1. 1 GREAT spectrograph and plugplate discussion Ian Parry Institute of Astronomy Cambridge University UK

2. 2 Spectral Resolution R max  PD max × [θ Fib × D Tel ] -1 R max is the maximum resolution of the spectrograph. θ Fib is the angular diameter on the sky of the fibres in the slit. D Tel is the telescope diameter. PD max is the largest optical path difference across the grating (related to beamsize).

3. 3 A-Omega of the spectrograph AΩ  {N fib × N SRE × A SRE } × Ω cam AΩ  A detector × Ω cam N fib is the total number of fibres. N SRE is the number of spectral resolution elements in one spectrum. A SRE is the area of one spectral resolution element. A SRE  D Tel 2 Ω cam is the solid angle of the light beam from the camera at the detector.

4. 4 Data packing on the detector N fib determines number of pixels in spatial direction. N SRE determines number of pixels in the dispersion direction. GREAT science requires more pixels in the dispersion direction than in the spatial direction but a single optical system works best when these two numbers are the same. Therefore need cross dispersion or several spectrographs or image slicing (several fibres per star).

5. 5 Spectrograph Cost Implications A smaller telescope helps because it reduces the beamsize for a specified spectral resolution. A smaller telescope also helps because it reduces the A-omega. For a fixed A-omega and spectral resolution, a spectrograph for a 4m is cheaper than one for an 8m because the beamsize is smaller. Image slicing (i.e. reducing θ Fib ) reduces the beamsize for a specified spectral resolution. Image slicing also helps with the data packing issues.

6. 6 Survey Cost and Duration Does one night on an 8m cost the same as 4 nights on a 4m? Can one get more nights per year on a 4m than on an 8m? For a fixed A-omega and resolution the survey time is independent of telescope time (e.g. a 4m telescope with a 3 deg FOV will complete the survey in the same number of nights as an 8m telescope with a 1.5 deg FOV).

7. 7 f/2.12 Collimator f/1.07 Camera Detectors 93mm x 93mm VPHG Collimated beam is 500mm in diameter Slit length is 184mm and has 800 fibres Ø930 Ø720 Ø592 Ø964 One of three WFMOS spectrographs shown in the GAHR mode Schmidt systems offer the most A-omega per dollar.

8. 8 DELZ Configuration

9. 9

10. 10

11. 11

12. 12

13. 13

14. 14

15. 15

16. 16 GAHR Configuration

17. 17 F/1.4 camera Fibres are 1 arcsec in diam = 57 microns, cwl = 850nm 233mm diameter beam Achieving R=5,000 with a VPH grating 4-m telescope (e.g. WHT 4.2m) A 538 l/mm VPHG. 260x240mm F/2.65 collimator 8192x8192 pixels CCD 15um pixels 123x123mm

18. 18 F/1.4 camera Fibres are 1 arcsec in diam = 57 microns, cwl = 625nm 233mm 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

19. 19 F/1.4 camera Fibres are 1 arcsec in diam = 57 microns, cwl = 625nm 233mm 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

20. 20 Information gathering power At R=5000 480nm – 885nm At R=40000616nm – 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.

21. 21 F/1.5, 200mm beam camera. Work in progress by Dave King (IoA). All the surfaces are spherical. Deimos camera design is f/1.3, FL~375mm BOSS/SLOAN spectrograph designs – smaller beam size, lower R

22. 22 HERMES (Anthony Horton, May 2009)

23. 23 HERMES Three 4kx4k detectors

24. 24 HERMES

25. 25 HERMES

26. 26 VISTA FOV is 1.65 deg diameter for the IR camera can maybe be ~2 deg for an optical system.

27. 27 Existing WHT PFC 1 degree diameter FOV Design includes an ADC Largest element is 450mm in diameter

28. 28 Plugplates

29. 29 Requirements-1 Science case requires ~1000 simultaneous spectra at optical wavelengths for discrete objects in a 1.5-2.0 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.

30. 30 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

31. 31 Possible plugplate solution The plugplate is basically a sheet of suitable material with accurately drilled holes into which the fibres are inserted. The plugplate modules are configured off the telescope. With exposures of ~2 hours, up to 5 pre- configured modules have to be ready for use at the start of a night (unlikely to be more). A special machine interchanges the modules on the telescope during the night. In each module the fibres are short (~500- 1000mm) and the output ends are arranged in a fixed pattern (a fibre-optic connector).

32. 32 Spectrographs PF corrector Permanently installed fibre run Interchangeable plugplate module A&G unit Telescope

33. 33 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.

34. 34 Module preparation facility This is a dedicated space (maybe at the telescope or maybe some other location). The plates are drilled on CNC machines. The facility has enough machines to accurately drill up to 5,000 holes in an 8 hour shift. For example, 4 machines are needed if the time to drill one hole is 20 seconds. The plates are non-metallic to reduce costs and reduce fitting tolerances. The fibres are removed from used plates and inserted into new plates by robots. The facility has enough robots to reconfigure 5000 fibres in an 8 hour shift. For example, 4 robots are needed if the time to move one fibre is 20 seconds. A special quality control machine is used to check and clean each module after it has been prepared. Each module is bar- coded so that the system can keep track of everything. The modules have to be transported between the telescope and the preparation facility each day.

35. 35 Next plugplate Previous plugplate with fibres inserted Robot that unplugs old plate and plugs new plate Multi-way fibre connector Plugplate module

36. 36 Illustration of unplugging an plugging sequence that robot carries out to reconfigure a plugplate module Start Finish

37. 37 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.

38. 38 Ferrule details

39. 39 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.

40. 40 Reliability Extra robots, modules and drilling machines will allow a routine maintenance program so that production always continues at the required pace. When fibres break, initially this only impacts on “sky” fibres so the surveys are not affected. Eventually, if too many fibres are broken in a module it is taken out of operation and repaired. Similarly, by having spare robots and drilling machines 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.

41. 41 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?