ASTR 511 (O'Connell) Lecture Notes
STATE OF THE ART SPECTROSCOPY
Two Degree Field Fiber-Fed Spectrograph
on the AAT
we discussed the design of optical spectrographs. Here, we show
some examples of state-of-the-art spectrographs, which come in a wide
variety of forms. Classical spectrograph designs employed circular or
long-slit entrance apertures, intended for a single target or a 1-D
slice through a single extended object. But this is inefficient in
the case of fields containing many comparable targets, and modern
designs usually feature multiple entrance apertures to make full use
of the spectrograph's focal plane.
Multi-object spectrographs are among the most sophisticated and
expensive instrumentation used by astronomers today, costing upwards
of $10M on a large ground-based telescope. Their primary application
has been to low-resolution spectroscopy of large samples of
. At the other end
of the design regime, the very high resolution, single-object
spectrographs have been essential to the discovery and exploration of
FIBER-FED MULTI-OBJECT SPECTROGRAPHS
Fiber-fed spectrographs use bundles of optical fibers to transfer
from arbitrary positions in the focal plane to the input of
the spectrograph. With care to minimize attenuation, fiber lengths
can be up to 10's of meters -- in order to feed a bench spectrograph
external to the telescope, for instance. Fiber systems typically
offer small (2-4 arcsec) entrance apertures, with a single fiber
assigned to each target. There is an "exclusion zone"
fiber, within which another fiber cannot be placed. 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. The Sloan
Digital Sky Survey (SDSS) uses specially-drilled "plug plates," into
which each fiber is inserted by hand. 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.
- 2dF (AAT 4-m)
Prime focus instrument (see
picture at top of page). Two spectrographs with 7 selectable gratings
each for R = 500/2000 observations in the 3700-9000 Å range.
Tektronix 1024x1024 CCD detectors.
2 degree FOV, requiring a large corrector lens; a prism compensator is
used to minimize light loss in fibers due to atmospheric dispersion at
larger zenith angles.
400 fibers per field; magnetically
attached to a field plate; positioned by a robot; 200 to each
spectrograph. Minimum fiber separation 30 arcsec.
Two fiber focal plane units (one undergoing fiber positioning while other
observes; flip to replace)
Two moderate resolution (R ~ 2000) spectrographs. Separate red and
blue light cameras and detectors in each; incoming beam is split by a
diagonal dichroic mirror (red light transmitted, blue reflected).
2048x2048 CCD detectors. Wavelength coverage: 3800-9200 Å.
640 fibers (320 for each spectrograph),
manually plugged into precision
drilled aperture plate covering a 3 degree FOV. Minimum fiber
separation is 55 arcsec. Fiber diameter on sky: 3 arcsec.
20-fiber cartridge, SDSS
APOGEE Spectrograph (2.5-m)
UVa-designed, bench-mounted, R ~ 22,500, infrared (1.5-1.7 µ)
spectrograph; 3-segment mosaic VPH grating; 3 2048x2048 HgCdTe
detectors. Intended to determine temperatures, gravities, and
15 elemental abundances in a large sample (over 150,000) of Galactic
stars with minimal interference from extinction from interstellar
dust (which is small in the IR).
Infrared operation requires a cryogenically-cooled, evacuated
enclosure; significantly complicates design.
300 40-m long fibers run from plugplates at the SDSS telescope
Cassegrain focus to a separate building holding the spectrograph.
APERTURE PLATE MULTI-OBJECT SPECTROGRAPHS
Aperture-plate spectrographs use small apertures in a focal-plane
at the spectrograph entrance to transmit light from selected
targets. Usually, computer-controlled devices (mechanical cutters,
lasers) are used to cut apertures in a thin, shaped, metallic mask.
Early designs used photographic masks. The apertures can be of
arbitrary shape and length within overall constraints set by the
spectrograph focal plane, but they normally have small widths in the
direction of spectral dispersion. The distribution of spectra in the
focal plane depends on the distribution of targets in the field.
The 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
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.
Telescope image is focussed on
an array of microlenses
which break up the 40x33 arcsec field into 1 arcsec images.
Grism disperser. Spectral range limited to 4800-5400 Å.
Slight rotation of dispersion direction with respect to lenslet array
achieves cross-dispersion so spectra don't overlap;
spectral length is limited by using an interference filter
Intended for areal studies (kinematics, populations, emission lines,
etc) of nearby galaxies. Example SAURON
mass-to-light ratios and Fundamental Plane for early-type
Uses the upgraded SDSS-III
BOSS spectrographs (3600-10300 Å).
For each field, can deploy up to 17 fiber-based hexagonal IFU units,
in sizes (see above) ranging from 19 up to 127 fibers (12-32" field
diameters; 2" fiber cores), across the 3o field. 1423
fibers total. For more information on the MaNGA fiber system, see
Intended for areal spectral studies of up to 10,000 galaxies
- JSWT NIRSpec
GRISM FULL-FIELD SLITLESS SPECTROSCOPY
A "grism" is a prism with a transmission grating mounted on its
entrance face. This combination produces a series of spectra of each
source in the focal plane, with the first and second orders of most
interest. There is also a zeroth-order image of each source, which
provides a means to determine the order positions and wavelength
scales for that source. Grisms are used only for low resolution
Grisms can be deployed in the same way as a filter, without a
collimator, ahead of the focal plane of an imaging camera. Grism
spectrographs are normally operated "slitlessly," without focal-plane
apertures, so they image the entire field. This captures spectra of
any object in the field
, which is their primary advantage. The main
operation/data-reduction complication is the overlap
of spectra from
different sources or even different parts of the same extended source.
To mitigate overlap, spectra are typically taken with the spectrograph
oriented in multiple (say up to 5) different position angles on the
sky. Even where there is no object overlap, the superposed spectrum of
the background sky limits signal-to-noise. Data reduction pipelines
are complex (e.g. see
HST Wide Field Camera 3 IR Grism Spectroscopy
WFC3 carries two IR grisms, with wavelength ranges of 8000-11500 Å and
10750-17000 Å, respectively. Corresponding resolutions are
210 and 130. The imaging field is about 130 arcsec square. The grism
mode is mainly used to obtain spectral energy distributions of high redshift
sources, where the information-rich UV/optical spectrum is shifted to the
near-IR. A sample data frame with high redshift emission line sources
identified is shown at the right. An important advantage of IR grisms used
on space telescopes is that they avoid the tremendously bright IR night
sky emission lines in the Earth's atmosphere. JWST will also carry
WFC3 IR Grism Frame
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
, where velocity differences of order 5 m/s
measured. Requires both high spectral resolution and great
mechanical/optical stability. Suitable designs employing digital
detectors have been around since the late 1970's (e.g.
) 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
- 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 (see
sample at right). The 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
Users must perform a cross-correlation analysis on a large number of
spectral segments of the star+iodine spectrum covering ~ 800 Å
to determine the wavelength shift of a target star
SNR ~ 200 in flux yields a velocity precision ~ 3 m/s. Since
world-class athletes can achieve ~ 10 m/s, we can now detect stars
moving at a human pace.
Sample Hi-Res Iodine Cell Spectrum
To date, most
have been first identified by the transit eclipse
method, especially from the
in space. However,
high resolution spectroscopy remains essential for verifying the
identifications using the Doppler technique and for determining the
physical characteristics of the planets and their parent stars.
November 2020 by rwo
Grism data frame from Pat McCarthy. Text copyright © 2001-2020
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.