**Solar Orbiter**

**Visible Imager / Magnetograph (VIM)**

Optical Concept

*Oskar von der Lühe^1^, Wolfgang Schmidt^1^, Achim Gandorfer^2^, and
Valentin Martínez Pillet^3^*

1)  Kiepenheuer-Institut für Sonnenphysik, Freiburg, Germany

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2)  Institut für Astronomie, Eidgenössische Technische Hochschule,
    Zürich, Switzerland

3)  Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain

June 2000

# Introduction

We describe the optical layout for the *Visible Imager and Magnetograph*
(**VIM**) for the Solar Orbiter Mission, based on a filtergraph concept
which includes a double Fabry-Pérot interferometer. The optics consists
of three subsystems:

1)  25 cm High Resolution Telescope (HRT)

2)  5 cm Full Disk Telescope (FDT)

3)  Filtergraph Optics (FO)

The two telescopes, HRT and FDT, deliver an image of the sun at a common
telescope focal plane. The FO produces a magnified image of the
telescope focus on the instrument\'s only detector. While the HRT
provides an image with a high resolution at its diffraction limit, the
FDT adjusts its focal length such as to fill the detector with the solar
disk. The filtergraph optics include a double Fabry-Pérot interferometer
equipped with a narrow-band interference filter as a prefilter, tuned to
the Fe I 630.2 nm line. This filtergraph is easily adjusted to the
lineshifts which occur due to sun-spacecraft relative velocities up to
40 km s^-1^. We describe all components in the following sections.

# High Resolution Telescope

## General Decription

The HRT is a 250 mm diameter Gregorian reflector. It provides an image
with a spatial resolution as high as 75 km on the solar surface during
the near-Sun orbital phase. A polarisation modulator is foreseen in the
optical path behind the main mirror in the F/17 beam and before the
first oblique reflection.

Figure 1 shows an annotated perspective view of the HRT and FO. Table 1
presents the main characteristics of the HRT, **Appendix A** shows the
surface design data. The results of a wavefront analysis of the design
is shown in **Appendix C**.

## Thermal Control

The HRT will have to withstand the heat load during the near-Sun orbital
phases. The primary mirror will be made from SiC and passively cooled to
dispose off the approximately 170W of radiation which is absorbed in its
reflective layer. SiC enjoys better static and dynamic thermal stability
than other mirror blank materials.

We consider the following two concepts for disposing off the remaining
heat load:

### Prefilter

A broad-band interference prefilter covers the entire telescope and
reflects most of the incident sunlight, with the exception of the
spectral range of interest. This makes a field stop at the primary focus
unnecessary. A combination of decentering and tilt of the secondary
mirror provides independent telescope pointing on the solar disk without
affecting the image quality. Baffling at the focus and at intermediate
pupils rejects remaining sunlight.

The prefilter must be of excellent optical quality. For reasons of
weight savings, a pellicle filter should be preferred. The feasibility
of such a filter needs further study.

### Primary Field Stop

The Gregorian design permits rejecting 96% of the incident radiation at
its primary focus with a field stop. The field stop is an inclined
mirror with a 2 mm central hole which transmits the observed field. The
reflected light is directed out of the spacecraft.

The field stop will also absorb about 170 W which will require passive
cooling.

A broadband interference filter will be included after M2 to reduce the
amount of light in the filtergraph optics.

![](media/image1.wmf){width="6.271527777777778in"
height="3.9194444444444443in"}

Figure 1: perspective view of the optical layout of high resolution
telescope and filtergraph.

+-------------------------------------------------+--------------------+
| Primary diameter \[mm\]                         | 250                |
+-------------------------------------------------+--------------------+
| Primary focal length \[mm\]                     | 500                |
+-------------------------------------------------+--------------------+
| Primary F ratio                                 | F/2                |
+-------------------------------------------------+--------------------+
| Solar image at primary focus \[mm\]             | 4.7                |
|                                                 |                    |
| at 1 AU                                         | 6.7                |
|                                                 |                    |
| at 0.7 AU                                       | 21.1               |
|                                                 |                    |
| at 0.22 AU                                      |                    |
+-------------------------------------------------+--------------------+
| Secondary diameter \[mm\]                       | 80                 |
+-------------------------------------------------+--------------------+
| Secondary focal length \[mm\]                   | 134.2              |
+-------------------------------------------------+--------------------+
| Secondary conic constant                        | -0.622684          |
+-------------------------------------------------+--------------------+
| Secondary magnification                         | 8.5                |
+-------------------------------------------------+--------------------+
| Secondary field diameter \[mm\]                 | 10.2               |
+-------------------------------------------------+--------------------+
| Effective focal length \[mm\]                   | 4250               |
+-------------------------------------------------+--------------------+
| Field of view \[arcsec\]                        | 500                |
+-------------------------------------------------+--------------------+
| Image scale at secondary focus \[Mm / mm\]      | 35672              |
|                                                 |                    |
| at 1 AU                                         | 24970              |
|                                                 |                    |
| at 0.7 AU                                       | 7848               |
|                                                 |                    |
| at 0.22 AU                                      |                    |
+-------------------------------------------------+--------------------+

Table 1: High Resolution Telescope characteristics.

