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--- science:imofmonth [2025/08/12 16:10] – [Images of the month] etienne
+++ science:imofmonth [2025/10/15 11:14] (current) – etienne
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-===== Images of the month =====
+ ===== Images of the month =====
+=== September 2025 ===
+<columns 100% l >
+<html> <a href="https://www.themis.iac.es/lib/exe/fetch.php?media=science:gallery:schema:opticalpath_tao.png
+"> <img src="https://www.themis.iac.es/lib/exe/fetch.php?media=science:gallery:schema:opticalpath_tao.png
+" alt="Diagram of THEMIS adaptive optic light-path." style="width: 80%; height: auto;"></a></html> \\
+THEMIS design, laid out in the 1980's, predate the revolution of adaptive optics (AO), and hence didn't foresee its installation. AO has since proved to be an indispensable technology of all modern high-resolution telescope. The core of the THEMIS overhaul that took place in the second half of the 2010's had the installation of a simple AO at the core of its objective.
+The installation of the [[technical:tao|THEMIS AO]] was thus particularly challenging, as it required to deal with the limited existing available space. The entry prism is thus located just inside the spectrograph container, close to its roof, in a location that is not easily accessible. In addition, as shown in the diagram of TAO, it imposed to place the wavefront sensor optical bench (as well as the scanning mirror) outside of the THEMIS spectrograph container, in a vertical layout.
+\\
+</columns>
+=== August 2025 ===
+<columns 100% l >
+<html> <a href="https://www.themis.iac.es/lib/exe/fetch.php?media=science:gallery:protu:limbprominenceobservation_20250506_withandwithoutao.jpg
+"> <img src="https://www.themis.iac.es/lib/exe/fetch.php?media=science:gallery:protu:limbprominenceobservation_20250506_withandwithoutao.jpg
+" alt="Prominence Halpha observations at limb: Adaptive Optics comparison" style="width: 80%; height: auto;"></a></html> \\
+During the 2025 observation campaign of Brigitte Schmieder and Arek Berlicki, THEMIS targeted a small prominence. Prominence are emitting structures that can be observed at the solar limb, beyond the outer edge of the Sun's disk. Prominences and filaments are two aspects of an unique physical feature: a domain of cold chromosphere like plasma, magnetically confined in the much hotter solar corona. While this structure appears as dark when seen in contrast with the disk, and is then called a filament, it appears bright in contrast to the plane of sky. As a magnetised structure filament/prominence can erupt, releasing plasma and material toward the solar system. Understanding how the magnetic structure them is fundamental to comprehend their stability or lack off.
+THEMIS is mainly a [[technical:mtr2 | scanning spectrograph instrument]], i.e. a very thin slit scans the region of interest, in order to obtain high resolution spectrograms. THEMIS images are thus reconstructed. While scanning, THEMIS is thus very sensitive to the effect of turbulence and the reconstructed image have necessarily a lower resolution that direct imaging. On the other hand, THEMIS can deliver simultaneous images at different wavelength.
+While adaptive optics can generally be used to significantly improve on disc observations, such as with our own [[technical:tao|Themis AO]], AOs fail when trying to observe over the limb because no structure there can be tracked by the AO. Recently our colleagues of the U.S. National Science Foundation National Solar Observatory and New Jersey Institute of Technology, observed [[https://nso.edu/press-release/new-adaptive-optics-shows-stunning-details-of-our-stars-atmosphere/ | beautiful prominence dynamics]] with the Goode Solar Telescope thanks to their [[https://www.nature.com/articles/s41550-025-02564-0 |new coronal adaptive optics]] in a direct imaging approach.
+At THEMIS, during our 2025 campaign we tried a different approach. While we were scanning a prominence at the limb, we use TAO on an offset region which is close to the limb while still on the disc. TAO is a simple AO which isoplanatic region (region where most of the AO correction is done) is limited. Since the isoplanatic region and the region of scientific interest are far away, the AO correction may be limited at the prominence. We were however very please to see that TAO still provide significant improvements. As can be seen in this image of the month, the turbulence induced motions (in the direction of the slit) which are present when TAO is off, are strongly reduced when TAO is switched on. The limb appears much smoother and the prominence better resolved.
+\\
+</columns>
 === July 2025 ===