An Echidna-style positioner for DESpec

1. An Echidna-style positioner for DESpec Will Saunders 8 March 2011

2. General considerations • Must compete with BigBOSS - 5000 fibres, 10,000 x 1000s exposures over 500 nights over 14,000 deg2 :  15M ELGs, 3M LRGs, 600k QSOs • But DESpec has better telescope, seeing, weather, photometry • DESpec throughput much better than BigBOSS: • Telescope collecting area is ~20% larger (smaller FOV and faster plate scale  smaller obstruction) • Blanco seeing ~0.7 vs Mayall 1.1 ! • Clear fraction 75% vs 62% (even excluding July/August)  Same S/N in less than half the time as BigBoss! • So significantly limited by read-time and read-noise, need to think about science case for going deeper than BigBOSS

3. Pitch and fibre numbers • Target densities are 2000+/deg2, high enough to fill any realistic number of fibres with the angular FoV (3.8deg2) of DECam. So FoV is large enough. • But number of fibres is critical; need several thousand just to compete, and could use any realistic number. Still not so many fibres compared with IFU-fed spectrographs. • DECam focal plane is only 450mm across (cf. 900mm for BigBOSS). • Pitch of 7mm ( 4300 fibres) between fibres is needed, 5mm ( 8500 fibres) would be grand. BigBOSS has 11.2mm, COBRA had 8mm, Echidna had 7.2, WFMOS had 11.2mm. LAMOST-style positioner probably ruled out, COBRA and Echidna probably ok.

4. DESpec WFC/ADC • DECam has flat focal plane, but not telecentric • Focal plane for fibres needs to be telecentric, but not flat • Need new field lens(es) • Last DECam lens is camera dewar window, must replace for fibres anyhow New field lenses Existing DECam lenses • New doublet design, FK5+BK7, all-spheric surfaces, gives <25μm rms images, FP has diameter 450mm (for 2.2 FOV), ROC ~7.5m. • Works fine to 350μm, but needs refocus Curved Focal Plane Space for ADC

5. Is an ADC needed? • ADC is maybe not needed for >500nm and ZD<45 (0.65 spread) • If no ADC, but good prior photometric-z, can tune fibre positions for expected redshift! • For BAO surveys with ELGs and LRGs, this should be satisfactory • ADC (and refocus) needed for all blue work - QSOs, LBGs, stars ZD=45 0.5μm< <1μm ZD=0 0.5μm< <1μm

6. A 2.5 Field of View • Can increase field of view, but with vignetting. • Vignetting starts at 2.2, is 22% at 2.5, 100% at 3 • Could use field overlaps to end up with equal exposure times • Image quality deteriorates sharply beyond 2.5, so stop there • 2.5  516mm diameter  29% more area  29% more spines 1.20 1.25 1.101.201.25 1.30

7. FMOS instrument for Subaru needed 400 fibres in 150mm diameter focal plane - so needed a completely new and very compact design. • Echidna uses 'spines’ – fibres in carbon-fibre spines, with a piezo-driven magnetic stick-slip to position fibres. • 7.2mm pitch between fibres • Fewμm positioning accuracy • Very short repositioning times

8. Echidna in operation

9. Echidna features • Echidna has 7.2mm pitch, few μm positioning, few seconds to reposition. • Spines introduce inevitable focal errors, up to 86μm. • Spines also introduce inevitable telecentricity errors, up to 2.75, small compare with f/2 telescope beam. • Patrol radius allows 3-fold covering of focal plane. So tolerant to clustering of targets – when # of targets ~ # of fibres, fibering/target efficiency is 95% versus 63% for LAMOST-style positioner Focal Pane NOT TO SCALE

10. Rest of Package: AAO provides whole data taking package, based on our 2dF, 6dF, AAOmega, Echidna, Ozpos, HERMES experience. Now routine to modify package for different instruments and telescopes. Package includes: • Observing planning software • Configuring software • Interface between instrument and telescope (instrument usually drives telescope) • Data reduction pipeline

11. Configuring software Configuring software includes guide stars, target priorities, checking for validity over range of Hour Angles, etc. Now based on simulated annealing, can't do better!

12. Configuring software Configuring software includes guide stars, target priorities, checking for validity over range of Hour Angles, etc. Now based on simulated annealing, can't do better!

13. Data reduction • AAO has state-of-the-art fibre spectroscopy data reduction pipeline. Very flexible and robust. • PCA (Principal Component Analysis) sky subtraction now routine  Poisson-limited sky subtraction with dedicated sky-fibres.

14. Data reduction • AAO has state-of-the-art fibre spectroscopy data reduction pipeline. Very flexible and robust. • PCA (Principal Component Analysis) sky subtraction now routine  Poisson-limited sky subtraction with dedicated sky-fibres.

15. WFMOS concept design • WFMOS proposed for large BAO (+ Galactic Archaeology) survey with Gemini or Subaru • 4000 spines over 500mm FOV with 11.2mm pitch • Curved focal plane (5m ROC) • Already prototyped at AAO

16. Proposed positioner design • Increase FOV to 2.5º diameter  4.9 deg2 area, 516mm diameter focal plane, 22% vignetting at edge • R&D program to determine minimum viable spine pitch: existing Echidna 7.2mm pitch  5400 spines; 6mm  7750 spines; 5mm  11000 spines • Overlapping patrol areas for Echidna-style positioners give ~15% higher fibre efficiency, ~25% higher target efficiency compared with LAMOST-style positioners • Required camera speeds • For small fibre-to-fibre pitches required, focal errors from spine non-telecentricity and are modest – <0.5 geometric blur, • AAO well placed for undertaking both design and construction (HERMES under construction, MANIFEST feasibility study almost complete)