The multi-object fibre-optic spectroscopy system on the UKST

Summary

Principal features of FLAIR

Characteristics of the thinned FLAIR CCD

FLAIR Performance

Galaxy observations at low resolution

Stellar observations at high resolution

Faint point source observations

The FLAIR Interim Upgrade

6dF: A fully automated robotic fibre positioner

Applying for FLAIR time

Current FLAIR allocations and schedule

SUMMARY

FLAIR is the wide-field, multi-object spectroscopic facility at the U.K. Schmidt telescope (UKST). It remains the worlds most powerful such facility in terms of area coverage (40.sq.deg) and multiplex advantage (90 objects can be observed simultaneously).

Over 25000 individual target observations have been made since 1992.

Typically 15 different projects are supported each year with many receiving multi allocations.

Currently 30-35% of UKST time goes to FLAIR.

Time allocated for photographic non-survey projects and FLAIR is based purely on scientific merit and is split between Australian and UK submitted proposals by awards from the relevant national panels.

The commissioning of a thinned, blue sensitive CCD during 1995 has resulted in dramatic performance gains. A wider range of projects can be envisaged to fainter magnitudes whilst support for exisiting projects can be accommodated with significantly lower impact on UKST time. Some results are presented which demonstrate these gains.

FLAIR's major draw-back is the laborious semi-manual fibre positioning system which precludes maximum productivity. An automated facility to be known as 6dF, based on magnetic buttons and a robotic R-theta positioner is currently the subject of a Phase A design study.

Links to the fully comprehensive FLAIR User Guide and FLAIR/IRAF data reduction guide are given.

Also the current FLAIR allocations and schedule can be found HERE

Principal features of FLAIR

Simultaneous multi-object spectroscopy over 40.sq.deg.

Exceedingly long field dwell times possible (e.g 9 x 3000sec).

Very stable instrumental configuration (floor mounted spectrograph) making FLAIR data easy to reduce (e.g. using IRAF).

Wide range of instrumental resolutions (12.5-1.2A/pix).

Observing overheads are small - arcs only required at beginning and end of field exposures, again due to system stability.

System is flexible and can be quickly interchanged with normal photographic operation (turn-around 10-20 minutes).

Two plateholders enable two fields to be observed per night (typical turn-around 30-40 minutes).

FLAIR is a very cost effective facility, was cheap to build and required no major modifications to the telescope.

FLAIR is an excellent statistical survey facility for object types with number densities in the range 1 < n < 10 and B<19.

Characteristics of the thinned FLAIR CCD

New FLAIR CCD characteristics

• Device type: EEV CCD02-06 Thinned & back-illuminated.

• Pixel size: 22 x 22 microns. Pixel array: 400 x 578

• Underscan: 11 pixels. Overscan: 4 pixels

• Operating temperature: 150K

• Gain: ~ 1electron/adu. Read-out noise: ~11 electrons

• Pin-out: 20. Digitization:16-bit

• Dark current: 3.57 x 10**-4 electrons/pix/sec

A plot of the DQE comparisons between the old and new CCD is given here in Figure.1

Figure.2 gives an empirical sensitivity curve derived by comparing data from the new CCD with the old CCD as a function of wavelength from real FLAIR exposures on the telescope. The Y scale indicates the factor improvement over the old CCD. This curve has been obtained by dividing an average dome flat field spectrum from all fibres obtained with the new CCD from an equivalent average from the old CCD with the same wavelength range, grating and exposure times. These dome flats obtained from a quartz-halogen lamp and flat-field screen have previously been shown to be very reproducible in overall shape though signal repeatability depends on the screen always being illuminated in exactly the same way. Hence although the curve shape is accurate there is some uncertainty in the exact values of the Y scale though constant screen illumination is always attempted. Note the dramatic improvement in sensitivity in the blue. Also note that the sensitivity ratio drops back to $\sim1.5$ at the extreme edges of the data. This is expected since there is some framing of the back thinned area at the edges of the CCD so that performance here should be similar to the old CCD. This also gives us confidence that the sensitivity comparison curve Y-axis is good to ~50%. The sensitivity comparison also includes the effects of benefits from a new AR coating on the CCD camera corrector window and the new SF5 prisms.

