Multi-slit spectroscopy

Multi-slit spectroscopy
In sky-noise dominated conditions (most interesting!) the use of slits
is essential:
eg: Faint object, extra-galactic, surveys:
Same instrument can do both imaging and spectroscopy – hence
multi-slit spectroscopy is “easy” (in principle).
Once you have a target list you have to design the mask (ie: select
the subset of objects you are going to observe spectroscopically) so
that the spectra
Multi-slits in the 4m era (1980s)
don’t overlap
Have sufficient sky either side of the object in each slit
and more recently LDSS-2 (WHT & Magellan)
Multi-slits in the 8-10m era (1990s)
LRIS (Keck) ; FORS (VLT) ; GMOS (Gemini)
and more recently: DEIMOS (Keck) ; VMOS (VLT)
IMACS mask (c2003)
Focal Plane
Dispersing element
Figure 3.1
Multislit spectroscopy
• Example of multislit
• Easier to achieve at
telescope (can use
holes in a mask) but
preparation and
reduction more
• Need to ensure
spectra don’t overlap
mask superimposed
on sky image
Great care has to be taken in
selecting objects to study so
that they don’t overlap in
wavelength direction.
Also need objects of similar
brightness so the SNRs are
Mask optimization is NOT
Field acquisition is NOT
Laser Cutting Machine
at Gemini
IMACS curved slit masks
IMACS spectrograph
(Magellan Telescope)
Multi-Object Spectroscopy
(what could be simpler than ….)
Slitless spectroscopy:
 Point-like sources (eg: stars, distant galaxies etc.) can
be observed spectroscopically without the use of a slit
to define their input aperture.
 In this case d is defined by the dispersive power
combined with the instrinsic (angular) size of the
object (usually defined by the seeing).
 Thus an imaging system incorperating a dispersive
element can, in principle, give the spectra of all
objects within the field. This can be enormously
Slitless Spectroscopy
Classic Example:
 Objective Prism on the UK Schmidt Telescope (full
aperture). Spectra of all objects (105 – 106) above sky
background can be obtained over the full 6º field of view.
But …
While light from point source is dispersed (and hence has lower
flux density), light from background remains the same. Contrast
between object and sky is reduced. This is why you need slits!
Spectra overlap in -direction (try rotating the prism by 90º)
Both argue for ultra-low dispersion and resolution
Palomar Schmidt system gives <1,000Å/mm (R ~200 at best) –
crude IDs and red-shifts of bright galaxies
Picture of Palomar Schmidt
prism with sample spectra
Corrector +
Objective Prisms
(full aperture)
Primary Mirror
Special case of slit-less spectroscopy
(emission-line point sources)
Emission-line point sources don’t overlap (in
general). Examples:
L galaxies at high redshift
Distant HII galaxies, HII regions, H-H Objects …
Classic case:
Planetary Nebulae
PNe are point sources?
So what about PNe in
External Galaxies?
Certainly emission line sources
 Also distant enough to be point sources
E-galaxy with kinematic
mass tracers
Globular Clusters?
Planetary Nebulae?
Detection of PNe in E-Galaxies and
their use as kinematic probes
PNe are very effective “standard candles”
Radial motion of PNe can be used to map
the dynamical motions within an E-Galaxies
Flux can be used to measure distances to EGalaxies
V. difficult otherwise since need high SNR
continuum spectra of faint outer envelope
With PNe can trace mass and “shape” of EGalaxies
PNe are point-like
PNe spectra are dominated by pure narrow
emission-lines which are easily contrasted
against the sky background
PNS optical configuration
PNS on the WHT (c2000)
The PN spectrograph
(How does it really work?)
Taylor and Douglas (1995)
Use slitless spectroscopy to detect
emission-line sources above the sky
Same detection threshold as for direct
Now the position of the PNe in the
slitless spectrograph image is a
function of:
But, we don’t know its “native”
position, so we don’t know its velocity
– right?
its “native” position in the sky,
modified by …
its velocity – its position
is deflected
in the dispersion dir
How do we disentangle the two
Easy … take 2 slitless spectral images
with the dispersion dirn reversed
Now a PNe detection
requires searching for image
• Pair separation = Velocity
• Pair centre = Position
• Pair flux gives distance
PN Spectrograph Images
Image [OIII]
y-slice through PN
y-slice through subtracted pair of images
Differencing image pair gives characteristic PNe signature. Can use optimized
pattern recognition software to pull out PNe signatures to determine
• (x0,y0)n ; PNe flux ; PNe systemic velocity
It works!