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AAOmega

Preparing Observations with AAOmega
Commissioning report
S/N Calculator (MOS Mode)
S/N Calculator (IFU Mode)
Grating Calculator
Grating Efficiencies
Raw low dispersion red image (1.25Mb)

AAOmega is the new spectrograph for the Anglo-Australian Telescope. It is a multi-purpose, bench mounted, fibre fed, double-beam spectrograph with two main observing modes:

Multiple-object spectroscopy using the existing 2dF top end (commissioned early 2006).
Integral field spectroscopy at Cassegrain focus (due for commissioning mid 2006).

AAOmega provides higher spectral resolving power, throughput, and stability than the decommissioned 2dF, RGO and SPIRAL spectrographs that it replaces. In particular, maximum resolution, coverage at given resolution, and throughput are all more than doubled as compared with 2dF (see HERE).

Contents

Overview
Spectrograph Mechanical Layout
Spectrograph Optical Layout
Important Numbers
Grating Set, Dispersion, Resolution
Grating calculator
Throughput Plots-MOS Mode
Throughput Plots-IFU Mode
S/N Calculations
Further Information

Overview

AAOmega is a general purpose facility providing multi-object and integral field spectroscopy. Multiple-object spectroscopy, of 392 objects, will be provided using the existing (but overhauled) 2dF top end. In MOS mode, spectral resolutions between 1200-10000 are be available. Integral field spectroscopy will be provided by the existing, 512-fibre, SPIRAL Integal Field Unit, with modifications to the slit end to allow it to feed the new AAOmega spectrograph. Spectropolarimetry may be available with the IFU mounted at the normal Cassegrain focus. Spectral resolutions between 2000-16000 will be available in IFU mode.

The spectrograph is a dual-beam system with separate blue and red arms, split by a dichroic at 570nm. AAOmega uses Volume Phase Holographic (VPH) gratings, which can be up to 50% more efficient than reflection gratings. The optics, coatings and detectors in each arm are optimised for the relevant working wavelength range. The new AAO-2 controllers will permit nod & shuffle observing. Scattered light has been minimised throughout the design, and in MOS mode, the fibres are very well resolved. The Point Spread Function (PSF) is extremely Gaussian and uniform, allowing accurate (1%) sky subtraction with ~25 dedicated sky fibres. The spectrograph is mounted in the mechanically and temperature stable Coude room, to ensure stability << 1 pixel over several hours..

Spectrograph Mechanical Layout

Overall Layout

Spectrograph Optical Layout

optical layout

Figure 2: Optical Layout of AAOmega Spectrograph. In order down the optical train is the slit, field lens, collimator mirror, dichroic, collimator corrector, grating, camera corrector, camera mirror, field-flattening lens, detector

Important Numbers

AAOmega - multiple-object mode
Field of view	2 degrees
Number of fibres	392
Angular size of fibre	2.0"
Wavelength range	0.37 - 0.95 microns
Dispersion	0.2 - 1.5 Å/pixel
Spectral resolution (FWHM)	3.5 pixels
Spectral resolution (λ/Δλ)	1300 - 10000
Peak system throughput (Blue/Red arm)	0.15/0.20¹
Limiting magnitude	B = 22.0²
AAOmega - Integral-Field Mode
Field of View	11x22 arcsec
Number of fibres	512
Angular size of fibre	0.7"
Wavelength range	0.37 - 0.95 microns
Dispersion	0.2 - 1.5 Å/pixel
Spectral resolution (FWHM)	2.1 pixels
Spectral resolution (λ/Δλ)	2000 - 15,000
Peak system throughput (Blue/Red arm)	0.23/0.27¹
Limiting magnitude	B = 22.0²

¹ End-to-end throughput, from top of atmosphere to CCD, including fibre seeing losses (no seeing losses for IFU).
² Calculated for a galaxy in 1" seeing, exposure time of 4 hours, with a S/N=10/Angstrom

Grating Set, Dispersion, Spectral Resolution

Name	Blaze	Useful wavelengths	Coverage	Angle	Dispersion	MOS Resolution
	nm	nm	nm	Degrees	nm/pix	R
580V	450	370 to 580	210	8	0.1	1300
385R	700	560 to 880	320	8	0.16	1300
1700B	400	370 to 450	65	18	0.033	3500
1500V	475	425 to 600	75	20 - 25	0.037	3700
1000R	675	550 to 800	110	18 - 22.5	0.057	3400
1000I	875	800 to 950	110	22.5 - 25	0.057	4400
3200B	400	360 to 450	25	37.5 - 45	0.014	8000
2500V	500	450 to 580	35	37.5 - 45	0.018	8000
2000R	650	580 to 725	45	37.5 - 45	0.023	8000
1700I	860	725 to 900	50	37.5 - 45	0.028	8000
1700D	860	845 to 870	40	47	0.024	10000

