# Solar Orbiter -- Payload Working Group

## Action Completion Form

***[Action ID Number:]{.underline}*** 1.1 (EUS) & 1.2 (EUS)

***[Responsible Working Group Member:]{.underline}*** Richard Harrison &
Louise Harra

***[Date of Completion:]{.underline}*** Var. 1 -- 6/9/02; Var. 2 --
12/9/02

***[Action:]{.underline}*** Thermal feasibility study for EUS
instrument/Adaptive Optics

***[Report:]{.underline}*** This report covers two areas. First we
discuss the basic outcome of thermal studies for EUS design concepts.
Secondly, recognising the variable thermal loads we discuss the possible
use of adaptive optics.

#### EUS Thermal Study

A preliminary thermal study has been performed of two proposed designs
for the EUS. This has been done through the RAL by a Cranfield
University MSc student (Mattieu Gasquet) working closely with the solar
group at RAL (see reference). The two designs include: (1) an off-axis
normal incidence (off-axis Ritchey-Chretien) design with 120 mm aperture
(due to Martin Caldwell, RAL), and (2) a 35x35 mm aperture stigmatic
grazing incidence Wolter II design (due to Luca Poletto, Padua). Both
systems use a variable line spaced grating in normal incidence. The
optical discussion is not included here.

To tackle the thermal situation it was assumed that a radiator area up
to the size of the instrument footprint may be used. Various surface
coatings were considered. For the NI design, the off-axis design allows
a significant heat stop between primary and secondary; thus, the
critical area to consider is the heat load on the primary. The Wolter II
design uses grazing incidence and, thus, a reduced aperture, which makes
the thermal load much more manageable. The heat loads on the primary
mirrors for the two designs are 41 W and 380 W for the Wolter II and the
NI design, respectively.

Refer to Gasquet (2002) for the full analysis. However, for comparison
let us examine a steady state situation at 0.2 AU, and fix the mirror
temperature at 61^0^ and the radiator temperature at 50^0^. For various
mirror coatings for the Wolter II option (e.g. gold, platinum,
silicon-carbon) the radiator size required ranged from 0.0313 to 0.0935
m^2^. The instrument footprint might be of order 0.3 to 0.5 m^2^.
Similarly, for the NI design, the radiator size ranged from 0.51 to 0.92
m^2^. These figures assume some absorption from the back of the
spacecraft heat-shield (because it extends beyond the edge of the
payload module). The NI figures, for example, come down by a factor of
up to three if this is negligible. Thus, for this static 0.2 AU case,
the Wolter II design is feasible and the off-axis NI design is
borderline, but feasible for certain materials if the absorption from
the heat-shield is not severe.

The off-axis design is clearly more challenging and was considered using
a time dependent model, which mimics the orbit. This model showed that
the primary mirror temperature varies considerably during the orbit. The
absolute temperatures could be controlled to some extent by using
different mirror coatings, different heat-shield parameters, different
radiator sizes and conductivities. However, considerable temperature
variations were found over the orbit, e.g. the orbital temperature
variation of the primary mirror ranged from 100^0^ to --50^0^ C for one
case, 180^0^ to 0^0^ C, and 31^0^ to --120^0^ C for others. A full range
of 150^0^ is typical over the half-orbit period. Preliminary
considerations of a heat-switch to the radiator show that some reduction
is possible in the extreme ranges, but we are still looking at a
significant range in temperature and a rather spikey temperature
profile. This could be brought down and smoothed by a more sophisticated
heat-switch arrangement and optimisation of the thermal design but, at
this stage, it is not clear that a NI approach is feasible from a
thermal point of view. It should be stressed that the current study has
concentrated on the optical components and the temperature of the
structure is of paramount importance. Further improvements could be made
from a spacecraft wide strategy, e.g. radiator viewing directions, heat
shield absorption minimisation, spacecraft/payload wide cooling rather
than instrument level cooling strategy etc...

### New Technologies: Adaptive Optics

One approach to cope with some aspects of the thermal situation, which
will be encountered by Orbiter is the use of adaptive optics. The Smart
Optics Faraday Partnership is investigating this and a number of areas
have potential for being used in space. Solar Orbiter will be in such an
extreme environment that it seems to be an ideal candidate for such
systems. For example, mass savings can be achieved from relaxation of
the mechanical constraints imposed by the requirement to align complex
optical systems prior to launch, by use of smart optics to correct for
alignment errors in a lightweight optical system post-launch. Savings
can also be obtained by relaxing constraints on the thermal design, by
using adaptive optics to correct for thermally introduced optical
aberrations in-flight.

