EST Calibration and Modulation Unit

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Technical Note

EST Calibration and Modulation Unit

Prepared by: Thomas J. Kentischer

10/2009 Draft

KIEPENHEUER-INSTITUT FR SONNENPHYSIKStiftung des ffentlichen Rechts des Landes Baden - Wrttemberg

Mitglied der Wissenschaftsgemeinschaft Gottfried Wilhelm Leibniz


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1. Polarizing and Retarding Devices ......................................................................................3

1.1. Piezo elastic polarizing Modulator (PEM) ............................................................3

1.2. Pockels Cell.........................................................................................................................3

1.3. Liquid Crystal Variable Retarders............................................................................5

1.4. Fixed retarders .................................................................................................................81.4.1. Fresnel Rhomb ........................................................................................................8

1.4.2. Crystal retarders ....................................................................................................9

1.4.3. Polymer Retarders ............................................................................................. 10

1.5. Making a Modulator out of Fixed Retarders..................................................... 10

1.6. Polarizers ......................................................................................................................... 11

1.6.1. Absorbing Polarizers.........................................................................................11

1.6.2. Reflection Polarizers ......................................................................................... 12

1.6.3. Birefringent Polarizers..................................................................................... 13

1.6.4. Thin Film Polarizers .......................................................................................... 13

2. A preliminary design............................................................................................................. 13

2.1. Basics ................................................................................................................................. 13

2.2. Modulator Wheels ........................................................................................................ 14

2.2.1. Base Modulator Selection................................................................................ 14

2.3. Calibration Wheels....................................................................................................... 15

2.4. Filter Wheel Setup........................................................................................................ 16

2.4.1. Necessary free Apertures................................................................................17

2.4.2. Chromatic Aberrations..................................................................................... 17

3. Open Questions .......................................................................................................................18

4. Literature................................................................................................................................... 18


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1. Polarizing and Retarding Devices

There are numerous optical elements which are capable to polarize light or/and

change its polarization state. Not all of them are suitable for the application in solar

polarimetry. Particular attention has to be drawn on the following characteristics:

Optical Quality Off axis sensitivity Temperature sensitivity Chromatic properties, aberrations Modulation waveform Modulation Speed Required modulation scheme Available Size Optical transparence

Durability

1.1. Piezo elastic polarizing Modulator (PEM)

A piezo elastic modulator (Kemp 1970, Kemp 1981) is a rod of non-birefringent

material (e.g. fused silica) which is excited via a piezoelectric transducer. It will

oscillate at its natural frequency which is given by:

l

c

=

2

Here cis the sound speed in the material and lis the length of the rod. Due to the

oscillations, stress and therefore birefringence is introduced in the material. Because

the maximum retardance is a function of time and position within the crystal, the

device can be tuned to any wavelength by varying the drive voltage.

Depending on the material used, the spectral coverage is excellent. For fused silica the

device can be tuned between 170 nm and 3.5 m. Achromatic designs are not possible

up to now. The polarimetric accuracy can reach 610 . The optical quality (wavefront

and transmission) is superior.

Unfortunately the maximum reachable clear aperture (20 45 mm) of such retarders

is far too small EST. Because PEMs have to be operated at their mechanicalresonance, the modulation speed (up to 50 kHz) may be much too fast for most

spectro polarimeters which will be operated at EST.

1.2. Pockels Cell

Pockels Cells are using the linear electro-optic effect (Pockels, 1893). Birefringent in

uni-axial crystals is introduced by applying a constant or varying electrical field. The

electrical field can be parallel (Longitudinal field modulator LFM) or perpendicular


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(Transverse field modulator TFM) to the light beam. In case of a LFM the retardance

between o and e beam is given by:

ijo rVn =

32

Hereo

n is the refractive index of the ordinary beam,ijr is the electro-optic

coefficient, V is the applied voltage and the wavelength.

