Page in construction ; Last Update December 2024
THEMIS is a versatile 90cm Ritchey-Chrétien optical solar telescope that can be used in daylight for solar or bright objects observations, or at night for fainter objects. THEMIS handle the (extremely) wide range of available light energy flux through a dedicated light distribution detailed below. Overall, the working characteristics of THEMIS are the following:
The figure opposite presents a diagram with diverse functional blocks of THEMIS systems. A more realistic cartoon (although simplified) of the light-path within THEMIS is presented hereafter. Below is a description of each of THEMIS functional blocks.
Text in orange indicates to interested THEMIS users the available observing options (if any) on each particular system. (1) Telescope AssemblyAs a majority of large professional research telescopes, THEMIS is a Ritchey-Chrétien type telescope, with primary (M1) and secondary (M2) mirrors having hyperbolic shapes. This design allows to eliminate off-axis optical errors (e.g. comatic aberration) and thus offers wider field of view free of optical errors compared to traditional telescope. The M1 mirror of THEMIS has a width of 95cm with an effective aperture of 92 cm. M1 is made in Zerodur (lithium-aluminosilicate glass-ceramic), hence having a near zero thermal expansion, and has a protected silver coating, permitting a near 99% light transmission. The M2 mirror has a width of 30cm and is also Zerodur made with protected silver coating. The entrance plate of the THEMIS telescope tube is slightly prismatic, which allows to remove some interferences, makes it slightly chromatic. A filter latter allow to deal with this slight chromatism. The telescope tube has a length of 4m. While originally design to be vacuum sealed, the telescope tube is filled with Helium at half local atmospheric pressure. This enables optimum performance in order to reduce turbulence within the telescope tube and stress on the entrance plate (which would have been excessively important under vacuum conditions. The THEMIS telescope is actively cooled by a water heat-exchanger system, having typical controlled working temperature of about 5-10°C. The telescope is supported by an alt-azimuthal mount. Tracking is permitted thanks to XXXX. In order to avoid the transmission of vibrations to the instrument suites from difference sources within the THEMIS building (generators, compressor, pumps, …), the whole THEMIS scientific instruments are isolated from the THEMIS building, resting on a fully distinct inner concrete tower. The relay instrumentation as well as the whole spectrograph are thus “hanging” on the THEMIS mount. The overall the weight of the THEMIS assembly is about 30-40 tons. It is supported and stabilized on the inner tower thanks to a XXXX. tbc
The telescope optical path has been modified in 2018 to allow for the simplification of the transfer optics from the first optical focus (F1) to a new secondary focus (F2'). The secondary mirror has been refigured, together with a change of the exit window (now an exit lens with optical power and positive chromatic effects). The main resulting characteristics are a new f/16.58 F1 (quite close to the former version), and a new position (lower on) for this focus. No user option available here. (2) Full-Sun guider
A full-sun guider has been setup on the telescope outer ring of the heat protection, near the 1m entrance plate. It uses a 45/500 mm objective, an Herschel prism, a neutral filter, a green continuum 540nm photosphere filter, and a ZWO ASI 178 mono (2kx3k) CMOS camera.The full-sun guider image is always available in the control room of THEMIS. Please note that given the location of the entrance pupil and depending on the telescope/dome relative positions, this guider may be momentarily obscured by the dome edge(for less than 30 seconds in any circumstance). The image from this camera is an available data product (cf. THEMIS data products). (3) First optical focus instrumentation, F1The original raison d'être of THEMIS was to be a polarisation free telescope thanks to polarimetric analysis being performed immediately at the first optical focus, while adaptive optics (AO) is standardly located at F1 in professional telescopes. Maintaining its excellent polarimetric sensitivity has been one of the main challenge of developing the THEMIS AO. Polarimetric analyser : Information to be added (4) Second optical focus instrumentation, F2'The creation of the F2' optical focus mainly results from the redesign of the THEMIS optical path and the need to shape the light beam onto the Adaptive optics system with the adequate size (on a 15mm pupil). A translation stage is present that presents different set-up conditions. Several optical elements are present in order to help for the optics alignment and adaptive optics calibration: pinhole, lens-slot & laser. These are not used in observation mode. During observation, a mask is placed at F2' so has to deliver a squared field-of-view of 2'x 2' on the Sun/plane of sky. No user option available here. (5) Adaptive Optics correctionThe THEMIS adaptive optics (TAO) system has been the main goal of the 2015-2018 THEMIS re-design. Since it's first light in 2020, TAO has permitted THEMIS to improve very significantly its imaging capacity and reach its diffraction limits (see dedicated TAO gallery). Users can perform THEMIS observation with or without TAO. TAO has been tested for solar disk observations, e.g. sunspots & granulation, with good results over significantly long periods of time (seeing dependent). At the moment it is not possible to use the AO over the solar limb (or for neighbouring prominences). For Mercury observations, a slowed-down (100Hz) version of the same system can be used to stabilise Mercury. More information about TAO is available on the dedicated page. (6) OBJ2 field scanningTo be completed No user option available here. (7) Beam splitters at F2THEMIS has currently no unique solution for a feeding a context camera in all the possible situations of flux. This is why, just ahead of the F2 optical focus, a translation stage is present with diverse beam-splitting options. THEMIS observer shall decide on the adequate option. The choice correspond to the relative % of light flux that feeds the context camera or the spectrograph. We recommend the user to choose one configuration for the whole run, as for now the amount of refocusing and adjusting the flux on the camera after a change is not precisely known.. A cartoon of the different option is presented on the right.
(8) Beam splitters filters & (9) Context cameraOur 3 beamsplitting options are splitting on a wideband and with very different range of flux. A choice of broad band interference filters and matching neutral densities to suit a particular request is available (list under construction). Context camera is currently a 2kx2k Andor Zyla (refs). Acqusition system and post-processing are available, specs TBW The images from this camera are an available data product (cf. THEMIS data products). (10) Spectrograph slit at F2We currently offer 2 slit configuration:
The mechanical slit is the “historic” Themis slit (since 2004 at least), and is suitable for solar observation. If the context camera is used, then the fov image is available and this slit can be used, with the advantage that it is continuously adjustable to any width. (10) Slit-jaw filters & (11) Slitjaw cameraCurrently a Pixelink PLA720 showing the full 2' field on a 1280×1024 max resolution specs TBW (12) Spectrographs(13) Spectral cameras
Spectrograph cameras are at the “camera focii”, which differ from the spectrograph focus (“SP2” focus), because the focal scale of the latter is way too large for the spectral image to fit over modern detectors. The de-magnification comes with a turn in the geometry: the SP2 output is directed toward the ceiling of the spectrograph, but the cameras are on a horizontal beam. The optical assy performing this function is call “barette” (in french) and tuning the barettes is a part of the user's setup. Typical de-magnification assuming the complete spatial field is on the detector is: ~3.8 for an iXon camera and ~2.25 for a Zyla. These numbers hold in spectroscopic or spectropolarimetric mode, but for spectropolarimetry the spatial field is reduced (stopped at the F2) to make space on the detector for the dual beam polarimetric output. |
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