The 6dF Galaxy Survey - The First Year

Will SaundersAnglo-Australian Observatory

Feb 2001: 6dF delivered to UKST

May 2001: first useful 6dFGRS data

June 2001: 6dF declared commissioned

Aug 2001: runz useable

Sept 2001: acquisition improved

Nov 2001: merged sample adopted

Dec 2001: 6dFDR workable

Dec 2001: much photography

Feb 2002: R-arm failure

Mar 2002: Robot repair and recalibration

Mar 2002: mirrors coated

Mar 2002: first fibre repairs

Apr 2002: adaptive tiling adopted

Jul 2002? Spectrograph VPH upgrade

Sep 2002? First data release

Salient Dates

Data Reduction

• Uses 2dFDR. Many teething problems adapting to 6dF data

• Now working very reliably, public version issued.• Automated fibre identification using known gaps

Outstanding concerns:• sky subtraction imperfect (as per 2dF)• wavelength calibration could be improved• need to splice V and R spectra• only tramline extractions for now

Acquisition

• Serious problems throughout first year.• Guiding uses 4 x 7 fibre guide bundles.• Fibres too fragile, many (~50%) broken during

commissioning and since.• Usually 2-3 useable, not enough• Repairs impossible, redesign completed and

remanufacture in progress.• May move to 75um fibres from current 100um

Fibre breakages

• Very sensitive to accurate robot calibration• ~30 fibres (10%) broken during commissioning• normal breakage rate 1/week• March/April breakage rate much larger due to

imperfect calibration after R-arm failure• Now much better, aim is max 2 fibres/lunation• Many fibres damaged, poor throughput or fringed.• Only one field plate in use currently while other

undergoes repair. • Repairs very difficult and slow but improving• Field Plate 1: 120 fibres, Field Plate 2: 90 fibres

Robot Positioning

• Accuracy mostly excellent. Problems with homing of encoders on startup caused instability and loss of few nights data.

• Current uncertainty in precise robot plate scale at 0.0001 level

• many manual fibering interventions required.• Configuring time 1 hour (with all fibres!), little

effort made to tune up.• New appointment Martin Oestreich will watch over

mechanics.

Quality Control

• Initially very erratic - just trying to sort out problems preventing data being taken at all.

• Many small changes to spectrograph alignment, focus, wavelength range.• Very little feedback possible prior to 6dFDR• Now more stable but still need real-time S/N calculator to determine

exposure times• Biggest problem acquisition.• No astronomer present for bulk of 6dFGS observations• poor feedback loop timescales• 6dF complex and delicate instrument, must expect variable quality at

some level.

Spectrograph Upgrade

• Current peak efficiency ~8% (cf 2dF 5%)• 6dF camera overfilled by spectra at ends of slit and

ends of wavelength range.• Existing reflection gratings good but still only 65%

efficiency at peak.• Will shortly switch to VPH gratings, 580V (3900A-

5600A) and 425R (5350A-7650A) at same resolution. Should reduce losses for both reasons, hope for 50% efficiency gain.

• Mirrors coated March 2002 but no efficiency gain.

Survey Progress 2001

May 11

Jun 17

Jul 0

Aug 19

Sep 19

Oct 19

Nov 20

Dec 2

2002

Jan 13

Feb 0

Mar 25

Apr 47

May 42

Total 232

6dFGS progress has been erratic with many interruptions due to other programs and instrumental problems. However, the rate at which fields are observed has continually increased. The last two months have had excellent weather, and have achieved record numbers of fields despite only having one field plate for May. However, both field plates have large numbers of broken fibres.• 20,000-25,000 galaxies observed.• Relatively few additional targets due to priority weighting

Data Quality

• 6dF capable of taking excellent data - average S/N > 10/pixel in both V and R in 1 hour + 1/2 hour, 10-20 objects/frame with >1000 electrons/pixel in V.

• Poor acquisition, robot calibration, seeing or transparency can easily halve S/N or worse

• No real-time S/N calculator developed yet.• Large fraction of data compromised by either

observational or instrumental problems.• Data quality and consistency generally improving.• Concern over whether quality is good enough for Dn-σ.

Data Reduction Progress

• reliable 6dFDR package only available Dec 2001.• 1/3 of data reduced• 1/5 of data put through runz• Further work needed to splice V and R data• Velocity dispersion techniques not yet in place• Completeness very variable, 60%-95%.

6dFGS prospects

• Minimum 1100 fields to observe in 3 years

=> 28/lunation => required average 4/clear night• Spectrograph upgrade may allow extra field/night, or

better higher S/N.

• Ways to streamline observing and/or trim the sample extremely desirable

• Doing better than 2dFGRS