6dF Data Release 2 The good, the bad and the ugly Will Saunders AAO 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 Possible to position slit vane not fully homed. Slit vane then jolted by shutter flag. Causes poor sky subtraction All data with shifts > 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: = 22km/s  2 =1.21 looks reasonable. Outliers:Q266% (Δv>500km/s)Q314% Q44% Q54%

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.

Enjoy