**Solar Orbiter**

**Visible Imager / magnetograph (VIM)**

Optical Layout

*Oskar von der Lühe^1^ and Achim Gandorfer^2^*

> ^1^) Kiepenheuer-Institut für Sonnenphysik, Freiburg, Germany
>
> ^2^) Institut für Astronomie, Eidgenössische Technische Hochschule,
> Zürich, Switzerland

June 2000

**Introduction**

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

1)  25 cm High Resolution Telescope (HRT)

2)  6 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 full
solar disk. The filtergraph optics include a tandem Fabry-Pérot
interferometer equipped with a narrow-band interference filter as a
prefilter, tuned to the Na D 589 nm line. The Magneto-Optical filter
which was originally included in the design is unable to adjust to the
lineshifts due to sun-spacecraft relative velocities up to 40 km s^-1^.
We describe all components in the following sections.

**High Resolution Telescope**

The HRT is a 25-cm-diameter Gregorian reflector. It provides the
high-resolution image, with a spatial resolution as high as 37 km on the
solar surface during the near-Sun orbital phase. The telescope has an
F/17 focus with a radius of 5.2 mm and an effective focal length of 4250
mm. The full field of view is 0.138 degrees or 8.3 arc minutes.

A broad-band interference filter 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. The F/2 primary mirror produces a solar image at its
(primary) focus whose diameter varies between 5 mm and 25 mm throughout
the orbital phases of the spacecraft. A combination of decentering and
tilt of the secondary mirror provides telescope pointing on the solar
disk without disturbing the image quality. Baffling at the focus and at
intermediate pupils rejects remaining sunlight. Secondary magnification
is 8.5.

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
shows the surface design data which are used in this document.

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

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

**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 6-cm diameter refractor with a variable focal
length which provides a full-disk image with the same diameter during
all orbital phases. A zoom range of 3.5:1 is needed to fully fill the
detector throughout the later orbital phases between 0.2 and 0.7 AU. A
separate mechanism will provide the zoom function.

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 preliminary surface design
data of the FDT are given in Table 1. The zoom lens consists of a PNP
triple lens made from singlets, because chromatic aberrations are of no
concern with a single wavelength narrow-band filtergraph. The zoom lens
may be replaced by an exchange mechanism for discrete objectives during
a refinement of the design.

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

Figure 2: perspective view of the optical layout of the full disk
telescope.

**Filtergraph Optics**

The FO consists of two singlet lenses which combine the functions of
collimator and camera witha 1:2 magnification. A system of two off-axis
paraboloid mirrors provide a 1:1 pupil relay function and produce an
F/132 telecentric intermediate image plane.

Two 50 mm diameter Fabry-Pérot etalons at the telecentric image plane
provide the required spectral resolution of typically 3 pm. If
necessary, a separate narrow band interference filter will block the
secondary transmission maxima of the combined etalons. The gap width of
the two etalons are electronically controlled to also provide spectral
scanning over a ±50 pm range which is sufficiently large to cover both
line width and sun-spacecraft velocity shifts.

The principle proposed here is the same as 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 etalons are
not included but their position indicated.

**Overall Configuration**

The combination of all optical subsystems is shown in Figure 4. 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.

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

Figure 3: optical arrangement of filtergraph optics.

![](media/image4.png){width="4.377083333333333in"
height="2.1743055555555557in"}![](media/image5.png){width="1.6875in"
height="2.140972222222222in"}![](media/image6.png){width="4.346527777777778in"
height="2.051388888888889in"}

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. The optics fit a 300 mm x
400 mm x 1600 mm envelope.

**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

**Solar Orbiter Magnetograph - Full Disk Telescope**

SRF RADIUS THICKNESS APERTURE RADIUS GLASS NOTE

OBJ \-- 1.4900e+14 3.1211e+12 AIR Solar Surface at 1 AU

1 \-- 400.000000 28.378806 AIR Entrance aperture

AST \-- \-- 20.000000 AIR

3 352.681541 8.000000 25.000000 BK7 First lens of zoom system

4 \-- 109.200000 25.000000 AIR

5 -149.042262 5.000000 25.000000 BK7 Second lens of zoom system

6 -275.699183 V 77.800000 25.000000 AIR

7 112.330000 8.000000 25.000000 BK7 Third lens of zoom system

8 300.726795 V 49.700000 25.000000 AIR

9 \-- 20.000000 20.000000 BK7 Modulator

10 \-- 50.000000 20.000000 AIR

11 \-- \-- 16.080690 AIR

12 \-- \-- 16.080690 REFL Folding mirror 1

13 \-- -100.000000 16.080690 AIR

14 \-- \-- 12.146739 AIR

15 \-- \-- 12.146739 REFL Folding mirror 2

16 \-- 100.000000 12.146739 AIR

17 \-- 1.023967 8.212789 AIR Intermediate focus

**Solar Orbiter Magnetograph - Filtergraph Optics**

SRF RADIUS THICKNESS APERTURE RADIUS GLASS NOTE

15 \-- 100.000000 5.118352 AIR Intermediate focus

16 113.628966 10.000000 15.000000 BK7 \| Collimator Lens

17 -98.316225 109.850909 15.000000 AIR \|

18 \-- 400.000000 3.048863 AIR Pupil 1

19 -800.000000 -400.000000 22.806033 REFL Off-axis paraboloid 1

20 \-- -400.000000 19.757170 AIR Telecentric image plane

21 800.000000 \-- 22.790932 REFL Off-axis paraboloid 2

22 \-- 400.000000 22.790932 AIR

23 \-- 190.149091 3.048863 AIR

24 225.582023 10.000000 15.000000 BK7 \| Camera Lens

25 -200.980693 202.520691 15.000000 AIR \|

26 \-- 1.931746 10.259100 AIR

IMS \-- \-- 10.236975 Detector surface

Table 1: optical design data of HRT, FDT, and FO.