AAOmega is intended to provide a stable, efficient, flexible spectrograph to replace the existing 2dF and SPIRAL spectrographs on the AAT. The principal AAOmega optical design considerations are as follows:

• To make use of VPH (Volume Phase Holographic) gratings, with their excellent efficiency and flexibility.
• To accommodate as many fully resolved fibres as possible.
• To maximise the number of spectroscopic resolution elements.
• To give excellent optical performance for 370nm-950nm.
• To allow use at R=1500-10,000 at any wavelength.
• To have excellent spectral stability.
• To minimise scattered light and ghosting.
• To be highly efficient.
• To use as few and as simple optical components as possible.
• To have uniform and well-sampled PSF.
• To have cameras as fast as readily achieved, to minimise the oversampling of the 2dF fibres.
• To do all the above within the limited budget and with minimal risk.

The main physical features are as follows:

• There is a single collimator feeding separate red and blue gratings and cameras.
• The beamsize is 190mm, determined by the competing considerations of obstruction losses and spectral resolution, versus the availability and cost of optics.
• There is a remotely operated slit wheel containing 4 slit units. These are one for each 2dF field-plate, one long-slit calibration slit and one spare. SPIRAL will be fitted with a new slit of the same design.
• There is a single flag-type shutter in front of the slit; this can be triggered by either of the two CCD controllers.
• The collimator is a Schmidt f/3.15, with a 145mm curved slit, spherical mirror, and a singlet BK7 corrector. The collimator speed is determined by the prime focus corrector of the AAT (f/3.5), with an allowance for Focal Ratio Degradation and the effects of non-telecentricity (up to 2°) at the fibre input.
• There are 2 Hartman shutters close to the collimator mirror.
• There is a single dichroic, which also acts as the order-sorting filter, specified to cut at 570nm.
• Each arm then has an f/1.3 Schmidt camera, with an identical N-FK5/LF5 doublet (which also acts as dewar window), a spherical mirror and a field-flattening lens of N-LAK33.
• The detectors are EEV 44-82 2k x 4k CCDs with 15micron pixels, back-illuminated/deep-depletion for blue/red arms respectively. The short direction is spectral; this is well matched to the angular bandwidth of VPH gratings, and allows nod&shuffle observations.
• Focus is changed by 3 motorised micrometers to the field-flattener/CCD assembly. However, the optical design is close to achromatic across all camera angles and wavelengths, and the required focal changes are small.
• The CCDs will be controlled by the new AAO controllers.
• All operations except grating changes will be remotely controllable.
• Usage will normally be Littrow (symmetric grating and camera angles), but non-Littrow usage is straightforward.

The figure shows the optical layout for AAOmega. The main features of the design, in order of the light train, are as follows:

A circularly curved fibre slit 145mm long. There are slit units for each 2dF plate and one for IFU use; each slit is made up of 40(MOS) or 32 (SPIRAL) individual flat slitlets. There is also a calibration slit for flat-fielding.

An f/3.15 Schmidt spherical collimator mirror. The collimator speed is determined by the 2dF prime focus optics, the amount of non-telecentricity in feeding the fibres, and the expected Focal Ratio Degradation within the fibres.

A fixed (but removable) dichroic beam-splitter within the collimator space, operating at 570nm.

For each of the two beams, there is then:

An identical BK7 collimator corrector plate.

A VPH grating, with allowed grating angles between 0 ° and 47°.

On each arm there is then an f/1.3, field-flattened Schmidt camera optimised appropriately (365-580nm for the blue arm and 570-950nm for the red arm), consisting of:

A flint/crown glass doublet corrector lens, also acting as camera dewar window.

A spherical f/1.3 mirror.

A dense, low dispersion field-flattening lens, positioned close to the CCD.

An EEV/Marconi 2K x 4K CCDs with 15micron pixels, thinned in the blue arm and deep depletion in the red arm. The long axis is in the spatial direction, since this allows N&S, and the field angles in the spectral direction are well matched to the efficiency peak of the VPH gratings.

The dichroic can be removed, allowing straight-through use in the V band with the red camera. The design is close to achromatic, refocussing may not be necessary when changing gratings. The projected fibre sizes are 58micron (MOS) and 35micron (IFU); the PSF sampling is 3.4 pixels/fwhm (MOS) and 2.1 pixels/fwhm (IFU).

The optics have been jointly optimised over the full range of anticipated wavelengths and grating angles. The theoretical optical pewrformance achieves the functional specification of 7.5micron rms radius for all fibres at all wavelengths in all confurations. The rationale for this specification is that the PSF for MOS fibres (3.4 pixel projected size), should vary by less than a few %, so that the variation per pixel is less than 1% of the total flux. The aim is to achieve 1% sky subtraction, in normal (sky fibres rather than N&S) mode. It is also crucial to prevent aberrations to large radii for the mini-shuffle observations.

The tolerances on all components are set so as not the degrade the optical performance by more than a few percent.

The dual-beam design allows simple coatings, since the wavelength range in each arm is less than an octave. All transmissive components will be MgF coated. AAO is developing the technology for solgel coating of large components; if this is successful, all components not in vacuum will be solgel overcoated.

Currently, it is envisaged the coatings for the collimator mirror and red camera mirror will be protected silver; while the blue camera mirror will be enhanced aluminium.

Scattered light is minimised by the coatings; by specifying high surface qualities for the components; by baffling stray light, and by the small number of optical surfaces and thickness of glass. The fibre arrays at the slits will have 8 small gaps to allow residual scattered light to be measured and subracted out.

Ghosting has been investigated carefully; the worst ghosts are expected to have intensities of 10^-4.

Calibration

Calibration facilities will include all existing 2dF arcs and flats. However, new colour balance filters and voltage control will be installed to allow enough flat-field counts at all wavelengths; there will also be a calibration long-slit, allowing proper pixel-to-pixel flatfielding of the CCD's.

Extra arc lamps may be installed to give enough arc line at all resolutions and spectrograph angles.