Visible-Light Imager and Magnetograph (VIM)

An instrument on the model payload of the future Solar-Orbiter mission

The Visible-Light Imager and Magnetograph (VIM) is a remote-sensing vector magnetograph and Doppler-graph. It will image at high resolution magnetic fields and flow motions in the solar photosphere. The instrument package contains two telescopes and a filtergraph.

	Science Objectives
	The Instrument
	MPS Contribution
	The Team
	Related Links
	VIM Homepage
	Solar Orbiter publications by MPS members

Science Objectives

The Visible-Light Imager and Magnetograph (VIM) will determine the boundary conditions for the MHD processes observed by other remote sensing instruments on Solar Orbiter. It will observe the morphology and dynamics of magnetic field structures and plasma motions at the photospheric level. Its resolution is consistent with the resolution of the EUV telescopes.

VIM will have vector magnetic field capabilities as this is of fundamental importance to understand the nature of photospheric magnetic fields. Having vector capabilities is also the only way in which quantitative inferences of the magnetic field in the transition region and corona can be made (from force-free or full 3D MHD extrapolations)

VIM will produce line-of-sight velocity maps by determining the Doppler shift of a spectral line. These maps can be used, through local helioseismology techniques, to investigate sub-surface flows. The internal structure and dynamics of the near-polar regions of the Sun is of paramount importance and perhaps THE key to our understanding of the solar cycle.

VIM will also provide images, Dopplergrams and magnetograms from the far side of the Sun (the side which is not seen from Earth).

The key scientific goals of VIM are:

Magnetic coupling science: Photospheric vector-magnetograms allow 3D field extrapolations into the transition region and corona
Polar view: Helioseismic and magnetic field measurements at the Sun's polar regions will provide crucial constraints on the solar dynamo theories
Stereoscopy: Simultaneous observation from different viewpoints (co-ordinated with near-Earth-orbit instruments) will establish a stereographic view of photospheric structures. VIM will provide even data from the Sun's far side
Co-rotation: Long-term observations of structures from almost non-variable vantage points will be enabled during perihelion passages
Helioseismology: VIM will continuously provide high-cadence velocity maps, excellently suitable for both local and global helioseismic studies of sub-surface flows and magnetic field structures

The Instrument

The VIM instrumental concept consists of:

HRT: a 125 - 160 mm diameter High Resolution Telescope which achieves an angular resolution of 1.00 arcsec (150 km on the Sun at minimum perihelion distance of 0.22 AU). Its field of view will be 1000 x 1000 arcsec²
FDT: a 13 - 17 mm diameter Full Disk Telescope which will obtain 2k x 2k full disk images
FO: a Filtergraph Optics based on two LiNbO3 Fabry-Pérot etalons. Its transmission passband will be between 7 and 10 pm wide and its central wavelength shall be tunable in a range of +/- 0.1 nm around the FeI 617.3 nm spectral line
PMP: two Polarisation Modulation Packages based on liquid crystals which will allow VIM to provide longitudinal and transversal magnetograms of the region being observed
ISS: Due to data processing requirements the VIM pointing needs to be extremely stable, better than 0.02 arcsec in 10 s, and, therefore, an Image Stabilisation System will significantly improve over the pointing accuracy of the spacecraft

The two telescopes alternatetly deliver images of the Sun's photosphere at a common focal plane. The Filtergraph Optics produces magnified images of narrow bandwidth using a double Fabry-Pérot interferometer. The wide tuning range of the FO allows scanning across the FeI 617.3 nm spectral line. In addition, Doppler shifts of +/-0.06 nm, emerging due to cruise speed variations of the spacecraft, have to be compensated for. The two PMP packages will produce modulation on an APS detector as a function of the polarisation state of the incoming light. The intensity change will be used to recover the Stokes vector of the solar light, which allows a determination of the full magnetic field vector and the line-of-sight velocity by means of Milne-Eddington inversion calculations.

MPS Contribution

The MPS will be assigned with the system responsibility for the implementation of the VIM instrument. This comprises the development of a stable housing and its connections to the spacecraft's main body. Due to the close solar proximity during Solar Orbiter's perihelion passages all instrument components are subject to stringent conditions regarding radiation hardness and thermal stability.

The MPS will be responsible for the full opto-mechanical development of the High Resolution Telescope. The prospective optical concept will be based on an off-axis Ritchey-Crètien telescope with an aperture diameter of 160 mm and a focal length of approximately 1000 mm. An off-axis design is preferred since it causes reduced thermal stabilisation complexities which pose the main challenges of the instrumental design.

The MPS will provide the focal plane assembly of the VIM instrument. This comprises the development of a fast 2k x 2k APS-detector system and the associated front-end electronics which can be qualified for the space mission.

The Team

Dr. Achim Gandorfer
Dr. Laurent Gizon
Hermann Hartwig
Dr. Johann Hirzberger
Dr. Andreas Lagg
Hendrik Preiß
Dr. Udo Schühle
Prof. Dr. Sami Solanki
Dr. Joachim Woch
Lotfi Yelles Chaouche