Information on mirror and general description to be added

As a majority of large professional research telescopes, THEMIS is a Ritchey-Chrétien type telescope, with primary and secondary 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 90cm primary mirror of THEMIS is made of … and coated with… The secondary mirror has a size of and is …

The telescope tube has a length of … While originally design to be vacuum sealed, the telescope tube is filled with Helium at 0.5 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 thanks to… This allows to have typical working temperature of about 5-10°C on the different system

The entrance pupil of the THEMIS telescope tube…

Overall the weight of the THEMIS telescope assembly is about…

The telescope optical path has been modified in 2018 to allow for the simplification of the transfer optics F1 → 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.

Information to be added

Status of Nighttime 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 image from this camera is an available data product (cf. THEMIS data products).

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).

To be completed

See the dedicated page about the THEMIS adaptive optics

We currently have no unique solution for a feeding a context camera in all the possible situations of flux. This is why we setup a translation stage with the following beam-splitting options:

• Passthrough (100% transmitted, 0% reflected). This option is for planetary and stellar spectropolarimetry requesting 100% of the flux to the spectrograph. Obviously there is no light on the context camera, but 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% transmitted, 100% reflected).This option is suitable for pure imaging program of (preferably) faint objects. It is also an engineering mode for tuning the adaptive optics NCP abberations.

• Custom wideband beamsplitter (BS) (80% transmitted, 20% reflected over 400-1100 nm). 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 is choosen because the 2nd surface coating of the BS plate shall provoke a (displaced) 2nd image in the range 0.5 to 1%, hence a ratio 20 to 40 with the first surface, requested to safely ignore this issue.

• Pellicle (96% transmitted, 4% reflected, wide band). This option is offered as a backup in the case the BS would not work properly; Pellicles 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 abberations.

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.

7. Beam splitters filters
Our 3 beamsplitting options are splitting on a wideband and with very different range of flux. A choice of broad band inteference filters and matching neutral densities to suit a particular request is available (list under construction).

8. Context camera
Context camera is currently a 2kx2k Andor Zyla (refs). Acqusition system and post-processing are available, specs TBW

9. F2 spectrograph slit
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. Slitjaw filters

11. Slitjaw camera

Currently a Pixelink PLA720 showing the full 2' field on a 1280×1024 max resolution specs TBW

12. Spectrographs

13. Spectral cameras

• 6 EMCCD Andor iXon DV897        (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      (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.