1. Concepts for a high resolution multi-object spectrograph for galactic archeology on the Anglo-Australian Telescope
Samuel C. Bardena, Joss Bland-Hawthornb, Vladimir Churilova, Simon Ellisa, Tony Farrella, Ken C. Freemanc, Roger Haynesa, Anthony Hortona, Damien J. Jonesd, Greg Knighte, Stan Miziarskia, William Rambolda, Greg Smitha, Lew Wallera
aAnglo-Australian Observatory; bThe University of Sydney; cMount Stromlo Observatory; dPrime Optics; eSinclair Knight Merz
* phone ; fax ; web
The activities for this project are funded by the Commonwealth of Australia through an agreement between the Department of Innovation, Industry, Science and Research (DIISR) and Astronomy Australia Ltd. (AAL) as an initiative of the Australian Government being conducted as part of the National Collaborative Research Infrastructure Strategy (NCRIS).
Table 1: HERMES Requirements
Item
Mode
Requirement
Comment
Field of View
Both
2 degrees
FoV and simultaneous targets per exposure are good match to existing 2dF facility.
Number of Objects
High Res
Very High Res
~400
~40
Spectral Resolving Power
30,000 nominal
40,000 to 50,000
λ/Δλ FWHM
Simultaneous λ Coverage
~400 Å
~2000 Å
Preferably in two comparable swaths
Operable λ Range Å
Non-simultaneous
Sensitivity
SNR=100 in 60 minutes at V=14
SNR is per resolution element
Science Objectives
The High Resolution Multi-Object Echelle Spectrograph (HERMES) is an instrument for a massive survey of 1.2 million stars in the Milky Way Galaxy over 10,000 square degrees on the sky to measure chemical abundances and kinematics. Such a survey will reveal families of stars that formed together, but were then dispersed by the gravitational well of the Galaxy. Tracing the kinematic behavior of each stellar family will yield insight into the formation processes that created the present day structure of the Milky Way [1],[2],[3].
HERMES is also expected to serve as a general purpose, facility class instrument. The fundamental requirements are shown in Table 1.
Design
The HERMES concept is based upon:
Use of existing 2dF positioner and fibers
Implementation of White Pupil Echelle
Utilization of existing AAOmega hardware
Instrument Modes
Low Resolution (Classical AAOmega)
Uses existing set of gratings to achieve existing AAOmega functionality
High Resolution
Single order echelle spectra in four channels with R=30,000
Very High Resolution
Cross-dispersed few order echelle spectra in two channels with R=~50,000
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Table 2: Design parameters
Number of Channels (Cameras) 4
Collimator Type
Houghton derivative White Pupil
Collimator Focal Ratio
9.0
Fiber Relay
f/3.15 to f/9.0
Camera Types
2 Schmidt
2 Transmissive
Camera Focal Ratio
1.3
CCD pixel size 15
microns
CCD format spectral
2048
pixels
CCD format spatial
4096
Fibre Sampling
3.8
Dispersing Element
echelle
Grating Frequency
110
l/mm
Grating Blaze Angle 71
degrees
Design Options
The following design options were considered:
Dual VPH Grating in AAOmega [4],[5],[6]
Double pass on grating to double dispersing power
Likely difficult to produce gratings
Classical non-white pupil echelle
Similar to Hectochelle [8],[9],[10],[11]
Likely cumbersome instrument for required wavelength coverage
Anamorphic collimator with VPH Grating
Stretch of beam to allow lower dispersion grating [12]
White pupil echelle
Implementation of white pupil collimator [13],[14] that allows possible utilization of existing AAOmega hardware
Preferred option – allows relatively easy multi-channel implementation for wavelength coverage
The background image is the Milky Way in Sagittarius taken from the AAT
© , Anglo-Australian Observatory, photograph by David Malin.
Table 3: Primary grating orders for High Resolution mode.
Spectral Order: m (λc)
30 (5730 Å)
28 (6140 Å)
26 (6612 Å)
20 (8596 Å)
Total
Detector Coverage
118 Å
127 Å
137 Å
178 Å
560 Å
HERMES configured for high dispersion mode.
Slit Curvature
The HERMES slit must be curved to counter second order dispersion along the slit length [15].
Slit to be curved as shown here.
Detected λ coverage non-uniform with straight slit.
