An Echidna-style positioner for

DESpec

Will Saunders

8 March 2011

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

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.

DESpec WFC/ADC

• 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

• 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 anyhowExisting DECam lensesNew field

lenses

Space for ADCCurved

Focal Plane

Is an ADC needed?

ZD=0

0.5μm<<1μm

ZD=45

0.5μm<<1μm

• 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

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

1.101.201.25 1.30

1.20 1.25

• 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

Echidna in operation

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

NOT TO SCALE

Focal Pane

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

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!

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!

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.

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.

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

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)