TAO stands for THEMIS Adaptive Optics. It is made of a classical solar AO hardware currently running at 1 kHz and combining a Shack-Hartmann wavefront sensor with 76 subapertures (10×10 geometry) and a deformable mirror DM97 from ALPAO with 97 actuators. See TAO system description for more technical details. It was implemented by the Themis team (IRL-FSLAC), after integration at CRAL (CNRS/UMR 5574). Innovative algorithms are used for solar wavefront sensing and the control loop, based on an end-to-end inverse approach and a minimum variance estimator. Commands are computed iteratively, without any matrix inversion, using a method developped for Extremely Large Telescopes.

On Dec 8th, 2020 TAO started running in closed-loop over the solar granulation and the existing sunspot at that moment (NOAA 12790). We present here some preliminary and extremely encouraging results, while the closed-loop algorithm is not yet fully implemented.

• This movie is one of our very first attempts, and the first on a sunspot.
• The wavefront sensor (WFS) is locked for 30s (as per operator’s choice) in the penumbra of NOAA 12790. Lock point is indicated by the green cross.
• Field size is ~23 arsecs, the WFS is running in white light and the field camera has a 650nm filter.
• TAO brings an impressive improvement in image quality, making penumbra filaments and neighboring granules quite visible. Most of the tip-tilt image motion is removed.
• Without AO, a visual estimate of seeing is $𝑟_0$≈ 4 - 5cm (daytime medium)

• For this movie, TAO runs alternatively in open loop (wavefront measurements only) and in closed-loop (measurements and commands sent to the deformable mirror)
• The switching period is about 4 seconds.
• This lets you appreciate the improvements brought by TAO over short times intervals and with comparable seeing condition.
• Field size is reduced to ~15 arcsecs
• Without AO, a visual estimate of seeing is $𝑟_0$≈ 2 - 4 cm (daytime BAD)

          
                      (click to play video         NB: AO is off for a few seconds at the beginning and at the end)

• This is just a hand-held telephone recording of our wavefront sensor and field camera displays, as long recordings at high speed are not yet implemented.
• The processed WFS frames can be seen on the left. The crosses spot the measured positions of the subimages, and the ellipses show the corresponding measurement errors. The subimage superimposed at the center shows the model of the granulation seen by the WFS and used to get the measurements.
• Our WFS stays locked for more than 3 min on pure solar granulation. Seeing changes are affecting the result, without never breaking the real-time loop. We are very proud of this result as locking a WFS on granulation with less than 2% contrast is among the most difficult achievement of any solar AO.
• Movie duration is purposely of several minutes to let you appreciate the changes in quality on the corrected images, coming from changes in the seeing conditions.
• The system stability over 10-15 min periods is excellent and compatible with the requirements of our slit scanning spectrograph for spectropolarimetric analysis.
• Without AO, a visual estimate of seeing is $𝑟_0$≈ 4 - 5cm (daytime medium)
  

A very simple tool to quantify the benefit of the AO is to look at solar granulation contrast. A classic proxy of the contrast of solar granulation, indicative of the image quality, is the relative standard deviation of the intensity $ 𝑠𝑡𝑑(𝐼) / <𝐼>$ .

                       

• We computed this proxy over 4 time-series of images taken on Dec 17th, with and without AO.
• The contrast differences between the “AO on” and “AO off” conditions maybe as large as 250% !
• 3 samples of 100 images taken a few minutes apart with the AO running show significant quality differences that we attribute to changes in the seeing conditions, which is a nominal daytime seeing behavior.
• The movies below are made with one 'AO off' and two 'AO on' time-series, and they emphasize a dramatic improvement in image quality using the AO in any case.

(Please allow time for videos to autoplay)

• Without AO, no solar granules can be seen, and a very large tip-tilt motion is obvious.
• AO brings a tremendous improvement in image quality, making granules visible and controlling the tip-tilt to a minute level.

• The smallest distinguishable structures on panels B & C are the intergranular bright points, (small scales magnetic features), that are ~7-8 px wide, equivalent to a spatial resolution of ~ 0.18” . For reference, Themis diffraction limit at 650nm is 0.177”.
• The granulation contrast as measured from the Hinode spacecraft is 11.7%.

TAO is:
IRL FSLAC: Gelly B., Douet R., Laforgue D. (Themis), and Le Men C. (now at CNRS LISB)
CRAL: Thiébaut E., Tallon M., Langlois M., Tallon-Bosc I. (Harissa), Moretto G. (Pôle Instrumentation)

— BG, MT, ET, ITB, RD 2020/12/16 10:40