# Full Disk Telescope

It is impractical to combine the functions of full-disk and high
resolution viewing of the sun into a single telescope. Therefore, a
separate telescope will provide a full disk image at the same position
as the secondary image of the HRT. The filtergraph is therefore shared
by both telescopes. A folding mirror which delivers the FDT image is
movable and can be inserted into the optical path of the HRT. The
mechanism will be such that the mirror will be removed from the optical
beam in the case of a mechanism failure, the high resolution function
will therefore have precedence.

The FDT consists of a refractor with a focal length which varies between
233 mm and 888 mm. The telescope provides a full-disk image of the Sun
of constant diameter on the detector during all orbital phases between
0.22 and 0.9 AU. The zoom lens consists of a classical afocal PNP
triplet followed by a fixed aperture stop and a camera lens which
provides a focus at a fixed position. All lenses will be singlets as
chromatic aberration is not a concern with the narrow-band filtergraph.
A mechanism which drives the middle lens of the triplet and the camera
lens provides the zoom function. The diameter of the entrance pupil
varies between 20 mm and 50 mm as the focal length increases, resulting
in a diffraction limited sampling of the detector at all distances from
the sun. The optical design is diffraction limited for all settings of
the focal length.

An interference filter at the entrance aperture reflects most of the
incident sunlight so that the radiation load is not a problem for the
FDT.

Figure 2 shows the annotated optical layout of the FDT from a similar
point of view as the HRT in Figure 1. Note that the layout is
\"horizontal\" (in the XZ plane) compared to the vertical (YZ plane)
layout of the HRT proper. A separate modulator is necessary for the FDT
in order to perform polarisation modulation within the centered optical
path before the first oblique reflection. The surface design data of the
FDT are given in **Appendix A**. The results of a wavefront analysis of
the design is shown in **Appendix C**.

![](media/image2.wmf){width="6.271527777777778in"
height="3.9194444444444443in"}

Figure 2: perspective view of the optical layout of one of seven
configurations of the full disk telescope. Drawings of all
configurations are shown in Appendix B.

  -------------------------------------------------- --------------------
  Entrance Pupil diameter \[mm\]                     20 \... 50

  Objective focal length \[mm\]                      233 \... 888

  Objective F ratio                                  F/11 \... F/18

  Solar image diameter at primary focus \[mm\]       10

  Field of view \[°\]                                0.64 \... 2.46
  -------------------------------------------------- --------------------

Table 2: Full Disk Telescope characteristics.

![](media/image3.wmf){width="6.271527777777778in"
height="3.9194444444444443in"}

**Figure 3:** arrangement of filtergraph optics.

# Filtergraph Optics

The FO consists of two singlet lenses which combine the functions of
collimator and camera with a 1:2 magnification. A system of two off-axis
ellipsoid and paraboloid mirrors provide a 1:1 pupil relay function and
produce an F/66 telecentric intermediate image plane.

Two 50 mm diameter Fabry-Pérot etalons at the telecentric image plane
provide the required spectral resolution of typically 5 pm. One etalon
will provide the spectral resolution while the other blocks the
secondary transmission maxima of the first. A separate interference
filter with a 1 nm band blocks the secondary transmission maxima of the
combined etalons.

We consider using two solid state etalons with fixed resonator widths
which are mounted on a common rotating stage for fine tuning. About 1
degree of rotation angle is required for a shift of the passband of 100
pm, which is sufficient to cover both line width and sun-spacecraft
velocity shifts.

As an alternative we consider variable gap width etalons which are
electronically controlled to provide spectral scanning. This principle
is the one which is used in the TESOS filtergraph which operates at the
Vacuum Tower Telescope of the Kiepenheuer-Institut on Tenerife.

Figure 3 shows a plane view of the filtergraph optics. The position of
the etalons in the telecentric beam is indicated. The design data are
given in **Appendix** **A** and results of a wavefront analysis of the
design is shown in **Appendix C**.

# Overall Configuration

The combination of all optical subsystems is shown in . The HRT is the
large cylinder on top of the Y and Z views. The FDT is to the lower left
of the Z view and near the bottom of the X view. The FO are to the lower
left of the Z view and near the top of the X view.

The overall envelope of the optics is 250 mm x 400 mm x 1100 mm.