Applicable technologies for Solar Orbiter might be:

-   **Aluminium mirrors**: robust and easy to control
    thermally---possibly as part of an isothermal design where the
    physical relationships between the optical components are
    self-correcting; Aluminium has been predominately used in infrared
    and X-ray space telescopes, but there has been little use of
    aluminium for optical ground or space based telescopes. Aluminium
    mirrors were considered for the VLT primaries and prototype mirrors
    were made (by REOSC) that were within the specs, though glass was
    eventually chosen;

-   **Deformable mirrors**: another method of controlling the focus and
    image quality. Not only can these mirrors be lighter than monolithic
    types they can also represent an overall system improvement when
    considered as part of an adaptable structure. A diagram showing a
    typical system is shown in Figure 1. It is very important to notice
    here that the closed-loop bandwidth requirements for a
    self-focussing system in the Solar Orbiter are trivial compared with
    the performance required to achieve real-time correction of
    atmospheric distortions in terrestrial telescopes. Atmospheric AO
    systems have an update rate of \~1000Hz (giving closed loop
    bandwidth of \~100Hz). An active optics systems for space (primary\
    mirror support system for example) can work very much slower, at
    0.1-10Hz perhaps. Various types of deformable mirror are currently
    available such as thin ceramics controlled by piezo-actuators,
    bimorph mirrors (two sheets of piezo electric material), and thin
    membrane electrostatically deformable mirrors. For the Solar Orbiter
    however, a metallic (perhaps aluminium) deformable mirror controlled
    by actuators would probably be preferable due to its high thermal
    conductance which allows heat to be conducted away easily and limits
    thermal distortions to large spatial frequencies, mainly defocus,
    which can be corrected by its adaptive nature. For the Solar Orbiter
    the number of actuators would probably be quite low (10--20) as the
    only low spatial frequencies would have to be corrected. Various
    low-precision actuators have already been used in space, but less
    work has been done so far on the high precision actuators likely to
    be needed for Solar Orbiter. Work is ongoing in this area, however,
    for example the high accuracy position actuators by Energen Inc.
    which are low power, light weight and capable of cryogenic
    operation; There would also need to be some sort of wavefront
    sensing in the system to measure the mirror distortion and provide
    feedback to the mirror. This could be achieved with a separate
    wavefront sensor or the science detector alone.

![](media/image1.jpeg){width="3.7819444444444446in"
height="4.0256944444444445in"}

Figure 1: "Classical" Adaptive Optics System

This diagram shows a typical layout of the components used for
correction of atmospheric turbulence. In this configuration a share of
the incoming light is picked off and distortions in the wave front are
measured by the wave front sensor, typically with reference to a guide
star. The processor system can then calculate the correction that is
needed in the optical path and the necessary shape is applied on the
deformable mirror.

-   **Thermochromic Coatings**: Scaling a radiator to keep a primary
    mirror cool enough during perihelion is likely to lead to too great
    an oscillation in temperature as the spacecraft progresses around
    its orbit. A thermochromic surface that can vary its emittance would
    solve this problem with no moving parts or control systems
    necessary.

With regard to the adaptive optics concept, the precise requirements
need to be considered in detail by any proposing instrument team. The
inclusion of the concept here does not necessarily imply that this
approach can provide the mass saving or the necessary responses (in time
and space) required for any particular design. This needs to be studied,
and is being considered by the UK-led consortium considering an EUS
proposal.

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

1.  Two EUS designs were considered. We believe that a Wolter II design
    option is feasible from a thermal point of view, but an off-axis NI
    design requires considerable work to demonstrate thermal
    feasibility.

2.  Further work is to be done on both options, including a full
    consideration of heat-switch use and extending the analysis to
    include a complete temperature profile of the structure in
    particular.

3.  Further work does depend on some spacecraft level input. What
    absorption can we expect from the back of the heat-shield, by
    radiation or even by conduction at the front of the instrument? What
    is the maximum size of the radiator for the EUS?

4.  Given the extreme thermal situation, it would seem sensible to
    embark on a study of thermal control options for the payload at a
    spacecraft level. We would suggest that we recommend this. For
    example, can we de-couple the heat-shield as much as possible from
    the instrument front bulkheads and is it possible to minimise
    radiation from the heat-shield to the instrument radiators? Is the
    spacecraft shape best suited to a system with multiple radiators
    facing space -- would a conical shape be better!? Would it be
    sensible to consider a payload-wide heat-pipe cooling system?

5.  Proposing teams should consider the feasibility of using adaptive
    systems for Solar Orbiter, e.g. deformable mirrors and
    thermo-chromatic mirrors.

***Actions:***

-   RAL-led EUS group is continuing thermal analysis in collaboration
    with GSFC and Padua.

-   Some spacecraft-level input is required from ESTEC to the payload
    groups to assist in refining these critical studies.

-   ESTEC and payload thermal engineers should study a payload-wide
    thermal strategy rather than an instrument level thermal strategy.
    What is to be gained by such an approach?

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

Gasquet, M., 2002, Cranfield University MSc Research Report, 'Solar
Orbiter: Thermal Analysis and Design of an Extreme Ultraviolet
Spectrometer'.

Smart Optics Faraday partnership -- http://www.smartoptics.org