Figure1:DifferentPockelscelllayouts.Left:Longitudinalfieldmodulators.Top:WasherelectrodeBottom:Cylindricalbandelectrode.Right:Transversefieldmodulator.

There are a lot of different uni-axial materials available with large electro-optic

coefficients, high transparency, optical quality and availability. The voltages which

have to be applied are in the kV regime. Because a Pockels cell acts electrically as a

capacitor, the demands on the drive circuits are very demanding. The voltages can begreatly reduced by the use of a stack of Pockels cells. Also the high temperature

sensitivity of such devices can be reduced by the use of more than one cell. However

an achromatic design is not possible.

Because of the wide variety of materials, electrode configurations and stack designs,

it seems to be possible to design a Pockels cell which is ideal for the use as a

polarization modulator for solar polarimetry. Up to now, there are already devices

with the required free aperture of 100 mm in diameter and with the needed optical

quality. The usable wavelength range is between 250 and 2000 nm.


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1.3. Liquid Crystal Variable Retarders

Liquid Crystal Variable Retarders (LCVR) are electro optical, tuneable retarders

made of liquid crystal molecules [1]. The molecules are sandwiched between two

optical flat windows made from fused silica, spaced a few microns apart. The

windows are coated with transparent conductive indium tin oxide (ITO). A thin

dielectric layer is applied over the ITO and gently rubbed, to provide for liquid crystal

molecular alignment. The cavity between both windows is filled with birefringent

nematic liquid crystal material. The anisotropic nematic crystal molecules form

uniaxial birefringent layers in the liquid crystal cell. An essential feature of nematic

material is that, on average, molecules are aligned with their long axes parallel, but

with their centres randomly distributed an shown in Figure 2 a. With no voltage

applied, the liquid crystal molecules lie parallel to the glass substrates and maximum

retardance is achieved. When voltage is applied, liquid crystal molecules begin to tip

perpendicular to the fused silica windows as shown in Figure 2 b. As voltage

increases, molecular tip further, causing a reduction in the effective birefringence andhence retardance. Molecules at the surface, however, are unable to rotate freely

because they are pinned at the alignment layer. This skin effect causes a residual

retardance of approx. 30 nm even at high voltage (20 V).

Figure2:LCVR construction showing molecular alignment (a) without and (b) with appliedvoltage.

The retardance of nematic liquid crystal retarders can be varied over a wide range

with only small voltages. Figure 3 shows the retardance of the two TESOS/VIP

retarders (Kentischer 2005, Beck et al. 2009). The driving voltage is a square wave

signal with a frequency of 2 kHz.

LCVRs can be operated between 450 and 1800 nm (VIS: 450 700 nm, IR1: 650

950 nm, IR 2: 900 1250 nm, IR 3: 1200 -1700 nm). The optical quality is only /4

which has to be taken into account when such devices have to be placed within the


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light beam. Up to now, clear apertures of 40 mm are available from stock. Larger

apertures are possible. The retardance is slightly temperature sensitive (max. 2

retardance per one degree temperature change (Kentischer 2005), so the devices have

to be temperature stabilized.

Figure3:Retardancevs.voltageforthetwoTESOS/VIPretarders.Theredcurveisforawavelengthof630nm,thegreencurveforawavelengthof530nm.

Unfortunately LCVR are slow. The birefringent effect within a liquid crystal device is

produced by the self organization of the molecules within the liquid crystal. This self

organization can be disturbed by a rapid changing external electrical field.

0 20 40 60 80 100

Time [msec]

0.0

0.2

0.4

0.6

0.8

1.0

rel.Retard

ance

6.5 - 2.1 V

6.5 - 3.5 V

3.5 - 2.4 V

2.4 - 2.1 V

6.5 - 2.4 V

3.5 - 2.1 V

Figure4:Reposefunctionof LCVRsfordifferentvoltagetransitions.

