The 6dF VPH Upgrade and other instrumental issues Will Saunders, Martin Ostreich, Allan Lankshear, Brendan Jones, Mick Kanonczuk, Kristin Fiegert, John Stevenson, Chris McCowage, Bob Dean, Peter Gillingham, Tony Farrell, Keith Shortridge, Brendan Jones, Mick Kanonczuk, Darren Stafford, John Collins, Fred Watson, Malcolm Hartley, Paul Cass, Ken Russell, Dionne James AAO 11/07/03

VPH Upgrade Volume-Phase Holographic gratings offer many advantages for astronomy - Efficient (90% vs 60%) Can specify dispersion and blaze Reduced pupil relief - smaller and slower optics Cheap, robust, large sizes available, multiple suppliers Can tune blaze in use (Prototype for AAOmega) BUT must articulate camera as well as grating

6dFGS Gratings Only 300 resolution elements on CCD - impossible to achieve required wavelength coverage (3900A-7600A) and resolution (~5A) in one hit Hence two gratings, two settings per field Need very rapid grating change =>Pair of gratings at same grating angle on a slide 580V gives 5A fwhm - just good enough for D n -σ. 425R gives 8A fwhm - shorter integration times, and can use twilight.

Results of VPH Upgrade No conflict with AAT use of 316R Much better PSF everywhere More constant PSF, especially across the chip Much more uniform throughput along the slit Not much gain in peak efficiency - gratings only 70% peak efficiency due to loss to zeroth order at such low dispersion Restricted range of other gratings now available: 580V, 425R, 1201B, 1516R, 1700I

Other instrumental issues - Fibre breakages Continues to be a major problem. Several fibres lost, per field plate, per lunation (improving?). Typically running with live fibres. Repairs very slow, difficult and risky. Often running with one field-plate at ~75% efficiency. Additional wear and tear from RAVE survey. All repaired fibres now jacketted with polyimide sleave. Positioning tolerances increased. Robot parking supervised. Other ideas?

Acquisition Guide fibres especially fragile (stripped). No way to repair in original design. Much of first 18 months of survey running with 3 or 2 incomplete (or scrambled!) guide bundles. Very serious effect on S/N and uniformity. Complete redesign and rebuild last year. Breakage rate reduced by sleeving. Now complete set of guide bundles. Guide fibres still oversized at 100μm. Have 75μm available but not high priority.

Camera oil contamination There is oil contamination in 6dF camera, from vacuum pump. Leads to progressive oil contamination of field-flattening lens during each lunation. Complete strip-down very lengthy and risky process. Maybe slow improvement with heat-jacketting?

Robot ‘comm2’ overhaul Complete hardware and software overhaul last year. Full maintenance schedule laid down and implemented. Robot reliability now very good. Configure also much improved (but slower!).