![](media/image4.png){width="4.21875in" height="2.1375in"}

Figure 4: plan views (top left: Y, top right: Z, bottom: X) of the
combined VIM. The HRT is arranged above the FDT and the FO, both of
which are mounted on a common optical bench.

**Appendix A: VIM optical Design Data**

**Solar Orbiter Magnetograph - High Resolution Telescope**

SRF RADIUS THICKNESS APERTURE RADIUS GLASS NOTE

OBJ \-- 1.4900e+14 1.7944e+11 AIR Solar Surface at 1 AU

AST \-- 700.000000 125.000000 AIR Entrance aperture

2 -1.0000e+03 -500.000000 125.842994 REFL Main Mirror

3 \-- -150.000000 0.602139 AIR Primary Focus

4 268.422000 150.000000 38.029882 REFL Secondary Mirror

5 \-- 500.000000 33.222977 AIR Primary Focus on return beam

6 \-- 100.000000 20.732507 AIR Primary Mirror on Return beam

7 \-- 50.000000 25.000000 BK7 Modulator

8 \-- 50.000000 25.000000 AIR

9 \-- \-- 16.161891 AIR

10 \-- \-- 16.161891 REFL Folding Mirror 1

11 \-- -200.000000 16.161891 AIR

12 \-- \-- 11.165703 AIR

13 \-- \-- 11.165703 REFL Folding Mirror 2

14 \-- 242.078619 11.165703 AIR

15 \-- 100.000000 5.118352 AIR Intermediate focus

CONIC AND POLYNOMIAL ASPHERIC DATA

SRF CC AD AE AF AG

2 -1.000000 \-- \-- \-- \--

4 -0.622684 \-- \-- \-- \--

**Solar Orbiter Magnetograph - Full Disk Telescope (Configuration 2)**

SRF RADIUS THICKNESS APERTURE RADIUS GLASS NOTE

OBJ \-- 1.4900e+14 2.6320e+12 AIR Solar Surface at 1 AU

1 \-- 400.000000 21.306463 AIR Entrance aperture

2 442.581348 8.000000 40.000000 BK7 \| Objective 1

3 -931.218609 \-- 40.000000 AIR \|

4 \-- 50.000000 14.059882 AIR Variable space 1

5 -205.814311 5.000000 25.000000 BK7 \| Zoom lens (movable)

6 194.505175 \-- 25.000000 AIR \|

7 \-- 250.000000 11.919031 AIR Variable space 2

8 2.5108e+03 8.000000 25.000000 BK7 \| Objective 2

9 -340.708620 \-- 25.000000 AIR \|

10 \-- 20.000000 16.874896 AIR

AST \-- 61.711000 22.000000 AIR Aperture stop

12 271.213023 10.000000 25.000000 BK7 \| Imaging lens (movable)

13 -1.8478e+03 \-- 25.000000 AIR \|

14 \-- 88.289000 17.266174 AIR

15 \-- 20.000000 22.000000 BK7 Modulator

16 \-- 60.000000 22.000000 AIR

17 \-- \-- 13.037993 AIR

18 \-- \-- 13.037993 REFL First folding mirror

19 \-- -100.000000 13.037993 AIR

20 \-- \-- 10.419743 AIR

21 \-- \-- 10.419743 REFL Second folding mirror

22 \-- 100.000000 10.419743 AIR

23 \-- 106.700000 7.801492 AIR Image plane

IMS \-- \-- 5.007819

GROUP THICKNESSES AND AIR SPACES FOR ZOOMING SYSTEMS

Group 1 consists of surf 2 to 4 Thickness = 8.000000

Group 2 consists of surf 5 to 7 Thickness = 5.000000

Group 3 consists of surf 8 to 11 Thickness = 28.000000

Group 4 consists of surf 12 to 14 Thickness = 10.000000

CFG OBJ\<-\>GRP1 GRP1\<-\>GRP2 GRP2\<-\>GRP3 GRP3\<-\>GRP4 GRP4\<-\>GRP5

1 1.490e+14 10.0000 290.0000 80.0000 70.0000

2 1.490e+14 50.0000 250.0000 61.7110 88.2890

3 1.490e+14 100.0000 200.0000 37.5000 112.5000

4 1.490e+14 150.0000 150.0000 15.8000 134.2000

5 1.490e+14 200.0000 100.0000 7.8400 142.1600

6 1.490e+14 250.0000 50.0000 32.7210 117.2790

7 1.490e+14 290.0000 10.0000 87.3850 62.6150

MAGNIFICATION CFG1 CFG2 CFG3 CFG4 CFG5 CFG6 CFG7

GRP1 -3.916e-12 -3.916e-12 -3.916e-12 -3.916e-12 -3.916e-12 -3.916e-12
-3.916e-12