Because self organization and its disturbance need time, there is a delay when we tryto change retardation. This delay will be a function of the magnitude of the voltage


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step as also on its direction. Figure 4 shows the transition from higher to lower

voltages. Depending on the position within the calibration curve, it can take up to 100

msec to reach the target retardance.

The response time is a function of the thickness of the liquid crystal gap. While

molecules nearby the alignment layer can react very fast, the bulk material within the

gab reacts slowly.

Figure5:(a)BulkLCdeviceshowingthefastsurfaceandtheslowbulkregion.(b)PolymerstabilizedLCdevicewithrandomalignmentoftheliquidcrystals.(c)SwiftLiquidcrystal

afterthealignmentprocess.This can be overcome by the introduction of small amounts of polymer material into

the bulk of the cell (Meadowlark, 2009). By this, every polymer sheet acts as an

alignment layer and the switching speed is largely reduced. Because the liquid

crystals are randomly orientated after infiltration into the polymers, there is no fastaxis defined in this state of production. To create such an axis, a mechanical shearing


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process is applied. This process aligns the LC molecules. Once this step is done, the

cell is locked in place and sealed (Figure 5).

Figure6:Responsetimeof aswiftretarder.While normal nematic LC retarders need operation voltages between zero and 10 V

(@ 2 kHz), a swift retarder requires voltages up to 100 Volts (@ 13 kHz). The

response time is about 100 sec which is a factor 1000 shorter than for normalLCVRs. The wavelength ranges are again between 450 and 1800 nm (4 different

retarders). Temperature stabilization is recommended. A temperature sensor is already

available in standard swift retarders.

A third types of LC retarders are ferroelectric devices. Here smectic liquid crystal

phases are characterized by well-defined layers. The retardance is defined by the gap

spacing and the material. If a voltage is applied, the optical fast axis can the switched

by e.g. o45 . They have excellent timing behaviour (approx. 1 kHz), but the

orientation of the fast axis is a function of temperature. Because only switching

between two polarisation states is possible they are not useful for polarimetry.

1.4. Fixed retarders

1.4.1. Fresnel Rhomb

A total internal reflection can be arranged to produce a retardance of /8. Two internal

reflections therefore provide a retardation of /4, /2 can be reached by 8 reflections.

Unlike such Fresnel rhombs are nearly achromatic (determined by the wavelength

variation of the glass), they can not be used for solar polarimetry, because the

retardance is a strong function of the angle of incidence (Hough 2005).


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1.4.2. Crystal retarders

Crystal retarders are commonly made from birefringent material as Quartz (180 nm

2.8 m), magnesium fluoride (150 nm 6 m), cadmium sulphide (5 m -15 m) and

sulphur free cadmium selenide (5 m 23 m).

Because of their small thickness, zero order retarders for the visible are very difficult

to manufacture. Enlarging the thickness to allow more then one order of retardance

(low order) gives steep variation of the retardance when the wavelength changes

(Figure 7).

Figure7:Wavelengthdependencesofdifferenttypesofretarders.


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Using two plates (each approx 0.5mm thick) with the difference in thickness of the

two plates equal the required retardance and put them together with the fast axis of

both plates orthogonal to each other gives a compound zero order plate. Such plates

can be manufactured easily with diameters up to 100 mm. If both plates are made out

of different birefringent materials the retarder can be made achromatic for two

wavelengths. The performance at other wavelength is still tolerable (Serkowski 1974),If three identical retarders are used together (outer two with parallel optical axis, inner

plate rotated by o60 ), the quality of achromatism is far better (Pancharatnam 1955).

Superachromats can be produced by combining three identical crystals in

Pancharatnam design. Wavelength ranges between 300 nm and 1100 nm with a

retardance change of only 0.005 are doable but the orientation of the fast axis also

varies with wavelength (max o2 between 300 an 110 nm). Theses values are valid

for a /2 plate; quarter wave plates have larger variations. Diameters up to 125 mm

are possible. They all have good thermal stability and a moderate angular acceptance

angle (Hough 2005). Surface flatness is between /5 and /10.

