AAOmega Gratings

AAOmega is a dual beam spectrograph. The Red and the blue arms each have E2V detectors optimized for the relevant wavelength settings (thinned blue detector for UV sensitivity, thick red detector to reduce fringing, see the AAOmega CCD page for details).

The slit units (one for each field plate) which each contains 392 science fibres are feed into a single collimator and then separate into the Blue and Red arms of the system via a dichroic beam splitter. While the dichroic is an interchangeable part of the system, currently only a single dichroic, which operates around 570nm, is available.

In principle the red (or blue) arms can be operated alone with the dichroic removed (or replaced by a plane mirror). However, in practice the need for an order sorting filters and the high quality of the dichroic mean that this mode is not currently supported.

Each arm of AAOmega uses a separate Volume Phase Holographic grating (VPH).

Selecting an AAOmega setup
There are several consideration the user should keep in mind when tunning AAOmega
The AAOmega grating set
Grating Efficiencies
An note of Blaze of the VPH gratings

Selecting an AAOmega setup

AAOmega can be configured to observe the entire optical spectrum over the wavelength range 370nm-900nm, with a small overlap between the red and blue arms at 570nm. The grating set available allows a range of resolutions between R~1,000 to R~10,000. The fibre spectra are recorded onto the 2Kx4K E2V CCDs with light dispersed along the 2K axis NOT the the 4K axis. Hence, at low resolution the entire accessible spectral range is recorded at once, but at higher resolutions the user must tune the wavelength range to that which best suites their requirements.

The full list of gratings can be found in the table below. An on-line AAOmega Grating calculator is available to simplify the process.

There are several considerations the user should keep in mind when tunning AAOmega:

Grating changes will not be performed during the night. Grating changes can only be undertaken during the afternoon. Wavelength changes can be incorporated into the nightly schedule, although there is an overhead and every effort should be made to minimize the number of changes required during any given night.
Where possible, the blue arm of the system should be set to allow the strong 570nm sky line to fall within the observed spectral range. This will allow sky subtraction to be performed without the need for twilight flat fields or dedicated offset sky frames.
It is not required to have the two arms of the system overlapping in wavelength. However, leaving some overlap allows spectra to be spliced together.
Spectral curvature. As with all spectrographs, the spectra follow curved paths on the CCDs and wavelength is not a constant function of X-pixel position between fibres. It low resolution this is barely noticed. However, at higher spectral resolutions there is a small mismatch between the observed wavelength of the central and outer fibres. This range of wavelengths is given in the AAOmega Grating calculator.
Departures from the standard default values for each grating are acceptable, but one should pause to ask if they are really required.
The Blue arm CCD as a number of regions with closely packed bad columns. For some projects it may be possible to tune the central wavelength to reduce the effects of these. The user should contact their support astronomer to discuss options.
Blaze angle. The collimator-to-VPH and VPH-to-camera angles are typical set to be equal. This gives the peak system through put at the central wavelength, with a slow role off to shorter and longer wavelengths. For certain application, one may wish to operate with an asymmetry in these angles which will boost the system sensitivity at shorter/longer wavelengths, but at the expense of sensitivity at longer/shorter wavelengths. Check out the notes below, or call your support astronomer for a discussion of this very important concept which is some what specific top VPH gratings.
Due to the long fibre run (38m prime focus to Coude West) and the optics of the 2dF prime focus corrector, the system throughput below 370nm is very poor and there is little point attempting to observe at shorter wavelengths.
High resolution CaIII observations. The 1700D grating is specifically designed for observation of the CaIII lines at ~860nm. It gives a better response at this wavelength than the 1700I grating. However, it cannot be used at any other central wavelength. When observing the CaIII there is no advantage to observing with the 1700I grating over 1700D. If one wishes to observe at high resolution at red wavelength, but away from CaIII, then 1700I must be used.
For Service Applications one might consider the use of standard grating configurations. While a highly ranked service proposal can to some extent dictate the configuration of AAOmega in advance, the probability of service observations being successfully undertaken is strongly coupled to the frequency of the requested grating set being mounted within AAOmega for other programs.

The AAOmega grating set

Grating	Blaze	Useful wavelengths	Coverage (single shot)	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*	750	725 to 850	50	37.5 - 45	0.028	8000
1700D*	860	845 to 900	40	47-48	0.024	10000

* Resolution with the SPIRAL IFU will be slightly higher due to the smaller fibres used in the IFU.
* For high resolutions observations in the CaIII region, 1700D is the appropriate grating. The table above shows information at the central super blaze grating wavelength. For CaIII observations, 1700I does not have an increased coverage over 1700D. Wavelength dependent resolution is a property of the VPH gratings.

Grating Efficiencies

Preliminary efficiency curves for the AAOmega gratings are available here. All measurements were taken prior to AR coating, so all quoted efficiencies should be increased by a factor 1.08. All curves are approximate, with a 25mm aperture used to test the efficiencies. The different curves for each grating correspond to different grating angles; users can select whatever grating angle is most suitable for their observations. Note that the test may have been done with the grating (and hence slant angle) reversed with respect to the grating calculator. Note that altering the grating angle also has a 2nd order effect on resolution and wavelength coverage; this becomes significant at high dispersion.

Low resolution	Medium resolution	High resolution
580V 385R	1700B 1500V (note angles +90deg) 1000R 1000I	3200B 2500V 2000R 1700I 1700D

An note of Blaze for the VPH gratings

The Volume-Phase Holographic (VPH) transmission gratings used with AAOmega have a number of interesting properties. The one that many user will be unfamiliar with is the flexible blaze angle. Each grating has a specific design blaze angle which will give the absolute maximum efficiency with that grating (the super blaze). This peak efficiency will then role off smoothly with wavelength away from that which corresponds to the blaze angle. The usual setup for most programs is therefore to have the grating set at it's super blaze angle and the camera at twice this angle to center the maximum efficiency wavelength on the CCD. the complication comes when the observer wishes to observe at an central wavelength which is some distance away from the super blaze angle for the grating. This would mean observing with the grating and camera angles highly asymmetric, and therefor operating on the low efficiency (and rapidly falling) part of the blaze envelope for the grating. The solution is to tune the grating and camera angles to new values. This shift in the grating angle will shift the blaze profile away from the super blaze, flattening the steep wings of the super blaze envelope and boosting system performance at the desired wavelength(s), with the expense of a slight reduction in overall peak performance in comparison to the super blaze setting.

Rob Sharp (rgs@aao.gov.au)