GRP2 -0.5155 -0.5771 -0.6784 -0.8228 -1.0453 -1.4326 -2.0364

GRP3 -92.4215 26.7474 11.1628 7.8442 7.1690 10.3293 -28.7577

GRP4 0.0084 -0.0314 -0.0833 -0.1289 -0.1453 -0.0914 0.0260

POWER EFL FNP SNP FF BF

GRP1 0.0017 583.4775 1.7043 -3.5860 -581.7731 579.8914

GRP2 -0.0052 -193.2866 1.6894 -1.5966 194.9760 -194.8831

GRP3 0.0017 582.8782 4.6535 -20.6315 -578.2247 562.2467

GRP4 0.0022 459.8018 0.8461 -5.7644 -458.9557 454.0374

IMAGE EFFECTIVE INFINITY IMAGE FIELD

CFG EFL DISTANCE f/# f/# HEIGHT ANGLE MAG

1 233.2185 106.7000 11.6609 11.6609 4.9975 1.2282 -1.565e-12

2 283.0539 106.7000 14.1527 14.1527 4.9973 1.0120 -1.900e-12

3 367.8664 106.7000 12.2622 12.2622 5.0008 0.7787 -2.469e-12

4 485.4600 106.7000 12.1365 12.1365 5.0074 0.5901 -3.258e-12

5 635.1369 106.7000 14.4349 14.4349 5.0115 0.4510 -4.263e-12

6 788.9404 106.7000 16.4363 16.4363 5.0105 0.3631 -5.295e-12

7 887.8449 106.7000 17.7569 17.7569 5.0038 0.3227 -5.959e-12

**Solar Orbiter Magnetograph - Filtergraph Optics**

SRF RADIUS THICKNESS APERTURE RADIUS GLASS NOTE

15 \-- 100.000000 4.973212 AIR Intermediate focus

16 165.103614 5.000000 15.000000 BK7 \| Collimator Lens

17 -77.429158 110.788664 15.000000 AIR

18 \-- 400.000000 3.022085 AIR Pupil 1

19 -800.000000 -400.000000 22.433712 REFL Off-axis paraboloid 1

20 \-- -400.000000 19.411627 AIR Telecentric image plane

21 800.000000 \-- 22.329705 REFL Off-axis paraboloid 2

22 \-- 400.000000 22.329705 AIR

23 \-- 189.211336 3.022085 AIR

24 239.644089 5.000000 15.000000 BK7 \| Camera Lens

25 -190.995523 \-- 15.000000 AIR \|

26 \-- 1.931746 10.259100 AIR

26 \-- 204.705538 12.277548 AIR

IMS \-- \-- 10.023568 Detector surface

CONIC AND POLYNOMIAL ASPHERIC DATA

SRF CC AD AE AF AG

2 -1.000000 \-- \-- \-- \--

4 -0.622684 \-- \-- \-- \--

19 -0.492733 \-- \-- \-- \--

21 -1.000000 \-- \-- \-- \--

**Appendix B: Full Disk Telescope Configurations**

![](media/image7.wmf){width="6.271527777777778in"
height="3.9194444444444443in"}

Configuration 1

![](media/image8.wmf){width="6.271527777777778in"
height="3.9194444444444443in"}

Configuration 2

![](media/image9.wmf){width="6.271527777777778in"
height="3.9194444444444443in"}

Configuration 3

![](media/image10.wmf){width="6.271527777777778in"
height="3.9194444444444443in"}

Configuration 4

![](media/image11.wmf){width="6.271527777777778in"
height="3.9194444444444443in"}

Configuration 5

![](media/image12.wmf){width="6.271527777777778in"
height="3.9194444444444443in"}

Configuration 6

![](media/image13.wmf){width="6.271527777777778in"
height="3.9194444444444443in"}

Configuration 7

**Appendix C: VIM Optical Performance**

![](media/image14.wmf){width="6.271527777777778in"
height="3.9194444444444443in"}

High Resolution Telescope Wavefront Analysis.

![](media/image15.wmf){width="6.271527777777778in"
height="3.9194444444444443in"}

Wavefront Analysis of HRT and Filtergraph Optics combination.

![](media/image16.wmf){width="6.271527777777778in"
height="3.9194444444444443in"}

Full disk Telescope Configuration 1 (longest focal length)

![](media/image17.wmf){width="6.271527777777778in"
height="3.9194444444444443in"}

Full disk Telescope Configuration 3

![](media/image18.wmf){width="6.271527777777778in"
height="3.9194444444444443in"}

Full disk Telescope Configuration 5

![](media/image19.wmf){width="6.271527777777778in"
height="3.9194444444444443in"}

Full disk Telescope Configuration 7 (shortest focal length)