1.4.3. Polymer Retarders

Polymer retarders consist of birefringent polymer material which is laminated

between two glass plates. They can be used between 450 and 2500 nm and

manufactured with very large apertures. They are true zero order retarders and can be

made achromatic between 425 and 675 nm using multilayer polymer stacks. Angle

acceptance is much better that for crystal plates. Within 10the retardation varies

only by 1 %. Furthermore such devices are much less temperature sensitive than

crystal retarders.

1.5. Making a Modulator out of Fixed Retarders

To use a fixed retarder as a modulation device it has to be rotated. This can be done

with a motorized rotation stage. To meet the science requirements, the rotation has to

be fast. In the case of POLIS the rotation rate is adjustable and can reach a maximum

speed of 60 rotations per second. One full stokes vector is measured with eight frames

during one half rotation. So, the maximum speed is 120 full stokes measurements per

second which is equivalent to an exposure time of approx. 1 msec per frame. Phasing

is done by the modulator drive. Every 1/16 rotation the camera receives a strobe and aposition mark from the modulator electronics (Schmidt 2003, Beck 2005).

Fixed retarders are plane parallel plates by nature. This can cause multiple reflections

inside the plates. By this etalon effect fringes are produced in the spectra which have

to be carefully removed by Fourier techniques later. To get rid of such interferences

the plates can be optical attached to a wedge. Here special care has to be taken on the

wedge material. Minimizing fringing is optimal if the refractive index of the wedge is

equal to the mean refraction index of the retarder (e.g.: 2/)( oe nn + ). POLIS uses

wedges made from BALF5 547526.

Wedging the retarder leads to a slight deflection of the light beam. If the modulator is

mounted in parallel light, an image wobble will be the result. If the modulator is

mounted near the focal plane, the pupil image will move. Both situations areintolerable. Beam wobble can be eliminated by adding another wedge for


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compensation. POLIS uses two additional wedges which were orientated by +60and

-60in respect to the retarder wedge. By rotating each individual compensator wedge,

the amplitude of the correction can be changed; rotating both compensators in respect

to the retarder changes the compensation phase. By careful adjusting of all angles the

beam wobble can be eliminated completely (Schmidt 2002).0 0 0 00 0 0 00 0 0 01 1 1 11 1 1 11 1 1 10 0 00 0 00 0 01 1 11 1 11 1 10 0 0 0 0 0 0 00 0 0 0 0 0 0 00 0 0 0 0 0 0 01 1 1 1 1 1 1 11 1 1 1 1 1 1 11 1 1 1 1 1 1 10 0 00 0 00 0 01 1 11 1 11 1 1 0 0 00 0 00 0 01 1 11 1 11 1 1

4 2 2 2 2

0 +60 60o o o

Material: Retarder: Quartz

Compensators: BALF5 547536 (Schott)

0 01 10 00 01 11 1

0 01 10 00 01 11 1

0 01 10 00 01 11 10 00 01 11 1

0 01 1 0 01 10 00 01 11 10 00 01 11 1

0 00 01 11 1 0 00 01 11 10 00 01 11 10 00 01 11 1

0 00 01 11 10 00 01 11 10 00 01 11 1

0 00 01 11 10 00 01 11 1

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1

5 5 5

6 6

30

48

ORing

2

Retarderd=1.3

Figure8:WedgedPOLISretarderwithwobblecompensation.

All outer surfaces have to be coated for minimum reflectivity at all operating

wavelength.

