1 WHT 2 Degree Field Forward Cass, Prime Focus correctors, multi-object spectrographs David King Institute of Astronomy University of Cambridge Science with the WHT London, 23 March 2010

2 Gaia Research for European Astronomy Training network/Gaia Chemo-Dynamical Surveys GREAT/GCDS aims to tackle some of the biggest galactic science challenges through the study of the chemical and dynamical properties of >10 6 stars. Go to Requires: –Large survey on 8m class telescopes –Large complimentary survey on 4m class telescopes –Development of suitable 4m class instruments for the northern and southern hemispheres to support these surveys > Large field (~2 degree) fibre-fed multi-object spectrographs covering R~

3 The Forward Cass option “standard” way of accessing a wide (>1.5 degree field is to do so at prime focus alternative, and possibly simpler solution is to use a forward Cass configuration entails having a new ~f/7 secondary tried using a doublet then triplet restricted wavelength range nm

4 Forward Cass with doublet

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6

7 Forward Cass with triplet

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10 Prime focus option

11 Prime focus option

12 Prime focus option

13 Forward Cass/Prime trades both require –multi-element correctors to get large field – atmospheric dispersion correction – not included in these preliminary models –one aspheric surface in each model larger elements in forward Cass (1060mm vs. 900mm ) new secondary (1200mm diameter) substantial mechanical redesign Prime corrector the way forward? – see Tibor’s presentation

14 Spectrograph parameters Assumptions: corrected WHT prime focus R = 5000, 20,000, 30,000, 40,000 f/2.65 collimator f/1.4 camera 8Kx8K CCD, 15  m pixels arcsecond fibres

15 GA surveys SurveyGALR-BGALR-RGAHR-1GAHR-2GAHR-3 GALR B+R start (nm) end (nm) Spectral R Spectrum Length (pixels) Min beam dia (mm)

16 F/1.4 camera Fibres are 1 arcsec in diam = 57 microns, cwl = 625nm 235mm diameter beam Achieving R=20,000 with one VPH grating (WHT 4.2m) One 2166 l/mm VPHG. 350x240mm F/2.65 collimator 8192x8192 pixels CCD 15um pixels 123x123mm

17 F/1.4 camera Fibres are 1 arcsec in diam = 57 microns, cwl = 625nm 235mm diameter beam Achieving R=40,000 by using two VPH gratings in series (WHT 4.2m) Two 2166 l/mm VPHG. Each is 350x240mm F/2.65 collimator 8192x8192 pixels CCD 15um pixels 123x123mm

18 F/1.4 camera Fibres are 1 arcsec in diam = 57 microns, cwl = 850nm 235mm diameter beam Achieving R=5,000 with one VPH grating (WHT 4.2m) A 538 l/mm VPHG. 260x240mm F/2.65 collimator 8192x8192 pixels CCD 15um pixels 123x123mm

19 Information gathering power At R= nm – 885nm At R= nm – 680nm, e.g. Spectrograph can have up to 1250 fibres At R=40000 can in principle trade wavelength coverage against number of fibres. e.g. 300 fibres with 250nm of coverage.

20 F/2.65, 240mm beam reversed collimator. All the surfaces are spherical.

21 F/2.65, 240mm beam reversed collimator matrix spot diagram.

22 F/2.65, 240mm beam reversed collimator ensquared energy diagram.

23 F/ mm beam camera. Work in progress

24 f/ mm beam camera spot diagram

25 f/ mm beam camera ensquared energy diagram

26 Plugplates

27 Requirements-1 Science case requires ~1000 simultaneous spectra at optical wavelengths for discrete objects in a deg FOV on a 4m telescope with up to 5 fields per night. The large FOV, the large wavelength coverage and the high spectral resolution are best addressed by a fibre-fed MOS approach with one fibre per object.

28 Requirements-2 Field change time has to be relatively fast, < 5 minutes from the end of one exposure to the start of another one on a new field. Up to 1000  5=5,000 fibres have to be placed in a 24 hour cycle. Each fibre tip has to be placed accurately with 5 degrees of freedom - X, Y- object position - Z- focus - tip, tilt- pupil aiming

29 Possible plugplate solution –plugplate - a sheet of suitable material with accurately drilled holes receive fibres. –plugplate modules - configured off telescope. –exposures of ~2 hours - up to 5 pre-configured modules per night –a machine interchanges modules on the telescope during the night. –in each module the fibres are short (<1m) and the output ends are arranged in a fixed pattern (a fibre-optic connector).

30 Spectrographs PF corrector Permanently installed fibre run Interchangeable plugplate module A&G unit Telescope

31 Module interchanging At the end of an exposure the telescope moves to a low elevation where the module interchanger can access the module. The permanently installed part of the fibre connector is backed off from the connector half on the plugplate module. The used module is removed and the next one is put on. The new module is referenced to the telescope focal plane. The permanently installed part of the fibre connector then mates with the other half of the connector on the module. The telescope slews to the next target. This entire process is the same for all modules.

32 Module preparation facility a dedicated facility (at the telescope or elsewhere). plates are drilled on CNC machines. – up to 5000 holes in an 8 hour shift ( 4 machines if the time to drill one hole is 20 seconds.) The plates are non-metallic to reduce costs and reduce fitting tolerances? The fibres exchange by robots. –reconfigure 5000 fibres in an 8 hour shift (4 robots if the time to move one fibre is 20 seconds.) quality control machine to check and clean each module after preparation. bar-coding for module housekeeping. modules transported between telescope and preparation facility each day.

33 Next plugplate Previous plugplate with fibres inserted Robot that unplugs old plate and plugs new plate Multi-way fibre connector Plugplate module

34 Illustration of unplugging an plugging sequence that robot carries out to reconfigure a plugplate module Start Finish

35 Ferrule is fluted and tapered at tip to make insertion easier. Plugplate is non-metallic. These features significantly reduce tolerance on diameter of drilled holes. Ferrule details A ferrule must not fall out of a hole because it’s too big or get stuck in a hole because it’s too small.

36 Ferrule details

37 Advantages of plugplates The least possible positional constraints: no magnets or prisms or crossing fibres. No positioning mechanisms attached to fibres. This is fundamental for scientific versatility! Operation during the night is extremely simple. Very low risk of telescope time being lost. The interface with the spectrographs is simplified (no need for fibre selection mechanisms at the slit end). All 5 axes for fibre placement (X, Y, Z, tip, tilt) can be controlled if required.

38 Reliability Extra robots, modules and drilling machines to allow a routine maintenance program. When a fibre breaks, impacts on “sky” fibres so the surveys are not affected. When many fibres are broken in a module it is taken out of operation and repaired. spare robots and drilling machines so there are always enough operational to meet the survey needs. About 2,500 plates (2.5 million holes) have to be drilled. 500 nights of observing.

39 Conclusions Plugplates have minimal positioning constraints and are therefore the best solution scientifically for applications which require the very highest target densities. The time required to get started is short (compared to fully robotic fibre positioners) and the overall spend profile is not heavily weighted up-front. The scheme presented here requires no new technology. We should develop the idea further and establish an accurate cost to complete the surveys (i.e. include all capital and operational costs). Maybe using humans instead of robots is cheaper?