Current AAOmega Status - as of March 2006
AAOmega was successfully commissioned between November 2005 and January 2006. The system is now undertaking routine operations. Limited development and alterations will be undertaken over the next 6-12months as more experience with AAOmega is gained.
As with 2dF before it, AAOmega is proving to be an extremely popular instrument. The UK AAOmega Fellow (Rob Sharp; rgs@aao.gov.au) now carries out observations in service mode for UK AAOmega programs (Rob is also Principal AAOmega Contact). Non-UK observers are required to send at least one experienced observer for their AAOmega runs. In line with observatory policy, this should not be an unsupervised PhD students. UK observers (particularly students) are still welcome to come out for observing, but they will have to find non-PATT funding for this. PIs should advise their Support Astronomer of the proposed observers and their travel plans, or of the contact person in the case of UK PATT programs, well in advance of their runs. Please see this page for further details of the AAOs policy for UK PATT observers .
We continue to carry out AAOmega service observations, for programs requiring 6 hours or less. All AAOmega proposals of more than 6 hours length should go through the normal twice-yearly AATAC proposal system. Please see this page for for more information about the AAO Service Observing Program .
2dF Positioner
The AAOmega system is based on the original 2dF fibre positioner. Currently, after steady technical improvements, the positioner is running reliably at 6-7 seconds/fibre. This figure is for going from one 400 fibre configuration to another, i.e. is an average over a typical 400 moves and 170 parks (parks are faster): so about 1 hour/field. Very compact fields will be slower to configure, due to the increased number of fibre parks that will be required between configurations.Reliability of 2dF has improved dramatically over the years. In some respects this is more important than raw speed for observing many fields in a night, as recovery from an error may involve an interruption to observing. The current error rate is about one failure in every four fields configured.
Local positioning accuracy remains at ~15 microns, equivalent to ~0.25 arcsec. However global astrometric accuracy can still be problematic. The astrometric model over a 2 degree field plate takes account of off-axis field rotation and atmospheric refraction and is very sensitive - fields must be observed close to the Hour Angle they are set up for and preferably within +/- 2.5 hours of the zenith.
The Atmospheric Dispersion Corrector (ADC)
The 2dF system is equipped with an ADC and a field plate rotation system. Over a 2degree field of view and +60min exposure, the earths atmosphere behaves as a time variable chromatic lens. Point sources above the atmosphere are dispersed into mini spectra as seen by 2dF. The ADC counter dispersed the objects and removes this effect.However, the ADC cannot account for the effects of atmospheric refraction. Refraction introduces three components to the astrometry of the field, a translation, a rotation and a change of plate scale. translation is entirely removed via the guiding process. Rotation is achieved via the 2dF field plate rotators. However, the only option for accounting for the change of plate scale is simply to position the fibres in the correct position for the proposed observations. Since plates scale varies with time (due to the changing airmass during an observation) it is therefore necessary to limit exposure to +/-2 hours either side of the Hour Angle (HA) for which the field was originally configured, preferably with the central HA=0. Observing at a HA significantly different to the configured HA will result in significant loss of object flux due to a mismatch between the fibre positions and the object apparent positions.
The AAOmega Spectrograph
The bench mounted AAOmega spectrograph is a duel-beam system with on-axis Schmidt collimator, a dichroic beam splitter, Volume Phase Holographic (VPH) gratings and blue/red optimized on-axis Schmidt cameras. Two allow simultaneous configuring and observing a mechanical slit exchanged (under full software control) swaps between the observing slit and the configuring slit, which is mounted inside a light tight unit to allow back illumination of the fibres which are to be repositioned. In its current implementation there is no evidence for a light leak from the back illumination unit, a problem which plagued the original 2dF system, and which would result in increased scattered light within the cameras.At the current time (March 2006) only one dichroic is available, splitting at ~570nm. The high quality dichroic performs the task of order sorting filter for high resolution observations. Due to the need to maintain path length within the camera, a transparent glass substrate is required in the beam if the Red arm alone is to be used for observations. A plane mirror can be inserted in the beam for blue arm only observation, however the quality of the dichroic is high enough that this is not recommended. Hence there is little reason to under undertaking single arm observations. At this time, all observations are undertaken in first order. A new grating set will be required for second order, due to the nature of the VPH gratings.
PLEASE NOTE: Grating changes are normally done during the day before observers are present so we will require your setup the night before. Last minute (i.e. during the afternoon) changes may lose you valuable time. Night-time grating changes are not supported.
PLEASE NOTE:
Wavelength changes with AAOmega should be rapid and stable (hence
setups can be derived during the previous afternoon). Wavelength
changes are
therefore possible during night time, but there are significant (15-20min) overheads for doing so.
Gratings
AAOmega uses transmissive Volume Phase Holographic (VPH) gratings. These typically have a higher throughput that reflection gratings, but have a host of complexities and subtleties that many astronomers will not be familiar with. the list of available gratings and wavelength coverages can be found here. Note that for the higher resolution gratings, the effective usable range of the grating is larger than the single shot coverage of the detector. A grating calculator is provided for the user to experiment with central wavelength and spectral coverage. Note that for most situations the blaze should be set equal to the central wavelength for maximum broad band efficiency. For work in the far red a special grating, 1700D, is optimised for the Calcium triplet. this grating offers higher throughput at CaIII at the expense of removing the option of any other central wavelength (use 1700I for an alternate central wavelength).Cameras
Both cameras have science-grade 2048x4096 CCDs, with focus of 3.4-3.5 pixels over the entire chip. Note that the spectral direction is aligned with the 2k axis NOT the 4k axis. While it is in principle possible to use on-chip binning, in practice this will under sample the PSF and simply using a lower resolution grating will result in better data quality. Both CCD are formally cosmetically good, however, the blue CCD has a number of bad column which are closely spaced and close to the central wavelength. There is hope that this will be corrected in software via bias and dark corrections, but the user should discuss the effect of these column with their support astronomer.Observing Control Software - tdfct
We have full, reliable, Graphical User Interface control of the telescope, positioner, the tumbler, ADC, Spectrograph and CCDs.Other Software
Configure
Configure, the software for allocating fibres to objects, is now in a very stable state. The origional allocation algorithm has been designed by Gavin Dalton as part of the 2dF Galaxy Redshift Survey, and is very efficient. A new algorithm, based on simulated annealing, is now available as the default. Tests indicate that under some usage conditions the annealing method can significantly improve the results of configure. Information about how to download and install the Configure software can be found on the 2dF WWW page .drcontrol
drcontrol is a heavily-automated package for the reduction of AAOmega data (as well as data from 2dF/6dF and FMOS), and is very easy to use. Information about how to download and install 2dfdr can also be found on the 2dF WWW page .Rob Sharp (rgs@aao.gov.au)