1.6. Polarizers

There are mainly four different types of polarizing optical components:

Absorbing polarizers (Polaroid, Polarcor,)

Reflection Polarizers

Birefringent Polarizers

Thin Film Polarizers

1.6.1. Absorbing Polarizers

Polaroid Films are stressed iodine doped polyvinyl alcohol foils embedded between

two glass plates or laminated in cellulose triacetate. Extinction ratios are at 1:4.000,

the transmission is at approx. 30 %. Much better Polarisators can bee made if

nanoparticles (elongated silver) embedded in thin glass plates. Here the extinction

ratio reaches 1: 10.0000 (Polarcor). Unfortunately the spectral range for these type

of polarizers is rather small.


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1.6.2. Reflection Polarizers

Reflection polarizers split the incident light beam into two beams of differing linear

polarization. At the Brewster angle no p-polarized light is reflected from the surface,

thus all reflected light has to be s-polarized. In order to achieve a high degree of

polarization several reflections have to be made in series. This is problematic from theoptical field of view. Extinction ratios are rather low.

Figure9:ExtinctionRatiosfordifferentlinearpolarizers.(Newport,2009).


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1.6.3. Birefringent Polarizers

Birefringent polarizers are splitting unpolarized light into two perpendicular linier

polarized beams. They use the difference in the refraction coefficients for s and p

polarized beams so:

One of the beams is reflected by total reflection. The two beams get different deflection angles. One or both beams are lateral displaced.

There are various designs for such polarization elements. They were mostly made

from Calcite because of its maximum birefringence. If only one linear polarized beam

is needed, many designs are nearly achromatic. They have excellent transmission over

a wide wavelength range and have excellent optical performance. E.g. Clan Thomson

prisms are widely used as polarizers. A modified version of this prism is the Marple

Hess prism. Here the acceptance angle is doubled in comparison with a Clan prism

and the damaging threshold is very high because of air gaps are used; there are no

cemented surfaces.

1.6.4. Thin Film Polarizers

Thin film polarizers are based on interference within a dielectric optical thin-film

coating on a thin glass substrate. They can be made with excellent environmental

reliability, the highest laser damage thresholds, and large aperture sizes (inches). And

they naturally function as beam splitters cubes with a 90 beam deviation of the

blocked polarization. Unlike birefringent crystal polarizers, thin-film plate polarizerstend to function over only a small range of wavelengths since because they are based

on multiwave interference, and thus they are best suited for laser applications or for

systems with limited signal band. A typical bandwidth is approx. 40 % of the central

wavelength.

2. A preliminary design

2.1. Basics

The modulator- and calibrator unit should be located as near as possible to thesecondary focal plain F2 before the symmetry is broken by the first oblique

reflection.

F in F2: 11 Plate scale: 5 arcsec/mm Unvignetted FOV: 3 arcmin The modulator wheel should include minimum four positions. The calibrator consists of a polarizer and a retarder wheel each with four

positions.


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Being near to a focal plane relaxes the requirements to the optical quality of the

components. As a lower limit maximum wavefront distortion of /4 is allowed.

Unfortunately it is not possible to mount the polarimetric unit directly into the focal

plane. So, due to the small F-number the components need to have large free aperture

which is challenging. Also it is impossible to include pinholes and targets into the

wheels.

2.2. Modulator Wheels

It is hard to use one single Modulator for all polarimetric post focus devices. Either

the modulators wavelength range is too small or the available speed does not fit to the

individual device. High efficient modulation at two different wavelengths for two

different polarimeters is challenging and depends on the particular wavelength. There

is a variety of different modulation schemata which can help to optimize the needed

retardance. So, in the moment, the idea is, to equip the modulator wheel with aselection of modulation devices which reasonable fit to all imaginable species of

polarimeters without the ambition to operate all devices together.

During development of the post focus devices and also later during telescope

operation new requirements might come up, so some modulators have to be replaced.

Two wheels are foreseen. So we have enough space for upgrades. Every wheel needs

to have a position for free field operations. In the first iteration one wheel has a

diameter of 600 mm and can hold five elements. If more space is needed (e.g. for

enlarged free apertures or temperature control devices, the number of elements can be

reduced to four.