Detected λ coverage uniform with curved slit.
HERMES configured for low dispersion mode (classical AAOmega mode).
Timeline
HERMES is a funded project and is expected to be commissioned in early 2012.
[9] Fabricant, D. G., Szentgyorgyi, A. H., and Epps, H. W., “Segmented Zero-Deviation Cross-Dispersion Prisms for the Hectochelle Multiobject Spectrograph”, Pub. Astron. Soc. Pacific, 115 (804), (2003).
[10] Cheimets, P., Szentgyorgyi, A. H., and Pieri, M. R., “Mosaicked echelle grating support for the Hectochelle fibre-fed spectrograph”, Proc. SPIE, 3355, (1998).
[11] Szentgyorgyi, A. H., Cheimets, P., Eng, R., Fabricant, D. G., Geary, J. C., Hartmann, L., Pieri, M. R., and Roll, J. B., “Hectochelle: a multiobject echelle spectrograph for the converted MMT”, Proc. SPIE, 3355, (1998).
[12] Spano, P., Zerbi, F. M., Norrie, C. J., Cunningham, C. R., Strassmeier, K. G., Bianco, A., Blanche, P. A., Bougoin, M., Ghigo, M., Hartmann, P., Zago, L., Atad-Ettedgui, E., Delabre, B., Dekker, H., Melozzi, M., Snyders, B., and Takke, R., “Challenges in optics for Extremely Large Telescope instrumentation”, Astron. Nachr., 327 (7), (2006).
[13] Baranne, A. and Duchesne, M., “Le spectrographe coudé "Echel.E.C.152" (Montage à pupille blanche pour caméra électronique)”, Proc. ESO/CERN Conf. on Auxiliary Instrum. for Large Telescopes, (1972).
[14] Baranne, A., “White Pupil Story or Evolution of a Spectrographic Mounting”, Proc. ESO Conf. on Very Large Telescopes and their Instrumentation, ed. Ulrich, 1195 (1988).
[15] Loewen, E. G., and Popov, E., [Diffraction Gratings and Applications], Marcel Dekker, New York (1997).
REFERENCES
[1] Bland-Hawthorn, J. and Freeman, K. C., “Galactic history: formation & evolution”, Memorie della Societa Astronomica Italiana, 77, (2006).
[2] Bland-Hawthorn, J. and Freeman, K. C., “Galaxy Genesis – Unravelling the Epoch of Dissipation in the Early Disk”, Pub. Astron. Soc. Australia, 21(2), (2004).
[3] Freeman, K. and Bland-Hawthorn, J., “The New Galaxy: Signatures of Its Formation”, Ann. Rev. Astron. and Astrophys., 40, (2002).
[4] Sharp, R., Saunders, W., Smith, G., Churilov, V., Correll, D., Dawson, J., Farrell, T., Frost, G., Haynes, R., Heald, R., Lankshear, Al, Mayfield, D., Waller, L., and Whittard, D., “Performance of AAOmega: the AAT multi-purpose fibre-fed spectrograph”, Proc. SPIE, 6269, 62690G-1-13 (2006).
[5] Smith, G. A., Saunders, W., Bridges, T., Churilov, V., Lankshear, A., Dawson, J., Correll, D., Waller, L., Haynes, R., and Frost, G., “AAOmega: a multipurpose fibre-fed spectrograph for the AAT”, Proc. SPIE, 5492, (2004).
[6] Saunders, W., Bridges, T., Gillingham, P., Haynes, R., Smith, G. A., Whittard, J. D., Churilov, V., Lankshear, A., Croom, S., Jones, D., and Boshuizen, C., “AAOmega: a scientific and optical overview”, Proc. SPIE, 5492, (2004).
[7] Szentgyorgyi, A., “Hectochelle: High resolution multiobject spectroscopy at the MMT”, New Astron. Rev., 50 (4-5), (2006).
[8] Szentgyorgyi, A. H., Fabricant, D. G., Brown, W. R., and Epps, H. W., “Cross dispersion and an integral field unit for Hectochelle”, Proc. SPIE, 4841, (2003).
Wavelength Coverage
Wavelength vs diffraction angle for 110 l/mm 71 degree echelle. Detector boundaries indicated for nominal grating tilt. The top priority spectral orders are indicated in the colored boxes.