# **[EUV Imager (EXI) for the Solar Orbiter]{.underline}**

Jean-Claude Vial^1^, Michel Decaudin^1^ & Richard Harrison^2^

1: Institut d'Astrophysique Spatiale, France.

2: Rutherford Appleton Lab., UK.

# 

# June 8, 2000

**[1. Introduction]{.underline}**

The EIT/SOHO and TRACE imagers have recently exemplified the power of
EUV imaging for the understanding of the solar corona at all different
spatial and temporal scales.

With the Full-Sun Imager (FSI) EIT, not only have we been able to survey
the general state of the corona (growing and decaying of active regions,
coronal holes, ..) but also to pinpoint special fine features such as
plumes or bright points or to record impulsive events such as flare,
prominence eruption and the start of CMEs (EIT waves).

With TRACE, the limited field of view (fov) allowed an increased spatial
resolution which revealed a lot of new features such as the filamentary
structure of coronal loops, their (apparent) constant cross-section
which puzzle (or dazzle ?) theoreticians. It also appeared that the 17.1
nm images are more filamentary than the 19.5 nm. Unfortunately, for
TRACE ( as for EIT), images taken at different wavelengths are also
taken at different times.

In the quiet Sun, a lot of \"events\" of various sizes ( bright points,
explosive events, jets, blinkers,... ) all evidence heating probably
related to magnetic reconnection. Their spatio-temporal behavior (power
laws) is interpreted in terms of intermittency due to, e.g., mhd
turbulence.

The results obtained lead to new questions concerning the structuring of
the corona above the poles, the real basal dimension of coronal
structures, the rôle played by nanoflares in the heating of the quiet
solar corona... Moreover, SOHO results point at the rôle of an activated
network from where high frequency waves, emitted to the outward corona,
could resonate with protons and heavy ions and provide the momentum for
the solar wind.

However, until combined remote sensing and *in-situ* measurements are
performed above the polar source regions of the fast wind, neither the
photospheric/chromospheric anchoring of polar plumes nor the exact
acceleration wind acceleration will be pinpointed.

Dramatic new results can be obtained from increasing the spatial
resolution by about one order of magnitude on one hand, from connecting
chromospheric/coronal phenomena with close *in-situ* measurements and
also from exploring the open field regions on the other hand.

This is where an EUV Imager on SO can fully exploit the unique
capabilities of the mission. Such an imager must provide EUV images with
an angular resolution increased by about one order of magnitude in order
to reveal the fine structure of most coronal features. To do so, it
benefits from the closeness of the Sun. It must also provide full disk
euv images of the Sun in order to reveal the global structure of regions
inaccessible such as the \"far side\" of the Sun and the polar regions.

**[2. Instrumental Considerations and Drivers]{.underline}**

A unique imager faces two difficulties : 1/ the best spatial resolution
cannot be obtained with a full disk imager for various reasons, the less
not being the limited number of pixels on current detectors, a
limitation coinciding with the (very) limited telemetry; 2/ it must cope
with a heavy thermal (radiative) load when at perihelion (25 solar
constants on the first optical element if the field-of-view (fov) is not
reduced). (It also must cope with the particle environment of such an
orbit).

This consideration leads to a separation between a High Resolution Image
(HRI) and a Full Sun Imager (FSI) all being part of EXI. In this way,
one can find specific solutions for the thermal load and have realistic
detectors.

**[3. Proposed HRI]{.underline}**

The driver of the design is the need of getting rid of the unwanted
solar radiation which goes along the direct light path. A first step
consists in reducing the entrance aperture of the instrument to about 2
cm and to fully use the available length of the PLM. In doing so, one
can cluster a few telescopes together, each one dedicated to one
wavelength, which eliminates any moving part, takes full use of the
aperture and allows for **simultaneous** observations in different
wavelengths.

Essentially one diminishes the fov through the use of an entrance baffle
and one sets the fov to 80 000 km ( or 500 arcsec when at perihelion)
for a 2\*2 K detector. This sets the equivalent pixel size to 40 km on
the Sun (or 0.25 arcsec). The pixel size of the detector is baselined at
9 microns, so the equivalent focal length is 7.2m.