FLAIR Performance

Galaxy observations at low resolution

Figure.4. gives a plot of average S/N as a function of COSMOS Bj magnitude for ~80 galaxies taken from a single 2000sec FLAIR exposure over the region 5000-5500Angstroms. This plot illustrates the good S/N values that can now be obtained in 2000sec with the new CCD. The error bars represent the standard deviation from each average at the given magnitude bin. Although a decreasing trend is evident one can see that average S/N is not a strong function of magnitude over the range Bj~15.8-17.5. This is because at Bj~15.8 not all of the galaxy's visible disk is typically being sampled by the large 6.7arcsec diameter fibres. As one goes fainter the situation is compensated by a larger fraction of a galaxy's projected diameter being sampled by the fibre. Consequently the average object surface brightness sampled by each fibre remains relatively constant over this magnitude range. Current FLAIR programmes can now be accommodated with a significantly lower impact on overall UKST time. Furthermore, entirely new types of FLAIR project become feasible such as determinations of galaxy velocity dispersion measures with the use of higher dispersion gratings. Likewise, previously difficult projects that required the very best conditions (such as QSO candidate observations) can now be confidently attempted.

Stellar observations at high resolution

These test observations were made with the high resolution grating 1200B at 1.34Angstroms/pix from a single 600second exposure. The data have been sky-subtracted and wavelength calibrated. The obj-id/magnitudes for these stars are given below.

Table.1: Object ID & Magnitudes for the 4 high resolution stellar spectra

Note the wide range in adu's for the 3 stars of the same magnitude - hence the large error bars in the S/N plot. The effect is probably caused by stars not quite in the full fibre due to: i) atmospheric refraction effects, ii) some minor proper motion component with some of the bright stars, iii) inherent fibre transmission efficiency differences, iv) small positioning errors.

Figure.6. gives S/N as a function of magnitude from 42 stars observed with G1200B calculated for the region 4500-4700Angstroms over the B magnitude range 9.0-10.4. Also for a star at 11th mag we typically obtain: 18.5adu/pixel/sec equivalent to 13.8electrons/Angstrom/sec with grating 1200B at 1.34Angstrom/pix resolution with gain=1 and 1adu=1electron.

Faint point source observations

Table.2: S/N values & Magnitudes for 3 faint point sources

These few individual fibre results are for a single 3000sec exposure with observations through 100micron (6.7arcsec) fibres and with grating 250B at 6.2A/pix resolution (no on-chip binning). The S/N results depend quite strongly on seeing, individual fibre transmission efficiency etc but are nevertheless indicative. Examples of a couple of AGN candidates confirmed during the latest Sealey run during September 1995 are given in figure.7.

These two AGN/QSO candidates were observed by Sealey et al. as part of the RBQS FLAIR project. The rightmost spectrum is a QSO with Z=2.44 and R=18.32, B=18.52.

The FLAIR Interim Upgrade

Several important developments have ocurred which significantly alter the way that FLAIR operates. These are all part of the so-called FLAIR interim upgrade.

Most of these upgrades are now in-place and will be available for Semester 1998B (August 1998 - January 1999).

Magnetic buttons

This new system replaces the rather messy and time-consuming UV curing cement process with magnetic button ferrules that still retains the benefits of the existing semi-automated postioning arrangements. use of magnetic buttons will save 25 seconds cure time plus 5-10 seconds glue application time per fibre or about 48 minutes for Plateholder 14/5.

New fibre-positioner Z-drive

New spectrograph remote-control features

Interim upgrade summary

6dF: A fully automated, robotic fibre positioner

Approval has now been given to proceed with the building of a fully automated, magnetic button based R-theta robotic fibre positioner. Called 6dF this system will replace the existing FLAIR system hopefully within 2 years. For the first time full-hemisphere spectroscopic surveys can be attempted over realistic timescales.

With the new CCD operational the biggest hurdle to a much more flexible and efficient mode of FLAIR operation lies in the semi-manual fibre-positioning system. This is such a large overhead that it precludes maximum productivity with FLAIR. The current FLAIR interim upgrade only partially abates this problem. A much more automatic system based on magnetic buttons and an R-theta robot would yield very considerable benefits. In particular it would be of enormous help with several very large surveys that have been proposed for FLAIR.

The scientific and operational benefits that a fully automated 6dF system would deliver are described in detail in various ancilliary documents which can all be found on the www HERE

None of the compelling large-scale science projects would be really practical without such an automated 6dF system in terms of timescales, efficiency and cost-effectiveness. This new facility, endorsed by ACIAAT and now fully supported by the AAT board, is an intended cornerstone of the UKST in the next decade. 6dF will offer a factor 10 increase in effective throughput over the existing FLAIR system at relatively modest cost. However even this cost is beyond the current budget of the AAO so alternative 3rd party and other novel funding arrangements are currently being actively pursued.

6dF Phase-A Design Study completed

Following on from the Phase A study, the fibre positioner should be completed within 2 years (Dec 1999). An accurate cost estimate in the region of 450K dollars has emerged. The funding to build 6dF is likely to come from a combination of AAO sources and third parties, for whom appropriate levels of guaranteed time on the instrument will be negotiated.

Applying for FLAIR time

Alternatively they can be accessed directly from HERE

Credits