Spectrographs: Integral Field Units
E
N C Y C L O P E D I A O F
A
S T R O N O M Y A N D
A
S T R O P H Y S I C S
slicing
mirror
image slice 3
image slice 2
image slice 1
field optics
to spectrograph collimator
imaging mirrors
from
pickup
mirrors
and
telescope
front view image plane
slice 1
slice 2
slice 3
image
plane
Figure 4.
Principle of the slicer IFU (from Content 1997).
system 2237 + 0305. An exposure taken with the IFU
TIGER (lenslet type) gave 400 spectra covering the central
part of the galaxy with a 0.4 arcsec sampling. The spectra
(figure 5, panel A) at the four quasar image locations show
a mixture of galactic light and quasar light. One can see
indeed the strong line emission of C III] 1909 Å redshifted
to 5200 Å as well as stellar absorption lines typical of
a galaxy. Integrating the flux over the whole spectral
range for each spectrum allows the reconstruction of the
equivalent of a direct image within that wavelength range.
This image (panel B) shows the well-known cross shape
of this object with five maxima, the central one being the
nucleus of the galaxy. This image is obviously dominated
by the continuum, which is an unknown combination of
the quasar and galaxy fluxes. However, it is quite easy to
properly subtract the continuum independently for each
spectrum. The continuum-free C III] reconstructed image,
displayed in panel C, is now free from most of the galactic
contribution and it no longer shows the galaxy nucleus.
Using simultaneously spectral and spatial information,
it was thus possible, in this example, to disentangle the
contributions of the quasar and of the galaxy (Fitte and
Adam 1994).
Figure 5.
IFU study of the Einstein cross. Panel A: spectrum at
one of the quaser image locations. Panel B: reconstructed image
with the whole spectral range. Panel C: reconstructed image of
the continuum-free C III] emission.
Prospects
Integral field spectrographs in operation are not very
common yet, but a number are being designed for the
new 8 m class telescopes. In parallel with multiobject
spectrographs, which are ideally suited to the study of
large collections of objects, IFUs will become more and
more popular for the detailed study of individual objects,
or alternatively for very dense fields such as
GLOBULAR
CLUSTERS
.
The success of adaptive focus on ground-based
telescopes and the launch of large-aperture space
telescopes has emphasized the need for high spatial
resolution spectroscopy. Decreasing the slit width is no
longer a valid solution and IFU capabilities then become
essential to understand the physics and the complex
morphology generally displayed by astronomical sources
at high spatial resolution.
Recently the first three-dimensional detector, with
direct measurement of the photon energy, has been tested
on a telescope (Peacock et al 1998). These superconducting
photon-counting cameras have an excellent quantum
efficiency over a broad wavelength range (UV to IR).
Although their spectral resolutions are still very modest
(
20) and their number of pixels quite small (8 × 8),
they have the potential to replace most of the existing
spectrographs in the future.
Bibliography
Afanasiev V L, Vlasiouk V V, and Green R F 1995 ASP Conf.
Ser. 71 p 266
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