2.2.1. Base Modulator Selection

Pockels cell: This device has excellent optical quality, works in the entire wavelength

range and can be operated with very high speed. Furthermore Pockels cells are

capable to modulate visible and infrared light simultaneously. It is not clear if the

required clear diameter and the modulation quality are sufficient for the requirements.

Special attention has to be drawn on the field dependent variation of the retardance. It

will be necessary to initiate a complete new, individual design in respect on the used

materials and the electrode configuration. If it is not possible to include a temperature

stabilisation onto the modulator wheel (e.g. due to room restrictions) a temperature

compensated tandem system is necessary.

Rotating Retarders: There are two coupled rotating stages for rotating retarders. The

retarders can be zero order retarders, superachromatic retarders or polymer retarders;

depending on the kind of polarimeters which ware used for EST. All retarders are

equipped with a wedge in optical contact and two wobble compensator wedges.

Wobble compensation is critical because the adaptive optics need stationary pupil

images. All retarders are driven by a single motor via a drive belt. The servo

controlled drive electronics delivers strobe and position signals to the polarimetric

cameras.

Swift liquid crystal retarders:There are four swift liquid crystal retarders (SLCR) for

the four different available wavelength regions (VIS: 450 700 nm, IR1: 650 950


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nm, IR 2: 900 1250 nm, IR 3: 1200 -1700 nm). Every retarder stage consists of two

SLCVRs with their fast axis orientated by 45in respect to each other. By this, four

steps modulation schemata are possible. Each tandem system has to be completed

with an UV blocking filter. All retarders have to be temperature stabilized.

Figure10:ModulatorWheels.Left:PockelsCell,tworotatingretarders, freefieldposition,twoemptypositionsforupgrade. Right:FourLCVRretarders,freefieldposition,one

emptypositionforupgrade.

2.3. Calibration Wheels

For any polarimetric calibration a polarizer and a quarter wave retarder is needed. Sowe arranged both types of elements in two different wheels: The polarizer- and the

retarder wheel.

All optical components have to be mounted on rotation stages. They all are driven by

a single dual, pre tensed gearwheel. So reversal backlash is minimized. Angle

resolution should be 0.1.

The polarizer wheel contains a Marple Hess prim which is a specific configuration of

a double Glan prism (Hofmann 2008). Due to its air gap, such a prism can withstand

the power density of approx. 10 W/cm easily. The extinction ratio is 510 . Marple

Hess Prisms have a large acceptance angle of 12. The transmission range (80%) is

between 300 and 2800 nm.

Furthermore the wheel holds two sheet polarizers, one for the visible and one for the

infrared. If Polarcorwith extinction ratios of510 are used, more than two of them


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are necessary. The damage threshold of such filters for laser radiation is quite above

several 100 W/cm. Two additional slots were free for upgrades.

The retarder wheel holds three superachromatic quarter wave retarders covering the

entire wavelength range of EST. Special care has to be taken on the maximum

acceptance angle of these devices.

Figure11:Calibratorwheels.Lestpolarizerwheelwith:MarpleHessPrism,twoabsorptionsheetpolarizers(VISandIR).Twopositionsarefreeforupdate.Right:retarderwheelwithtreesuperachromaticretarderscoveringtheentireESTwavelengthrange.

2.4. Filter Wheel Setup

All filter wheels are housed in a 760 mm long cylinder with an outer diameter of 1200

mm. The minimum distance between F2 and the first mechanical element is 350 mm.

Because of their limited available free aperture, the both modulator wheels are

mounted nearest to F2. The manufacture of superachromatic retarders and sheet

polarizers with clear apertures of more than 100 mm is feasible.

The filter wheels are mounted to hollow axels and connected to the motors by drive

belts. The rotation accuracy should be better than 0.5. Component exchange speed is

not an issue.


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Figure12:SetupofthefourfilterwheelsinfrontofthefocalplainF2.

