgrade

1992: FLAIR II in operation—but still manual

1998: AAT Board approves 6dF at ~$A0.6M

2001: 6dF commissioning begins Feb 2001

6dF commissioning team

6dF in action

UK Schmidt Telescope usage

Year 6dF Photo. CCD

2001 70% 20% 10%

2002 <90% Override 10%

2003 <90% Override 10%

2004 <90% Override 10%

~75% dedicated to 6dF galaxy survey

~15% available for non-survey spectroscopy

6dF operations

Summary

• Multi-object spectroscopy is cool

• LDSS++, 2dF and 6dF are fab

• Australia is a leading player

• Bonzer, mate

observational techniques workshop 19 april 2001 fred watson, anglo-australian observatory (with...

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