ASTR 511 (O'Connell) Lecture Notes
STATE OF THE ART SPECTROSCOPY
Two Degree Field Fiber Spectrograph on AAT
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
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 the fiber unit is
usually a linear (slit-like) array at the spectrograph input, yielding
a fixed position on the focal plane for each spectrum.
- SDSS-I Spectrograph (2.5-m)
Two dual spectrographs (R ~ 2000). Red/blue light split in each by
diagonal dichroic mirror [red light transmitted, blue reflected]
640 fibers (320 each spectrograph) manually plugged into precision
drilled aperture plate covering 3 degree FOV
APERTURE PLATE MULTI-OBJECT INSTRUMENTS
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) are
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. The distribution of spectra in the focal plane depends on
the distribution of targets in the field.
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 fixed and optimally packed).
INTEGRAL FIELD UNITS
An integral field unit (IFU) produces distinct spectra for many
contiguous elements in a given compact field. Powerful for the study
of extended objects like globular clusters or nearby galaxies.
Relative 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.
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 since the late 1970's 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
- Marcy-Butler Technique
Cross-dispersed echelle spectrometer, R ~ 60000
Employs a gaseous iodine cell at the entrance slit to impress a calibration
absorption spectrum on each stellar spectrum taken. Dense molecular
spectrum yields ~10
wavelength standard lines per Å
The calibration signal passes through the optics in exactly the same
way as stellar light and simultaneously with it
Must perform a cross-correlation analysis on a large number of spectral
segments of star+iodine spectrum covering ~ 800 Å to
determine the wavelength shift of target 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!
- Most exoplanets to date have been first identified by the reflex
Doppler technique, but it is anticipated that many more will be found
by transit eclipses in the future. However, spectroscopy will remain
essential for determining the physical characteristics of planets
and their parent stars.
August 2010 by rwo
Text copyright © 2001-2010 Robert W. O'Connell.
All rights reserved. These notes are intended for the private,
noncommercial use of students enrolled in Astronomy 511 at the
University of Virginia.