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

don’t overlap Have sufficient sky either side of the object in each slit

Multi-slits in the 4m era (1980s) Cryocam (KPNO); LDSS, LDSS++ (AAT) ; EFOSC (NTT/ESO) 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)

Telescope

Focal Plane

Slit

Spectrograph

Spectrographcollimator

Dispersing element

camera

detector

Figure 3.1

Multislit spectroscopy

• Example of multislit spectrometer

• Easier to achieve at telescope (can use holes in a mask) but preparation and reduction more complex

• Need to ensure spectra don’t overlap

LDSS-2mask

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 similar.

Mask optimization is NOT trivial!

Field acquisition is NOT trivial

Laser Cutting Machineat 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 powerful.

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 spectraCorrector +

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 inExternal 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”

Flux can be used to measure distances to E-Galaxies

Radial motion of PNe can be used to map the dynamical motions within an E-Galaxies V. difficult otherwise since need high SNR

continuum spectra of faint outer envelope With PNe can trace mass and “shape” of E-

Galaxies How?

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 background: Same detection threshold as

for direct imaging Now the position of the PNe

in the slitless spectrograph image is a function of:

its “native” position in the sky, modified by …

its velocity – its position is deflected in the dispersion dirn

But, we don’t know its “native” position, so we don’t know its velocity – right?

How do we disentangle the two parameters?

Easy … take 2 slitless spectral images with the dispersion dirn reversed

Now a PNe detection requires searching for image pairs:• Pair separation = Velocity• Pair centre = Position• Pair flux gives distance

PN Spectrograph Images

y

x

x

y

y-slice through PN

y-slice through subtracted pair of images

Narrow-bandImage [OIII]

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!

PNe

PNe