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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