We baseline a set of three telescopes working at 13.3, 17.3, and 30.4
nm. These three wavelengths cover a very wide range of temperatures
(from 1.6 10^7^ down to 5 10^4^ K) and targets (from flares to the quiet
Sun). Note that the 13.3 band includes a very hot line (Fe XXIII)
visible only during flares and that the 17.3 band only selects the 17.4
Fe X line, excluding the cooler Fe IX line at 17.1.

Characteristics of the HRI are given in the Table.

+----------------------------------+-----------------------------------+
| 3 Telescopes                     | \(1\) Off-axis Gregory; 20 mm     |
|                                  | diam.                             |
+----------------------------------+-----------------------------------+
| Spatial resolving element        | 0.25 arcsec (40 km on Sun at      |
|                                  | perihelion)                       |
+----------------------------------+-----------------------------------+
| Pointing                         | Common pointing mechanism         |
+----------------------------------+-----------------------------------+
| Detector                         | \(2\) Baselined on a 9 microns,   |
|                                  | 2K\*\*2                           |
+----------------------------------+-----------------------------------+
| Field of View                    | 80 000 km at perihelion           |
+----------------------------------+-----------------------------------+
| Peak Telemetry                   | \(3\) 3 \* 100 kbit/s (with       |
|                                  | compression 50)                   |
| Average Telemetry                |                                   |
|                                  | 20 kbit/s                         |
+----------------------------------+-----------------------------------+
| Mass                             | 24 kg                             |
+----------------------------------+-----------------------------------+
| Power                            | 15 W (essentially detector; with  |
|                                  | shared electronics)               |
+----------------------------------+-----------------------------------+
| Size                             | \(4\) Instrument = 2500 mm x 150  |
|                                  | mm x 240 mm                       |
+----------------------------------+-----------------------------------+
| Thermal                          | \(5\) Operating temperature       |
|                                  | 20^o^C with passive cooling; 140  |
|                                  | cm long baffle at the entrance of |
|                                  | each telescope                    |
+----------------------------------+-----------------------------------+
| Stability                        | \(6\) We assume a pointing        |
|                                  | stability of about 0.2 arcsec.    |
+----------------------------------+-----------------------------------+

\(1\) Possibility to put stops at the focal plane and at the image of
the entrance aperture

(2) One can think of APS (see EUS), or diamond or GaN new generation
    detectors (robust, solar-blind, ..). Shielding of the detector is
    included in the mass budget.

\(3\) The average rate of 20 kbit/s can be obtained by observing
**only** two days during the \"nominal mission\". This duration may be
increased by on-board binning (e.g. 2\*2).

\(4\) Depends on implementation on the PLM

\(5\) No major optical constraints (aberrations, ..). Structure in C-C
fiber.

\(6\) Active image stabilisation system at secondary may be considered
(adds mass and power).

The basic feature of each telescope relies upon the use of a long baffle
to reduce the fov and the stray light, an entrance metallic filter and
an off-axis Gregory telescope with multilayers fitted to each
wavelength.

The electronics (including DPU) is common to the three detectors and to
the FSI telescope. Its volume should fit within the overall volume given
in the Table.

For a typical exposure time (a few ten seconds), the bit rate for each
telescope is very high and it is assumed that we can compress data by a
factor 50 with a Rosetta-type compression scheme (Langevin, 2000)
without loss, as tests being performed now on solar images seem to
indicate.

The overall **mass budget** is given in the Table below.

The power requirements concern the detector feeding (it should be
remembered that voltage conversion is not lossless), electronics and the
pointing actuator (common to the three telescopes). For a given
telescope, we anticipate about permanently 4 W for the cycling \[power
on, integration, dump to memory\]. With margin and power for pointing,
we reach no less than 5 W. If the three telescopes are working
simultaneously, they will consume about 15 W for basic functions (above)
to which one should add about 2 W for data compression and dump by the
common DPU.

(Non-simultaneous (sequential) observations of course lead to a smaller
power consumption).

  ------------------------------------------------ ----------------------
  **Component**                                    **Mass (kg)**

  Structure (including the baffle) :               6.3

  Optics (including mounts) :                      0.2

  Detector (including electronics and shielding) : 0.5

  Miscellaneous (harness, ..)                      0.1

  Margin                                           0.8

                                                   

  Total                                            7.9

  Total 3 telescopes                               23.7

  Pointing mechanism                               2\.

