1. Meeting 11 Polarimetry: Beamsplitter; data formats Ken Nordsieck
Astronomy 920 Commissioning of the Robert Stobie Prime Focus Imaging Spectrograph
Meeting 11
Polarimetry: Beamsplitter; data formats
Ken Nordsieck
Dec 2, 2005
Ast 920 Meeting 11

2. How is this going? Data to play with, on class website Exercise #2
Imaging,
1-5 Biases
Tuc, 629 nm filter
Spectroscopy,
30: Ne Lamp
31: Xe Lamp
35-37 Biases
38, , 60 sec on BD
Exercise #2
pbias biases
pireduce, pmosaic 47 Tuc, Ne, Xe, BD
extra credit: extract spectrum, wavelength calibrate
Dec 2, 2005
Ast 920 Meeting 11

3. Polarimetric Optics
Dec 2, 2005
Ast 920 Meeting 11

4. Polarizing Beamsplitter
Calcite
E (y)
O (x)
In Collimated beam, just before camera
Mosaic of 9 Calcite “Wollaston” Prisms
Splitting 4.8 – 5.8 deg (color dependent): 240 – 280 arcsec at CCD
Dec 2, 2005
Ast 920 Meeting 11

5. Grating Spectropolarimetry
No Beamsplitter
Beamsplitter E
Unpolarized O
600 nm
900 nm
Setup
Tungsten Continuum
Test slitmask masked to center 4’
Polaroid screwed to slitmask
Polarization signal = (E-O)/(E+O)
Dual Beam cancels out transparency changes
HN32 Polaroid
Dec 2, 2005
Ast 920 Meeting 11

6. Imaging SpectroPolarimetry
Setup
HgAr Arc Lamp
Clear Filter (320 – 1000 nm)
Pinhole slitmask masked to central 4’
Entire PFIS accessible spectrum spread out into 25 arcsec mini-spectrum by chromatic dependence of beamsplitter
Low resolution spectropolarimetry of all stars in field E O
Dec 2, 2005
Ast 920 Meeting 11

7. Fabry-Perot Spectropolarimetry
No Beamsplitter
Beamsplitter E O
Setup
LR FP tuned to Ne line
Polaroid
Data product: FP scan => data cube of line polarization profile
Never before tried
Dec 2, 2005
Ast 920 Meeting 11

8. Modulating the Signal with Waveplates
1/2 and 1/4 wave retardation plates
switch which beam has which sense of polarization to remove response variations with time by rotating plate
remove polarization sensitivity of optical elements after waveplates
measure different Stokes parameters:
(Q,U): linear; V: circular
Q/I = (I(y) – I(x)) / (I(y) + I(x)) (measured directly by beamsplitter)
U/I = (I(+45) – I(-45)) / (I(+45) + I(-45))
V/I = (I(rcp) – I(lcp)) / (I(rcp + I(lcp)
Dec 2, 2005
Ast 920 Meeting 11

9. Waveplate Optics Pancharatnam “superachromatic” waveplates
at beginning of collimator
sandwich of 6 very thin pieces of crystal quartz and magnesium fluoride (birefringent materials)
320 nm – 1.7 microns to accommodate visible and NIR beams
1/2 wave: 100 mm diameter (full field
1/4 wave: 60 mm diameter (3 arcmin field)
Dec 2, 2005
Ast 920 Meeting 11

10. Dec 2, 2005
Ast 920 Meeting 11