1. 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)

2. Overview Basics Slitless spectroscopy Multislit spectroscopy Multislit spectroscopy with LDSS++ Multifibre spectroscopy Multifibre spectroscopy with 2dF & 6dF

3. 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.

4. Slitless spectroscopy Drawbacks: Large prisms required Overlapping spectra (may need orthogonal exposures) No comparison spectra (use red cut-off) Resolution strongly seeing-dependent Sky undispersed Objective prism True 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~10 5 ob jects in 40 deg 2

5. Slitless spectroscopy Drawbacks: Overlapping spectra (may need orthogonal exposures) No comparison spectra Resolution strongly seeing-dependent Sky undispersed Slitless spectrograph Again 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

6. 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).

7. LDSS AAT’s Low Dispersion Survey Spectrograph for faint multi-slit spectroscopy (12 arcmin FOV) Collimator Grism slide Filter slide Camera Aperture wheel CCD

8. 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/deg 2 or 600 per LDSS slit area

9. 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

10. `Va et Vient’ `Nod and shuffle’ CCD controller Instrument Sequencer Telescope Drives

11. 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]

12. AC114 9 arcmin

13. AC114 Mask

14. Detector Spectrograph Slit The answer to life, the Universe and everything... Multifibre spectroscopy

15. 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).

16. Multifibre spectroscopy Bulk transmission losses low from 0.5  m to 2.0  m Low-OH (“dry”) fibres excellent for > 0.55  m High-OH (“wet”) fibres good for > 0.35  m But OH absorption features at 0.73, 0.95 and 1.37  m 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

17. Multifibre spectroscopy 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

18. 2dF on the AAT Light from telescope 2dF corrector 2  FOV ADC Plate 0 - setting up Plate 1 - observing Robot gripper Spectrographs 4 metres

19. 2dF on the AAT Two spectrographs Robot positioner 400 fibres

20. Robot positioning fibres

21. Fibres on Plate

22. Configure

23. 2dfdr

24. 2dF spectra

25. Extracted 2dF spectra

26. 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

27. 2dF Nod and Shuffle (July 1999) Raw arc spectra Shuffled exposure

28. Results on cloudy sky Raw Normal sky subtract N&S sky subtract

29. 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

30. The 6dF instrument Specification: 6-deg field-of-view pick-place fibre system Off-telescope robot 150×7” science fibres FLAIR spectrograph Marconi 1024 2 CCD 4 acquisition fibres Two field plate units Turn-round ~20 min Reconfig. time ~40m Up to ~6 fields/night S/g 6dF

31. 6dF Robot (r,  ) robot positions on spherical field-plate. Air-bearings throughout; pneumatic gripper.

32. Gripper uses air-bearings for z-motion, jaws open/shut and frictionless rotation. Robot assembly—gripper

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

34. 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.)

35. Field-plate unit 40 cm

36. Field-plate unit

37. 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

38. 6dF commissioning team

39. 6dF in action

40. UK Schmidt Telescope usage Year6dFPhoto. CCD 200170%20%10% 2002<90%Override10% 2003<90%Override10% 2004<90%Override10% ~75% dedicated to 6dF galaxy survey ~15% available for non-survey spectroscopy 6dF operations

41. Summary Multi-object spectroscopy is cool LDSS++, 2dF and 6dF are fab Australia is a leading player Bonzer, mate