The SAMNET station instrument called SAMM is made up by a suite of four remotely operated 23.5cm diameter telescopes, each feeding a separate MOF (Magneto Optical Filter) assembly. The instrument will take line-of-sight (LoS) velocity field and magnetic field images of the Sun.
A single station instrument will be a four channel MOF telescope. Each channel will be tuned to one of the Fraunhofer absorption lines indicated in the following table. The table also shows the approximate formation height of these lines in the solar atmosphere. As each line is formed at a different height, this will allow a tomography of the solar atmosphere to be performed.
||Diffraction limit resolution with 23.5cm telescope (arcsec)
Each channel will provide a LoS velocity field image (Dopplergram) and/or a magnetic field image (magnetogram) of the solar atmosphere at that height. Output frequency is expected to be equal to 4 magnetrograms (1 for each observation line) every 5 s. This will provide high temporal resolution sampling of the solar atmosphere.
The Field of View (FOV) of the instrument will be 500", imaging the centre of the solar disk.
The scientific cameras will provide a 0.25" pixel scale. The cameras and computers used will provide high-speed acquisition, allowing a magnetogram/Dopplergram to be acquired every 5 s. The cameras will have very low electron noise (ca. 1 e-/s) so as to be essentially photon-noise limited.
To provide compensation of atmospheric aberrations, a high speed (1 kHz) tip-tilt corrector will be installed. It is expected that a 23,5 cm diameter telescope will not sample any order of atmospheric aberrations higher than the tip-tilt one. This will allow exposure times greater than the millisecond timescale typical of atmospheric coherence times and will allow the instrument to be essentially diffraction-limited.
A SAMNET station instrument is made up of four channels, one for each observation wavelength. Each channel assembly will be mounted on a purposely built telescope mount: each mount will be able to support two channel assemblies. The mount will include pointing and tracking systems to allow correct tracking of the Sun with the necessary angular resolution and stability.
Each 2-channel system (2 channels + 1 mount) will be housed in a remotely operated SAMNET dome, for a total of 2 domes per SAMNET station.
Each dome will be composed of a ca. 200x200x180 cm enclosure with an opening roof; rain sensors will allow automatic closure of the dome; a windspeed sensor will also be provided; a surveillance camera will allow remote visual inspection of the inside of the dome and of sky conditions. Management of the sensors, of the opening/closing mechanism of the dome and of the mount will be provided by a separate computer.
Each single channel assembly will comprise:
- rejection filter (to reduce solar heating in the instrument)
- bandpass filter (centred around the observed line)
- two MOF filters
- two Liquid Crystal Variable Retarders (LCVR)
- two polarizers
- tip-tilt mirror
- relay optics
The MOF filters will be contained in a thermally controlled enclosure to ensure their optimal performance. A thermal regulation system will ensure the correct operating temperature of the MOFs and the LCVRs. The cameras will be cooled to reach the correct temperature for low-electron-noise operation. The components will be assembled in an enclosing container to provide protection from dust. The whole channel assembly will be connected to the mount. Each dual channel set will have a separate computer which will provide control signals for the LCVRs and image acquisition from the camera.
Two domes will be consructed per SAMNET station, each for the observation of two absorption lines. Timing synchronisation between the acquisition computers will be provided to ensure correct timestamping of the images and simultaneity of acquisitions in different lines. This will allow correct tomography of the solar atmosphere at a given instant.