Early Commissioning Results from the SALT RSS Spectropolarimeter
Kenneth Nordsieck*a
* [email protected] a Department of Astronomy, University of
Wisconsin-Madison, 475 N. Charter Street, Madison WI 53706-1582
ABSTRACT The Robert Stobie Spectrograph is currently the prime
spectroscopic instrument on the 11m Southern African Large
Telescope (SALT), which is now in commissioning. The current
visible beam of RSS was designed with spectropolarimetric versions
of all its modes, imaging, grating spectroscopy (with Multi Object
Spectroscopy capability) from 3200 to 9000 Å, and Fabry-Perot
Spectroscopy from 4300 to 9000 Å, with spectral resolution up to R
= 10000. The polarimetric field of view is 4 × 8 arcmin. The
instrument was designed with an upgrade path leading to a
simultaneous Near InfraRed beam (out to 1.7μ), now in development,
which will duplicate the spectropolarimetric modes in the NIR. We
will describe results from early polarimetric commissioning of the
visible beam.
THE ROBERT STOBIE SPECTROGRAPH
COMMISSIONING
Filters
CCD
Camera
Collim
ator
Focal Plane
Collimated Beam
TripletSilica/NaCl/Silica
QuadrupletSilica/CaF /
Silica/CaF
2
2
DoubletSilica/CaF2
TripletCaF /
NaCl/CaF
2
2
Field LensSilica/CaF2
Beamsplitter
Grating/FP etalons
Shutter
Waveplates T Pyxidis: a convenient Commissioning Lamp
The Robert Stobie Spectrograph (RSS)1 is an imaging spectrograph
designed to occupy the Prime Focus Instrument Package of the
Southern African Large Telescope (SALT)2. The optical design of
RSS3 features - spectral range 320 - 900 nm - all-refractive
collimator and camera using fused silica, CaF2 and NaCl elements -
collimated beam diameter of 150 mm and field of view of eight
arcmin - low to medium resolution grating spectroscopy using one
conventional and five Volume Phase
Holographic (VPH) transmission gratings - an articulating camera
giving resolution R = 500 - 5500 (1.2" slit) and R up to 11,000
(0.6" slit) - a dual etalon Fabry-Perot mode for R = 320 - 9000
over the full FOV for wavelengths 450 - 900 nm - A simultaneous NIR
beam for 900 nm – 1.7 microns, with the fold flat replaced by a
dichroic, and which duplicates all
the visible modes, is under development5
- spectropolarimetry4 for both grating and Fabry-Perot modes,
using rotating quarter- and half- waveplates near the telescope
focal plane and a calcite mosaic beamsplitter in the collimated
space.
POLARIMETRIC OPTICS
REFERENCES
[1] Kobulnicky, H.A., Nordsieck, K.H., Burgh, E.B. Smith, M.P.,
Percival, J.W., Williams, T.B., and O'Donoghue, D., "Prime Focus
Imaging Spectrograph for the Southern African Large Telescope:
Operational Modes," Proc. SPIE 4841, 1634 (2003). *2+ Buckley,
D.A.H., Swart G.P., Meiring, J.G., “Completion and commissioning of
the Southern African Large Telescope”, Proc SPIE 6267, 62670Z
(2006). [3] Burgh, E. B., Nordsieck, K. H., Kobulnicky, H. A.,
Williams, T. B., O'Donoghue, D., Smith, M. P., and Percival, J. W.,
"The Prime Focus Imaging Spectrograph for the Southern African
Large Telescope: Optical Design", Proc SPIE 4841, 1463 (2002). [4]
Nordsieck, K.H., Jaehnig, K.P., Burgh, E.B., Kobulnicky, H.A.,
Percival, J.W., and Smith, M.P., "Instrumentation for
