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Page in construction ; Last Update December 2024
===== THEMIS technical description =====
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:
* Observational electromagnetic spectrum range: 400-1100 nm
* Imaging field-of-view : 2’x2'; square shaped
* Overall focal ratio : f/62
* Effective focal length: ~57m
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. \\
Text in blue highlights a data product and shall redirect to the dedicated [[observation:data|THEMIS data products webpage]].
==== (1) Telescope Assembly ====
As 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 [[observation:data#Full-sun guider images| (cf. THEMIS data products)]].
==== (3) First optical focus instrumentation, F1 ====
The 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 correction ====
The [[technical:tao|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 [[results:gallery:tao | (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 [[technical:tao|dedicated page]].
==== (6) OBJ2 field scanning ====
To be completed
No user option available here.
==== (7) Beam splitters at F2 ====
THEMIS 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.
* Passthrough/Hole: 100% light transmitted to spectrograph; 0% reflected to context camera (no context camera data). This option is for planetary and stellar spectropolarimetry requesting 100% of the flux to the spectrograph. Obviously there is then no light on the context camera, hence no context camera data. However, there is light on the slitjaw camera with a mirror decker slit, providing a low quality image of the field,and showing the slit position. \\
* Plain lambda/10 mirror: 0% light transmitted to spectrograph (no slitjaw and spectrograph data); 0% reflected to context camera.This option is suitable for pure imaging program of (preferably) faint objects, e.g. planets, stars. It is also an engineering mode for tuning the adaptive optics NCP abberations. \\
* Custom wideband beamsplitter (BS): 80% light transmitted to spectrograph ; 20% reflected to context camera. This option allows the context camera to run in parallel with the spectrograph, providing a high quality field image together with the spectropolarimetric analysis. The ratio 20/80 has been chosen because the 2nd surface coating of the BS plate shall provoke a (displaced) 2nd image in the range 0.5 to 1%. A ratio of 20 to 40 % of reflection with the first surface is thus requested to safely ignore this issue, so that the flux of the first surface reflexion remains significantly larger than the second one. \\
* Wideband Film : 80% light transmitted to spectrograph ; 20% reflected to context camera. This option is offered as a backup in the case the BS would not work properly; Films have no 2nd image and a much larger transmitted fraction, the reflected 4% is largely enough to feed the context camera. However they have a poor optical quality and cannot be used for tuning the AO for NCP aberrations.
==== (8) Beam splitters filters & (9) Context camera ====
Our 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 [[observation:data#Full-sun guider images| (cf. THEMIS data products)]].
=== (10) Spectrograph slit at F2===
We currently offer 2 slit configuration:
* a mecanical adjustble slit
* a 45° mirror slit
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.\\
The 45° slit is primarily for faint objects observations that require 100% of the incoming light to be sent to the specrograph. In this case, the context camera is useless. This mirror decker diverts all the light that does not enter the spectrogaph to a side slitjaw camera. This camera can then provide the fov image, with the actual slit superimposed.
=== (10) Slit-jaw filters & (11) Slitjaw camera ===
Currently a Pixelink PLA720 showing the full 2' field on a 1280x1024 max resolution **//specs TBW//**
=== (12) Spectrographs ===
=== (13) Spectral cameras===
* 6 EMCCD Andor iXon DV897\ \ \ \ {{:call22:andor-emccd-ixon897.pdf| (iXon tech specs)}} \\ These are our first batch of moderm cameras, still very useful and mandatory for Mercury observations. A typical setup on these camera with a standard de-magnification will give a spectral pixel of ~ 0.0123 A/px (12.3 mA/px), and a spatial pixel of ~0.234 "/px; the full spectral range on the detector is about 6.3 A ,
* 2 sCMOS Andor Zyla 4.2 Plus\ \ \ {{:call22:zyla-4.2-specifications-physical-science.pdf| (Zyla tech specs)}} \\ Our Zylas are interfaced and tested, both optomechanically and as to software, but they haven't been yet used for science at this moment. A typical setup shall give a spatial pixel of ~ 0.06" and a corresponding spectral pixel about 3mA/px, which may be preferable to rebin (by a factor of 2 at least) in the lambda direction.
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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F1 instrumentation \\
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Cartoon of the different beam splitting options available at F2. \\
{{:call22:f2_slit_conf.png?direct}} \\
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