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).

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

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. The light beam has then a power of about 15 Watts.

No user option available here.

The 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.

The OBJ2 movable mirror allow a fine scanning of the field of view one the Sun. This allows to move the field of view on the target without the need to move the telescope (the latter being much less precise). This scanning mirror enables displacement with increment as small as 0.01“. This would however be smaller than the seeing limits even with adaptive optics working. Typical observations uses scanning steps of the range 0.1” to a few “.

Users not performing sit-and-stare observations shall define the scanning properties for their observations.

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 slit-jaw 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 slit-jaw 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.

If illuminated, the context camera offers a high-resolution image of THEMIS with a square field-of-view of about ~55”x55“ in the red continuum. A red filter, with a ~10nm passband centered around 650 nm is placed upstream of the camera.

The context camera is currently a 2000 x 2000 pixels Andor Zyla camera. This camera relies on the scientific CMOS technology (active-pixel sensor). This offers a high quality image of a subset of the THEMIS field-of-view with a very fast read-out, enabling the capture of a burst of images capture (up to 40 images/second) that are ideal for post image reconstruction.

The red continuum images from this camera are an available data product. Post-observation image processing methods & routines (Knox-Thompson reconstruction method) are available (cf. THEMIS data products).

The second optical focus F2 hosts the entrance slit of the spectrograph. THEMIS currently offers 2 slit configurations.

• mechanically adjustable slit: The mechanical slit is the “historical” THEMIS slit (since 2004 at least. It is suitable for solar observation. This slit has the advantage that it is continuously adjustable to any width, with a precision of about 0.1”. The width of the slit affects resolution; the narrower the slit, the higher the spectral resolution. However, narrower slits also decrease signal strength. When choosing the slit width, users shall must be balanced these two factors. The slit width also possibly has an impact on the choice of the scanning steps if the users when to do a spatial scanning of a given region.

• Slit-jaw configuration / 45° mirrors slit: two aligned mirrors separated by a gap that constitute the entrance slit of the spectrograph. With these slit-jaw mirrors, the width of the slit cannot be adjusted and is about ~0.33“. The mirrors have a 45° inclination relatively to the incoming light beam, enabling to divert the light that does not enter the spectrograph to a side slit-jaw camera. This configuration has the advantage that the observers can precisely see where the slit is located on the THEMIS field-of-view. The slit-jaw configuration is thus essential for planetary observations to ensure that slit is precisely on the target.

A cartoon of the two configurations is presented on the right. The F2 slit is illuminated by a 2'x2' field of view. When polarimetric observations are performed, because of the latter need of a dual beam polarimetric output, the field of view must be reduced to 2'x 1' (along the slit length). The orientation of the slit relatively to the field of view can be modified. Typical solar observation permits to put the slit align with the solar north or parallel to the solar equator.

If the slit-jaw slit configuration has been chosen at F2, the light not entering the spectrograph is captured by the slit-jaw camera. Slit-jaw images offers the context images of the spectrograph slit and allows a precise knowledge of the localisation of the slit.

As almost all the remaining 15 watts of radiative power are then directed toward the camera, filters shall be place upstream of the camera to reduce the light flux. Presently a green continuum filter at XXX nm, with a 10 nm passband is used.

The slit-jaw camera is currently a ZWO ASI 178MM camera capturing the F2 field-of-view on a 3000×2000 pixels array. It's a CMOS camera. Without polarimetry, the field of view is 2'x2', while it is reduced to 2'x1' when polarimetric analysis are carried.

The green-light continuum images from the slit-jaw camera are an available data product. (cf. THEMIS data products)

The THEMIS spectrograph is an essential block of the THEMIS instrumentation that enable the formation of the solar spectrum and its detailed analysis. MTR2 is the advanced version of the original MulTi-Ray spectrograph of THEMIS. MTR2 enables the production of high-spectral resolution spectrograms simultaneously in different wavelengths. This enables the study of different physical properties of the distinct layers of the solar atmosphere.

Numerous user options are available here concerning the choice of the combination of spectral lines. More complete information shall be obtain on the MTR2 description webpage. In particular, user can check the list of existing masks (the physical wavelengths selector within the spectrograph) to have an idea of possible line observation combination.

At the exit of the spectrograph, several cameras are placed so that to record simultaneous spectrograms (lambda,y) in different wavebands. The spectrograms have a typical spectral range of 6-7 Å (0.6-0.7 nm) depending of the wavelength. The spatial direction has an extend of 1' when performing polarised observations, 2' otherwise. When performing polarimetric observations, two spectrograms are fitted on the camera field-of-view, each including the opposite polarimetric state (e.g. I+V and I-V Stokes).

The spectrographic images from the spectral cameras are the main data product of THEMIS MTR2 spectrograph observation mode. (cf. THEMIS data products)

The different cameras used are:

• 6 EMCCD Andor iXon DV897        (iXon tech specs)
  These CCD cameras are THEMIS first batch of modern 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 Å/px (12.3 mÅ/px), and a spatial pixel of ~0.234 ”/px; the full spectral range on the detector is about 6.3 Å.
• 2 sCMOS Andor Zyla 4.2 Plus      (Zyla tech specs)
  The cameras are THEMIS more modern CMOS technology spectral cameras. A typical setup shall give a spatial pixel of ~ 0.06“ and a corresponding spectral pixel about 3mÅ/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 assembly 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.