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--- technical:description [2026/03/05 17:37] – etienne
+++ technical:description [2026/03/17 15:57] (current) – etienne
@@ Line 12: / Line 12: @@
 Overall, the working characteristics of THEMIS are the following:
   * Observational electromagnetic spectrum range: 400-1100 nm
-  * Imaging field-of-view : ~2’x2’; square shaped
+  * Imaging field-of-view : ~2’x2’ square shaped without polarisation analysis; ~1’x1’ with polarisation
   * Overall focal ratio : f/62
   * Effective aperture: 92 cm
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 <newcolumn *40%*>
-<html> <a href="https://www.themis.iac.es/lib/exe/fetch.php?media=science:gallery:schema:themis_optpath_functionaldiagram2.jpg"><img src="https://www.themis.iac.es/lib/exe/fetch.php?media=science:gallery:schema:themis_optpath_functionaldiagram2.jpg" style="width:90%; height: auto;"></a></html>
+<html> <a href="https://www.themis.iac.es/lib/exe/fetch.php?media=science:gallery:schema:themis_optpath_functionaldiagram.jpg"><img src="https://www.themis.iac.es/lib/exe/fetch.php?media=science:gallery:schema:themis_optpath_functionaldiagram.jpg" style="width:90%; height: auto;"></a></html>
 </columns>
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 ==== (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 broadband (10 nm) green continuum 540nm photosphere filter (Baader Planetarium GmbH), and a ZWO ASI 178 mono (2000x3000 pixel²) CMOS camera. The full-sun guider image is always available in the control room of THEMIS. The full-Sun guider images are not meant for pure scientific analysis. 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). <html><font color="#ff9900"> Camera settings can be changed by the user in the control room. Acquisition is left to the needs and appreciation of the user. </font></html>\\
+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 broadband (10 nm) green continuum 540nm photosphere filter (Baader Planetarium GmbH), and a ZWO ASI 178 mono (2000x3000 pixel²) CMOS camera. The full-sun guider image is always available in the control room of THEMIS. The full-Sun guider images are not meant for pure scientific analysis. 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). <html><font color="#ff9900"> Camera settings can be changed by the user in the control room. Acquisition is left to the needs and appreciation of the user. </font></html>\\
  <html><font color="#0000ff"> The image from this camera is an available data product </font></html> [[observation:data#Full-sun  guider images| (cf. THEMIS data products)]].
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-==== (3) First optical focus instrumentation, F1 ====
+==== (3) First optical focus (F1) : light polarization analysis instrumentation ====
-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.
+The original fundamental //raison d'être// and strength of THEMIS is to be a polarisation-calibration-free telescope thanks to the polarimetric analysis being performed immediately at the first optical focus (F1). This contrasts with imagery-driven telescope for which adaptive optics (AO) is usually located at F1.
+Maintaining THEMIS excellent polarimetric sensitivity has been one of the main challenge of the upgrade of THEMIS and the development of [[technical:tao |THEMIS adpative optics (TAO)]]. The original optical design of THEMIS was composed of a double beam, each of them carrying the opposite polarisation state and having its own optical path. This design was not compatible with the existence of an AO in which the wavefront correction requires an unique light beam. The inclusion of TAO has thus required the complete redisign of THEMIS optical path as well as polarisation analysis instrumentation.
+As of today, a Full-Stokes analyzer (An4) is located at F1, producing superposed dual-beam polarimetry with beam exchange. The analyser is composed of  double Savart plates. The first Savart plates generate the dual beam, each of them carrying opposite polarisation state, e.g. I±S, with S the Stokes polarization parameter. The second Savart plates then superimpose both beams, each of them in a polarisation state that is orthogonal to each other. Both beams then do behave as one, differing only by their linear polarization state.
+Thanks to THEMIS “polarization friendly” new optical path (geometry of the elevation axis, field rotator, coatings), polarizer output can travel through the telescope and reach the spectrograph cameras “minimally perturbed”. Latter, just ahead of each of the spectral cameras, a Wollaston prism splitter separates the superimposed beam into complementary Stokes components to form the spectral focal plane.
-Polarimetric analyser : <html><font color="#ff0000" size="1.5"> Information to be added</font></html>
+<html><font color="#ff0000"> As of today, THEMIS remains a strongly polarization-calibration-free telescope, ideal for excellent spectropolarimetric measurements.</font></html>. Further information, such as THEMIS Mueller matrix can be consulted on the dedicated page about [[technical:polarisation | THEMIS polarimetric analysis scheme.]]
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 ==== (8-9) Broadband Imaging (BBI) filter & camera ====
-If illuminated, the BBI camera offers a broadband high-resolution image with a square field-of-view of about ~55"x55". A broadband red filter, with a ~3nm passband centred around 656 nm is placed upstream of the camera. THEMIS BBI camera thus currently provides images or the red visible continuum.
+If illuminated, the BBI camera offers a broadband high-resolution image with a square field-of-view of about ~55"x55". A broadband red filter, with a ~3nm passband centred around 630 nm is placed upstream of the camera. THEMIS BBI camera thus currently provides images or the red visible continuum.
 THEMIS BBI 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. <html><font color="#ff9900"> Camera acquisition settings can be changed by the user in the THEMIS control room. Acquisition is left to the needs and appreciation of the user. </font></html>\\
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 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.
+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 540 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 3000x2000 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.