high-resolution spectropolarimetry in the visible and
far-ultraviolet." Proc SPIE 4843, 170 (2003). [5] Sheinis, Andrew
I.; Wolf, Marsha J.; Bershady, Matthew A.; Buckley, David A. H.;
Nordsieck, Kenneth H.; Williams, Ted B., “The NIR upgrade to the
SALT Robert Stobie Spectrograph”, Proc SPIE 6269, 153 (2006). [6]
Buckley, D. A. H.; Barnes, S. I.; Burgh, E. B.; Crawford, S.;
Cottrell, P. L.; Kniazev, A.; Nordsieck, K. H.; O'Donoghue, D.;
Rangwala, N.; Zánmar Sánchez, R.; Sharples, R. M.; Sheinis, A. I.;
Väisänen, P.; Williams, T. B., “Commissioning of the Southern
African Large Telescopes (SALT) first-generation instruments” Proc
SPIE 7014, 6 (2008). [7] Williams, R. 2011 Private
Communication
WAVEPLATES
BEAMSPLITTER
SN2006mr 300 l/mm
M30 unfiltered
Orion Neb Hα
T Pyx 20110519
0.0
0.5
1.0
1.5
Po
l (%
)
80
90
100
110
Po
sit
ion
An
gle
(d
eg
)P
ola
rized
Flu
x (
arb
un
its)
-1.0%
-0.5%
0.0%
0.5%
1.0%
-2.0% -1.5% -1.0% -0.5%
%U
% Q
0.0%
Wavelength (Ang)
O III Bowen
O III Bowen
H I
H I
N III Bowen
N III Bowen
N II
N II
1.8
1.0
0.2
Inte
nsit
y (
arb
un
its
)
3000 3500 4000 4500 5000 5500 6000 6500
0
On 14 April, 2011, (one week after lift of RSS onto the
telescope) the Recurrent
Nova T Pyxidis underwent its first outburst since 1967. With a
declination within
one degree of the SALT latitude, it was ideal for observation.
Being quite bright
(Vmax ~ 7 mag) it was ideal as a first test for RSS high S/N
modes:
spectropolarimetry, high-speed, high-resolution
spectroscopy.
20110519 Data (the rest has not yet been analyzed!)
Observational Details:
- Grating: 900 l/mm VPH grating, 1.5 arcsec slit, R = 800.
- Polarimetry: Four 8 waveplate position linear polarization
runs (minus the last
2 images, for 30 images), 1600 secs total.
- Calibration: Not flux calibrated (roughly straightened). 2006
polarimetric
efficiency and PA calibration, extrapolated to 3100 D. No
instrumental
polarization calibration yet (2006 data said ~0.2%)
Spectrum (top plot):
- Strong P-Cyg lines of HI (Balmer), He I, N II (Williams
(2011)7 He/N phase).
- Strong Bowen lines, esp OIII 3133 (!).
Spectropolarimetry (plots 2, 3):
- Very high S/N. Data above 4300 D is unbinned.
- H I, He I, N II P Cyg lines all show depolarization =>
intrinsic polarization.
(magnitude 0.6%). Likely electron scattering polarization in
distorted Nova
atmosphere (like in supernova polarization).
- Strong polarization features in Bowen fluorescence features.
Not seen before?
- Rough IS Polarization removal: remove P Cyg emission features
in net
polarized flux (plot 4, 5): 0.7% at PA within 5E of intrinsic.
Kind of small IS
polarization for supposed distance of 3 kpc. May provide upper
limit to
distance.
Polarized flux features remaining after IS polarization
removal:
- P Cyg absorption, remains in absorption: => line absorption
is not patchy,
which could cause intrinsic polarization enhancements as in
SNe).
- Bowen NIII features. Complex intrinsic polarization feature.
Suggests looking
for other fluorescence lines in polarization. Some likely to be
magnetically
sensitive (Lazarian, 2010).
Demonstrates future possibilities of UV-sensitive
spectropolarimeter on 10m class
telescope.