Cables are guided in a groove along the outer circumference of each wheel and

tightened by cable wrappers.

2.4.1. Necessary free Apertures

To reach a free, unvignetted field of view of 3 arc minutes within a F# = 11 beam, the

following clear diameters are necessary:

Distance to F2 Free Aperture

Modulator Wheel 1 510 mm 82 mm

Modulator Wheel 2 620 mm 92 mm

Retarder Wheel 740 mm 103 mm

Polarizer Wheel 940 mm 121 mm

Perhaps the distances between the individual wheels (Figure 12) can be furtherdecreased so, the above values are upper limits.

2.4.2. Chromatic Aberrations

Due to the glass thickness of the polarizing elements focus shifts are introduced. E.g.

if a plan parallel BK7 plate is introduced in front of F2 a focus shift of 200 mm in the

science focus F4 is introduced. This can be compensated by a movement of M2 by

approx. 100 m. Spherical aberrations are negligible. The correction movement of

M2 can only be done for one wavelength. Chromatic focus differences have to be

corrected on instrument level. Very thick optical elements (as the polarization prism)


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can cause very large chromatic focus shifts but incomplete compensation is not

crucial for polarization calibration.

3. Open Questions

There still are some open questions to be answered.

What element sizes are available in 10 years and what will be the optical andpolarimetric quality of such devices?

What uniformity of the retardation/polarization is needed across the FOV andwhat variations are present in the actual available devices.

Which design of superachromatic retarders gives the required quality over abeam angle of 5.2?

Which element has to be temperature stabilized to which degree?

Are their thermal issues within the calibration/modulation unit due to theimpact of sun light?

The elements we choose for the different wheels are only preliminary. As soon as

there are designs of all polarimetric devices for EST a selection can be done in more

detail. The actual design is open for all individual needs.

Also simultaneous efficient modulation at different wavelength is a problem. Here a

lot of different modulation schemata are thinkable to solve this problem. From science

requirements and predefined observation sequences it should be possible to define

retardances which are able to account for this.

4. Literature

Beck C., Bellot Rubio L.R., Kentischer T.J., del Toro Iniesta J.C. and Tritschler A.

2009 Astron. Astrophs. (submitted)

Hofmann, A. 2008 Polarimetric Projects with GREGOR, Cent. Eur. Astrophys.

Bull. 32, 1, 17-24

Hough J.H. 2005 Polarimetry Techniques at Optical and Infrared Wavelength

Astronomical Polarimetry: Current Status and Future Directions

ASP Conference Series, Vol. 343Adamson, Aspin, Davis and Fujiyoshi

Kemp, J.C. 1970 J. Opt. Soc. Am., 59, 950

Kemp, J.C. 1981 SPIE Proc., 307, 270

Kentischer, T.J. 2005 Calibration of the Meadowlark LCVRs for the TESOS

Full Stokes Polarimeter, Technical Report, Kiepenheuer Institute

Meadowlark 2004 Optics Catalogue, 25hAnniversary Edition

Meadowlark 2009 Optics Catalogue

Pancharatnam, S. 1955 Proc. Indian Acad. Sci A41, 137

Schmidt, W., Kentischer, T.J. 2002 German Patent Nr. 10236999, Vorrichtung zur

Untersuchung von polarisiertem Licht

Schmidt, W., Beck, C., Kentischer, T., Elmore, D., Lites, B. 2003 POLIS: Aspectropolarimeter for the VTT and for GREGOR,Astron. Nachr./AN 324,


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300301

Beck, C., Schmidt, W., Kentischer, T., Elmore, D. 2005 Polarimetric Littrow

Spectrograph - instrument calibration and first measurements,Astron.

Astrophys.437, 11591167Serkowski, K. 1974, Methods of Experimental Physics, ed. N.P. Carleton, 12:

Astrophysics, Part A (New York: Academic Press), 361

Documents

EST Calibration and Modulation Unit