6dF Data Release 2The good, the badand the uglyWill SaundersAAO 27/04/05
SDSS $100M 2MASS $40M 6dFGS $3M Mostly 6dFGS data quality is good – better and more consistent than 2dFGRS. But ergonomics very poor, and all depends on a giddy edifice of flaky hardware and software. • VPH Ghosts • Fluxing issues • Splicing issue • (Wrong extensions) • Fibre crosstalk • Loose slit vanes • Fringed fibres Caveat emptor
VPH Ghosts • VPH gratings used in Littrow exhibit a zeroth order ghost image of the fibre slit • Caused by dispersed light reflected off detector, back through camera optics, then undispersed by the grating in -1R mode • Intensity few % • Is in ALL 6dF VPH data • Most AAOmega gratings have slanted fringes to avoid this problem
Fluxing issues • Fluxing is very crude. • Single spectrophotometric standard star observed with each grating through a single fibre and reduced in normal way. • Reduced spectrum used to calculate Transfer (Instrumental) Function for that setup. • This Transfer Function applied to all data for that grating. • No airmass or atmospheric transparency correction. • Assumes average Transfer Function for 6dF constant for both field plates and for all time. • No Atmospheric Dispersion Corrector in 6dF. • Scattered light subtraction very crude => fluxing at blue end of spectra especially uncertain
Splicing Issues • Splicing uses overlap between red and blue spectra around 5577Å. • Fits a continuum to each spectrum and matches levels. • Mostly works remarkably well! • No bad pixel mask in early data – ‘pedestal effect’ at ends of spectra due to differences in object/sky fibre coverage. • Sometimes very little (or no!) overlap. • Occasional drastic differences in red/blue S/N due to acquisition changes.
Fibre Crosstalk • Camera optics have strong coma at corners of field. • Strong features in a spectrum can contaminate neighbour(s). • Ragged fibre slit + wavelength calibration –> redshift of contaminated neighbour is different to contaminating spectrum! • Usually caused by massively bright stars in sky (or parked) fibres in early data, or by Planetary Nebulae
Loose slit vane data… • Original slit vane screw mount replaced by a magnetic system at end of 2003. • Possible to position slit vane not fully homed. • Slit vane then jolted by shutter flag. • Causes poor sky subtraction • All data with shifts > 1Å @ 5577Å held back from DR2 for rereduction.
Fringed Fibres • Buttons often damaged during fibre positioning, ferrule bent up (‘Turkish coffee pot syndrome’) but not broken. • Fibre often fractured by this, forms perfect Fabry-Perot Interferometer. • Usually few-20 cycles
Velocity precision • runz now assigns individual redshift errors based on noise in cross-correlation function. Better than Tonry+Davis formula. • But DR2 stills assigns errors based on quality flag – 100/80/50km/s for Q=3/4/5 Cross-correlation done with latest ZCAT. 17,000 matches. Excluding Δv>500km/s gives: • <V6dF – VZCAT> = 22km/s • 2 =1.21 looks reasonable. Outliers: Q2 66% (Δv>500km/s) Q3 14% Q4 4% Q5 4%
Template errors? • Same templates used as for 2dFGRS • For 2dFGRS, systematic offsets (~100km/s) found for each template in use. • Some templates show offsets up to 50km/s with respect to ZCAT– under investigation. • 2 looks reasonable, ~1 for all templates.