Date R λ1 λ2 Comment
20110511 880 3800 10000
20110512 5700 3030 3852
20110519 800 3076 6212
20110603 806 2991 3817
20110605 10900 6144 6963 60 sec cadence
20110614 800 3077 6213
Modes: There are many RSS polarimetric modes, broken down into
the three major instrument configurations (data format to the
right:) - Grating spectropolarimetry. Each spectrum is split by
Wollaston prism by ~ 4 arcmin into E and O spectrum. Can be
done
in Multi-Object Spectroscopy (MOS) mode. - Imaging. Either
classical imaging polarimetry (through filters), or unfiltered
imaging spectropolarimetry where each image
is dispersed by chromatic dependence of Wollaston prism into ~20
arcsec objective prism-like spectrum. A unique mode. - Fabry Perot.
Fabry Perot rings are split into E and O rings. Waveplate positions
and Fabry Perot scanning is done in double
Do-Loop. Another unique mode. Status: - Some commissioning of
all these modes was done during one week in 2006 6. - To correct
spectrograph UV throughout problems and telescope imaging problems,
it was off line until re-commissioning of
the telescope and spectrograph in Mar-June 2011. -
Re-commissioning of the grating spectropolarimetry mode (including
UV performance ) is now in progress, with data now
taken for linear polarization efficiency, position angle,
instrumental polarization (unpolarized standard), and absolute
position angle.
Prospects - Grating spectropolarimetry: We expect commissioning
to be complete by September 2011, and SALT will then be able to
issue a announcement of opportunity for this mode. - Imaging,
Fabry Perot. At best, end of 2011. Instrumental polarization
effects that need to be understood: - Position in the field of
view, and on the position of the telescope tracker. - Grating
polarimetric properties. - Effect of slit on pupil uniformity
(compare slitless with slit-limited polarimetry). Later
polarimetric modes: - Circular and All-Stokes polarimetry with
quarter-wave/ halfwave plate combination. - High Speed
spectropolarimetry (up to 10 Hz) using detector fast readout modes.
- Drift imaging spectropolarimetry.
The rotating waveplate subsystem consists of two slides, a
halfwave plate slide and a quarterwave plate slide, together with
plane-parallel windows when not in use (the waveplates are in the
diverging beam of the collimator, requiring focus compensation). -
The halfwave plate alone is used for linear polarimetry, and
the
combination of the linear- and quarterwave plates are used for
circular and all-Stokes polarimetry (below).
- Each plate (fabricated by Bernhard Halle, Berlin) consists of
a Pancharatnam “superachromatic” stack of six thin pieces of
crystal quartz and magnesium fluoride, glued between two fused
quartz substrates.
- The Pancharatnam design covers 3100 Ang – 1.7 microns (for the
future NIR beam), with the F/4.2 diverging beam accounted for.
- The waveplates are rotated by miniature stepper motors, and
actively detented at each rotation position, for angular
repeatability.
The polarizing beamsplitter consists of a 3x3 mosaic of calcite
Wollaston prisms, each 62 x 73 x 25 mm, mounted in a slide just
before the first element of the camera, so that it articulates with
the camera. - When the beamsplitter is inserted, the collimated
beam is split
perpendicular to the grating dispersion direction into two beams
with polarization parallel and perpendicular to the articulation
axis of the camera, with a separation of 5 degrees, which projects
to four arcminutes on sky.
- The two prism wedges of each Wollaston mosaic element are lens
fluid coupled, both to reduce ghost images at the prism interface
and to allow for coalignment of the mosaic by adjusting the fluid
wedges individually until the mosaic lines up.
- The calcite is UV grade Mexican material, with internal
transmission at 3200 Ang > 65%.
Mode Linear Circular All-Stokes
Plate ½ ½ ¼ ½ ¼
1 0 0 +45 0 0
2 45 0 -45 45 67.5
3 22.5 22.5 -45 90 135
4 67.5 22.5 +45 135 202.5
5 11.25 45 +45 180 270
6 56.25 45 -45 225 337.5
7 33.75 67.5 -45 270 45
8 78.75 67.5 +45 315 112
Grating Spectropolarimetry
Imaging Spectropolarimetry
Fabry Perot Spectropolarimetry