Note that for VPH gratings, the efficiency characteristics depend on the grating angle. The uncoated efficiencies of the gratings as a function of grating angle are shown in the gratings page . The grating angle also affects the dispersion, resolution, and wavelength coverage. The wavelength coverage depends also on the camera angle, and on the fibre position on the slit. Finally, for any given setup, the resolution (l/dl) varies significantly with wavelength. Phew. The actual wavelength coverage and resolution for any particular setting is given in the Grating setup calculator

Throughput Plots

MOS Mode

IFU Mode

S/N Calculations

S/N calculator (MOS mode)

S/N calculator (IFU mode)

The table below gives indicitive, preliminary S/N values (per Angstrom) based on the AAOmega throughputs given above. We give the S/N for dark, gray, and bright sky, and for seeing of 1.0, 1.5, and 2.0 arc-second. For each filter and magnitude combination, the S/N is given for both 1.0 and 4.0 hours exposure time. Note that these calculations have been done only for the MOS mode at present, and that the S/N calculator will always contain more up-to-date throughput estimates.

Sky	Seeing = 1.0"	1.5"	2.0"
V=18, S/N per Angstrom in 1/4 hours
Dark	64.2/129.2	54.6/110.0	44.5/90.0
Gray	61.0/122.7	51.2/103.1	41.0/82.8
Bright	38.6/77.4	30.3/60.8	22.4/45.0
V=20
Dark	19.5/39.9	15.7/32.3	12.0/24.7
Gray	16.5/33.5	13.1/26.6	9.7/19.8
Bright	7.1/14.3	5.4/10.9	3.9/7.8
V=22
Dark	4.1/8.5	3.1/6.5	2.3/4.7
Gray	3.1/6.4	2.4/4.9	1.7/3.5
Bright	1.2/2.3	0.9/1.8	0.6/1.3
B=22
Dark	6.5/13.7	5.0/10.8	3.7/8.0
Gray	4.4/9.0	3.4/6.9	2.4/5.0
Bright	1.5/3.1	1.2/2.3	0.8/1.6
I=20
Dark	7.0/14.1	5.3/10.8	3.9/7.8
Gray	6.4/13.0	4.9/9.9	3.5/7.2
Bright	5.2/10.6	4.0/8.0	2.9/5.8

Further Information

Planning Observations with AAOmega
Instrument concept and overview
Spectrograph mechanical layout
AAOmega spectrograph physical/optical layout
AAOmega grating info (grating set, spectral resolution, and wavelength coverage)
More general information about VPH gratings.
CCD detectors for use with AAOmega
Optical fibre information.
Overview of the AAOmega IFU observing mode.
Overview of multiple-object observing mode.
Integral field spectropolarimetry with AAOmega
Information regarding the spectrophotometric performance of the SPIRAL integral field unit can be found in Appendix A of the AAO Users Committee report (PostScript, July 2000).
AAOmega Functional Specification (Word, January 2003)
AAOmega Strategic Science Drivers and Science Requirements (Word, September 2002)
AAOmega Software Requirements Document (Word, May 2003)
AAOmega/2dF Throughput Comparison (Word, October 2003)

AAOmega Large Extragalactic Programs Workshop, 18th February 2005

Full report

UCL AAOmega meeting:

28 June 2004

Program

AAOmega Science Workshop:

1 October 2003

Full Report

Project team:

AAOmega Project Manager: Gabriella Frost
AAOmega Project Scientists: Terry Bridges (2000-2003), Will Saunders (2003-present), Scott Croom (2004-present)
AAOmega Instrument Scientist: Rob Sharp
AAOmega Project Engineer: Greg Smith
AAOmega Mechanical: Vlad Churilov, John Dawson, Mike Kanonczuk, BrendanJones, Allan Lankshear, Dennis Whittard, Greg Smith
AAOmega Optical Design: Peter Gillingham, Will Saunders , Damien Jones (PrimeOptics)
AAOmega Electronics: David Correll, Don Mayfield, Rolf Muller, Ed Penny, Lew Waller
AAOmega Software: Tony Farrell, Ron Heald
Data Reduction Software: Scott Croom, , Ron Heald Will Saunders
AAOmega Fibre Optics: Kristin Fiegert, Roger Haynes, John Stevenson
2dF Upgrade/AAOmega Infrastructure: Steve Lee, Chris McCowage, Martin Oestreich, Jonathan Pogson, Rob Patterson

Last modified: 5th Sept 2005, Will Saunders