# Solar Orbiter -- Payload Working Group

## Action Completion Form

***[Action ID Number:]{.underline}*** 2.2

***[Responsible Working Group Member:]{.underline}*** Valentín Martínez
Pillet

***[Date of Completion:]{.underline}*** 23 September 2002

***[Action:]{.underline}*** To identify the suitability of Liquid
Crystal Variable Retarders for polarization modulation on the
Visible-light Imaging Magnetograph (VIM).

***[Report:]{.underline}***

VIM measures the magnetic field on the Sun through the detection of the
polarization state of the light. Doppler measurements may use as well
polarization optics. For this purposes VIM needs a polarization
modulator. Traditionally this has been made by using wave-plates
(optical retarders) that are mounted on a rotating mechanism.
Polarization modulation is, then, achieved by mechanical rotations.
These continuous or stepped rotations need of a heavy (compared to the
retarder weight itself) and power consuming device that must show no
mechanical degradation (bearing degradation is a concern) during the
whole mission time frame. Liquid Crystal Variable Retarders (LCVRs)
offer an interesting alternative as polarization modulators.

The well developed Liquid Crystal Display (LCD) technology in use in a
myriad of applications can be used as a polarization modulation system.
Two types of LCVRs are available for this purpose, those based on
nematic materials (change of birrefringe) or ferroelectric materials
(change in orientation). Here, we refer them generically as LCVRs and
make no further distinction.

LCVRs are retarders whose properties can be changed electronically by
means of a simple (few volts) driving signals. The response times are in
the range of tens of milliseconds or better. LCVRs are easy to
synchronize to a detector readout due to their electro-optical nature,
simplifying the instrument control. No moving parts are needed, offering
a clear advantage for a space mission. Their power consumption is
negligible and only the driving electronics (which can be efficiently
designed for low consumption) should be considered. In addition they do
not need heavy mechanical mountings. They are commercially available
today and products manufactured 7 years ago are still working under
specs (IAC experience). They have been used for ground polarimeters
(Martínez Pillet et al., 1999) with a satisfactory performance and in
stratospheric balloon experiments (Flare Genesis Experiment, Bernasconi
et al., 2000) working in vacuum.

LCVRs main disadvantage is their sensitivity to UV light which causes
malfunctioning of the devices. For an experiment like VIM, where UV
light will be avoided by all means way before reaching the LCVRs, this
should not be a real problem. LCVRs need to be temperature controlled to
within 1 °C only.

LCVRs have been space qualified to various degrees by different teams.
Here we show present levels of characterization (integrity and
functionality in some cases) of LCVRs produced by Meadowlark (Colorado)
as made by EADS-CASA (see report for more details):

Vacuum integrity: 10^-5^ mbar

Heating/Cooling: \[-80, 85\] °C

Vibrations: 20-2000 Hz up to 7 g

Launch Shock: 15 g in 10 ms

Outgassing: ESA standard PSS-01-705

Ionizing radiation: up 17 Krad

Ionizing e- radiation: up to 2 Mrad (source Meadolwark Optics)

UV exposures: up to 50 ESH

all with positive results except the last one as commented before.

IAC has been since 2001 collaborating with an italo-spanish LCD company
(TECDIS Display,

[www.tecdis.com](http://www.tecdis.com/)) to produce LCVRs for use in
the stratospheric Antarctica balloon experiment SUNRISE. IAC is PI
institution of a VIM-like instrument, the Imaging Magnetograph
eXperiment IMaX, for the SUNRISE project that will produce polarization
modulation using the LCVRs produced under the collaboration IAC+TECDIS
(funded by the Spanish space program). TECDIS contribute with all the
LCD experience an IAC sets the requirements for optical quality of the
devices (/4 is expected to be achieved using fused silica substrates).
Prototypes for the IMaX instrument have been produced by TECDIS and
calibrated at IAC with most satisfactory results. Final prototypes with
the specified optical quality will be produced before the end of the
year. IAC together with INTA (Spain) plan to carry out the qualification
for the balloon experiment in year 2003.

LCVRs need further studies for use in an instrument like VIM. The
radiation levels the spacecraft will encounter should be considered.
Functionality under the cruise + nominal/extended mission conditions
should be studied. No LCVR has been in vacuum for a long period of time
and put to work after a large lapse of time. But the technology seems to
be ready for a full characterization (in particular, response to small
amounts of UV light). This technology is particularly useful for a space
mission which has very strong constrains of mass, power and complexity.
We note that the UVC instrument is also considering the use of LCVRs in
his visible part.

***[Conclusion:]{.underline}***

LCVRs technology (basically LCD technology) has been proven on ground
instrument and qualified for a suite of space applications. This
technology offers the possibility of producing large mass and power
savings compared to mechanically driven devices. It also offers a more
simple solution for synchronization purposes than rotating devices.
LCVRs are commercially available and are being manufactured for optical
and space programs in several parts of the world.

We thus recommend that ESA considers this technology for a study that
could lead to its application in Solar Orbiter (VIM+UVC) and, probably,
in many other applications, as it is happening on the ground.

***[References:]{.underline}***

Bernasconi, P., Rust, D., et al., 2000, Proc. SPIE 4014, 214

EADS-CASA CAS-LCV-RPT-0002

Martínez Pillet, V. Collados, M. et al., 1999, ASP Conference Series
#183, ISBN:1-58381-009-9, p.264