Two Degree Field Fiber Spectrograph on AAT
(J. Pogson)
Multi-beam spectrographs are among the most sophisticated and expensive
instrumentation used by astronomers today, costing upwards of $5M on a
large telescope. Here are some state-of-the-art examples.
The primary application of these systems has been to large samples of
faint galaxies.
FIBER-FED MULTI-OBJECT INSTRUMENTS
Fiber-optic transfer devices typically offer small (2-4 arcsec) entrance
apertures for each target. Fibers must be repositioned with high precision for each new
field. This is usually done by mechanical robots. In most designs,
individual fiber apertures are clamped magnetically on a flat focal-plane plate.
Output of fiber unit is usually a linear (slit-like) array at spectrograph
input.
Prime focus instrument; 2 spectrographs (R ~ 1000)
2 degree FOV
400 fibers per field; magnetically attached to field plate; positioned by robot;
200 to each spectrograph
Two fiber focal plane units (one positioning while other observes; flip to replace)
Aperture-plate spectrographs use small apertures in a focal-plane
mask at the spectrograph entrance to transmit light from selected
targets. Usually, computer-controlled devices (mechanical cutters, lasers) used to
cut slits in a thin, shaped, metallic mask.
Early designs used photographic masks. The slits can be of
arbitrary shape and length within overall constraints set by the
spectrograph focal plane.
Main operational problem is to avoid overlap of spectra and to maximize
use of the detector area in a given field; this requires
special optimizing software. In principle, aperture plate designs
should have better throughput, better sky background subtraction, and
better flux calibration than fiber designs. Fiber designs can
accommodate more targets, however (because the output format on the
detector is independent of the spatial distribution of targets).
An integral field unit (IFU) produces distinct spectra for many contiguous elements in
a given field. Powerful for the study of extended objects like globular clusters or
nearby galaxies. Aperture positions and sizes are fixed and generally cover
a square/retangular area. IFU's have been designed using fiber bundles, lenslet
arrays, and configurable microaperture or micromirror arrays.
Design
Telescope image is focussed on array of
microlenses which break up 40x33 arcsec field into 1 arcsec
images.
Grism disperser
Slight rotation of dispersion direction wrt lenslet array achieves
cross-dispersion so spectra (if short) don't overlap; limit spectral
length using IF filter
Example SAURON science:
kinematics and absorption line maps in early-type galaxies
HIGH PRECISION DOPPLER SHIFT SPECTROSCOPY (Planet Detection)
The most conspicuous use of high precision spectroscopy has been in the
detection of extra-solar planets through stellar reflex Doppler
shifts, where velocity differences of order 5 m/s must be
measured. Requires both high spectral resolution and great
mechanical/optical stability in spectrograph. Suitable designs
employing digital detectors have been around for > 20 years, but were
not energetically exploited until the surprising detection of a Jupiter mass
planet in a sub-AU orbit made in 1995 by
Mayor & Queloz at Haute-Provence
Observatory.
Cross-dispersed echelle spectrometer, R ~ 60000
Use a gaseous iodine cell at entrance slit to impress a calibration
absorption spectrum on each stellar spectrum taken. Dense molecular
spectrum yields ~10
wavelength standard lines per Å
Wavelength standard passes through optics in exactly same way as stellar
light and simultaneously with it
Perform cross-correlation analysis on large number of spectral
segments of star+iodine spectrum covering ~ 800 Å to
determine wavelength shift of star
SNR ~ 200 in flux yields velocity precision ~ 3 m/s. Since world-class athletes
can achieve ~ 10 m/s, we can now detect stars moving at a human pace.
NB: 3 m/s precision corresponds to effective Doppler shift resolution
of ~ c/(3 m/s) = 108!
All exoplanets to date have been detected by the reflex Doppler technique,
but it is anticipated that many more will be found by transit eclipses in
the future
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