  Electronics shared between the 4 telescopes      1.5

                                                   

  Grand total for the HRI                          27.2
  ------------------------------------------------ ----------------------

[Stability/Pointing]{.underline}

The three telescopes cluster has its own pointing mechanism since the
limitation of the fov prevents any independent internal pointing.

We anticipate a platform stability much better than 1 arcsec/15 min
(this is 15 years after SOHO, which achieved 0.99 arcsec/15 min). Given
our plan to achieve 0.25 arcsec resolution elements, we have chosen not
to include an image stabilisation system, assuming that the variations
of the spacecraft stability occur on time scales much less than the
exposure time.

An image stabilisation system could be included with a small guide
telescope driving adjustments to the three secondary mirrors.

[Counts number]{.underline}

For the 17.3 and 30.4 channels, the small aperture allows for a few
counts only per pixel per second. One can cope with this low number by
increasing the exposure time up to at least 10 s (as compatible with the
pointing stability) and/or on-board binning (at least a factor 2\*2).
(the counts number for the 133 channel can be much higher but only on
very active regions and flares).

The heritage of HRI comes from the SOHO/EIT, STEREO/SECCHI projects.
Some studies could be shared with teams involved in the Remote Sensing
Package of the Solar Probe Mission.

**[4. Proposed FSI]{.underline}**

The Full Sun Imager needs a wide field (about 5 °) when at perihelion in
order to cover the full lower solar corona with about 50 % margin in
each direction. For a 2K\*\*2 detector, this means a pixel size of 9
arcsec (or 1250 km on the Sun at perihelion). In order to solve the
issue of thermal (radiative) load on the entrance filter of the Imager,
different schemes have been envisaged, which have shown deceiving
properties in terms of optical properties or transmissivity (e.g.
pinhole camera).

The only safe possibility is to move back the entrance filter as much as
possible (1800 mm) and as close as possible to the M1 mirror. Then the
mirrors combination (all off-axis) allows for an image quality of the
order of the pixel size. The entrance filter is crossed twice by the
light. This system could work e.g. in the 17.3 line with the appropriate
multilayer (Mo/Si).

Another solution exists if one works above 120 nm (e.g. in the Lyman
alpha line at 121.6 nm) where lenses allow for a better combination
after the M1 mirror.

In any case, the dimensions of the instrument are of the order of 2000 x
250 mm x 350 mm, shorter than HRI but wider.

The mass is of the order of 8-10 kg (essentially the weight of the
structure).

The power needed is only related to the detector (4 W) since the
electronics is common with HRI.

The FSI relies on the pointing stability of the PLM.

**[5. Operations]{.underline}**

We envisage a semi-synoptic operation - i.e. a basic set of operation
modes which are run in a pre-defined sequence or over long periods. Day
to day planning will be minimised.

Observing sequences should be defined/designed well ahead of time and
include target coordinates, sequence of operating telescopes, along with
the transmission of a few sample data. These sequences would be uplinked
and stored on board in a deferred command store. The operations for a
defined period would be stored in this way and sequences would be run
autonomously. Planning will be done more on a monthly and weekly basis;
however, we feel necessary to have daily samples of the data (at least a
full-Sun image and an image in each wavelength, i.e. the equivalent of 4
Mbits) in order to check the acquisition of scientific data and possibly
to change the pointing or the sequence of observations (or safing the
instruments).

**[6. Participations and Costs]{.underline}**

The estimated total cost of EXI is of order 25 MEuros. ESA countries
(France, Belgium, Italy, UK, Germany, ..) have a large range of
expertise on most components (optics, multilyers, calibration,
detectors, baffling, ..). The project could include some collaboration
with non-ESA teams (especially in the U.S.A.). This could bring the ESA
member states contribution down depending on the nature of the
collaboration.

**[7. Further Options]{.underline}**

There are several options for making savings in cost and mass. These may
not be appropriate for a strawman payload but include the following:

\- Common electronics box with the EUS instrument (such as with the SOHO
EIT and LASCO instruments); however, this option may not be valid from
the standpoint of reliability (5 instrument outputs depending on the
electronics);

\- Common structure with the EUS and/or the Coronagraph (such as with
the STEREO 'SKIP' package); but this can only be valid for the
non-moving FSI;

\- Reducing the number of HRI telescopes.

\- Non-implementation of FSI.