oct 26, 2007salt workshop ukzn1 robert stobie prime focus imaging spectrograph science rationale...

Oct 26, 2007 SALT Workshop UKZN 1

Robert StobiePrime Focus Imaging

Spectrograph

• Science Rationale• Modes

– Fabry-Perot Spectral Imaging

– Grating Spectroscopy; VPH Gratings, MOS

– Spectropolarimetry

– Mode table

• Status

Oct 26, 2007 SALT Workshop UKZN 2

What science should RSS do?

• SALT features– very large aperture

– good field of view (8 arcmin)

– excellent ultraviolet throughput

– queue scheduling

– modest imaging

• RSS: emphasize unique observing modes that do not require excellent imaging– UV spectroscopy down to 3200 Ǻ (rare on large telescopes)

– higher resolution: high-throughput grating 3200 - 9000 Ǻ; Fabry-Perot 4300 - 9000 Ǻ (Visible Beam); later - 1.7 μm (NIR)

– polarimetry (circular and linear) (very rare)

– high-speed detector mode (very rare)

Oct 26, 2007 SALT Workshop UKZN 3

Optics

• All-refractive UV optics for high throughput• At prime focus for UV and full-field access• NIR upgrade path: simultaneous 3200 Ǻ – 1.7 μ

C o llim a to rVis ib le C am era

N IR B eam (fu tu re )

F o ca l p lan e

A rticu la tio nA x is

F o ld / D ich ro icF o ld

Oct 26, 2007 SALT Workshop UKZN 4

Mechanisms

2 Fabry-Perot Etalons (slides)

Polarizing Beamsplitter

(slide)

2 Polarimeter Waveplates (slides)

6-Grating Magazine

40-Slitmask Magazine

Camera Articulation 0 - 100°

Shutter

20-Filter Magazine

3 CCD Mosaic

Detector

Oct 26, 2007 SALT Workshop UKZN 5

RSS on SALT

Articulation Rail

Camera

Detector

Etalons

Collimator

Oct 26, 2007 SALT Workshop UKZN 6

Perseus Cluster of Galaxies

Perseus A in Hydrogen Light

Imaging/ Fabry-Perot Spectroscopy

Filters

Fabry-Perot etalons(scans wavelength) M

any

wavele

ngths

Oct 26, 2007 SALT Workshop UKZN 7

Hubble Deep Field

(South)

Up to 40 Spectra

Slitmasksselects objects

Filters

"VPH" Grating disperses wavelengths

Multi-Object Grating Spectroscopy

Oct 26, 2007 SALT Workshop UKZN 8

Hubble Deep Field (South)

Waveplate rotates polarization

Beamsplitter splits 2 polarizations

Polarization of 20 Spectra

Spectropolarimetry Submode

Also works in Imaging/ Fabry-Perot Modes

Oct 26, 2007 SALT Workshop UKZN 9

High Efficiency with Volume Phase Holographic Gratings

R

6 0 0 0

4 0 0 0

2 0 0 0

3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0W av e len g th (n m )

[O II] H β [O III] H α [S II] C a II

3 0 0 0 l/m m2 3 0 0 l/m m

1 8 0 0 l/m m

1 3 0 0 l/m m

9 0 0 l/m m

1 5 °

3 0 ° 3 0 ° 3 0 °2 5 °

4 0 ° 4 0 °4 0 °

3 5 °

5 0 ° 5 0 ° 5 0 °

= λ/Δλ

Resolution vs wavelength for 1.2 arcsec slit

Constant Efficiency Contours

Oct 26, 2007 SALT Workshop UKZN 10

How VPH Gratings Work

• Created by exposing holograph material (“DiChromated Gelatin") to interference pattern from laser

• Index of refraction of DCG is modulated in space• Large; inexpensive; custom design; efficient at high groove density

{ 17 {

{ 30 {

VPH G ratings. F rom Barden, e t a l, 2000, PASP

Astronom ical G ratings

Oct 26, 2007 SALT Workshop UKZN 11

One RSS VPH

1300 gr/ mm VPH

Oct 26, 2007 SALT Workshop UKZN 12

Multi-Object Spectroscopy with custom masks

• Laser-cut slit masks• Standard set of longslits• Unique mask cut for each individual

MOS observation• Slit mask magazine will hold 10

longslits, 30 MOS masks• Reload once/ week

Giant Elliptical Galaxy NGC1399

at focus of telescope:

Mask with slits cut in it to let through selected nebulae in galaxy

"Longslit" mask to get spectrum along one position angle

Oct 26, 2007 SALT Workshop UKZN 13

MOS Test Field

Courtesy: Petri Vaisanen

Oct 26, 2007 SALT Workshop UKZN 14

NGC6822 – Oct 20

4080 A 7180 A

MOS Test Run Spectra

Position Reference Stars

HII and PNe

Oct 26, 2007 SALT Workshop UKZN 15

17 PFIS ModesOptics configuration Detector Configuration

Config Pol Slit Normal Hi Spd Shuffle Drift

Imaging No No X x x

L,C,S No x

Multislit X x

VPH Spectro- scopy

No No x x

Longslit X X x

Multislit X X

L,C,S Longslit X x

Multislit X

Fabry- Perot

No No X x

L,C,S No xTried: 6 Team Commissioned: 9 User Commissioned: 8

Oct 26, 2007 SALT Workshop UKZN 16

0

5

10

15

20

25

3000 4000 5000 6000 7000 8000 9000

Wavelength (Ang)

e/se

c n

m f

or

AB

=20

0

5

10

15

20

25

3000 4000 5000 6000 7000 8000 9000

Wavelength (Ang)

e/se

c n

m f

or

AB

=20

Status

• On telescope Nov 2005 - Oct 2006– longslit mode, polarimetry

commissioning

– other mode tests

– on-sky throughput tests

• Off telescope Nov 2006 - present– improving UV throughput, ghosts

– Measured throughput to be 75 - 100% of design, sensitivity comparable or better than design throughput of other large telescope spectrographs

• Back on telescope for further commissioning April 2008