Improvement of Doppler measurements thanks to adaptive optics: the case of JOVIAL+AOC

The JOVIAL instrument is a Doppler imager, dedicated to the study of atmospheric dynamics at the surface of the planets of the Solar System, and in particle Jupiter and Saturn. The principle of this instrument is fully described in several papers presented at previous SPIE meetings 1-4 (Gonçalves et al, 2016; Soulat et al, 2017; Underwood et al, 2017, Schmider et al; 2020). Recently, we obtained instrumental and astrophysical results, showing the performances of the instrument and for the first time, giving zonal and vertical winds measurements at the surface of Jupiter (Gonçalves et al, 2019, Schmider et al 2024). 5, 6 The terrestrial atmospheric turbulence remains a strong limitation for this type of measurements. Indeed, the velocity field at the surface is intricately linked with the photometry through the PSF of the instrument, as the measured Doppler shift is the product of the actual velocity by the intensity and convoluted by the PSF 7 (Civeit et al, 2005). Therefore, the large PSF corresponding to long exposure time causes spurious Doppler shift, affecting the accuracy of the measurements. This problem affects any spectroscopic velocity measurements of resolved objects, but is particularly important in the case of Jupiter, which presents strong photometric variations at its surface coupled with a large gradient of velocity due to the fast rotation. In a recent paper, 6 we show the effect of the PSF and calculate the bias that it introduces. We show the data processing that was used to minimize this effect and recover unbiased measurements.

However, this treatment cannot be fully satisfactory, as the PSF itself always varies with time and its shape is not well known, making illusory any type of deconvolution. A best solution would consists in solving the problem at the beginning, i.e. when the images are formed in the instrument. That is why we developed the Adaptive Optics system AOC (Adaptive Optics at C2PU), placed at the entrance of the instrument, and able to correct the wavefront on a field as large as 1 arcmin, sufficient for Jupiter. This field is much larger than the isoplanetic angle, so the correction is limited to the ground layer up to 1000 m, which contains most of the turbulence. Still, the preliminary results show a nice improvement of the quality of the PSF and, as a consequence, a better accuracy of the measured velocity field.

We will illustrate the results of the AOC on observations of Jupiter realized in November 2023. We present how it could improve the precision of the velocity measurement. We will also present the recent improvements of the AOC system and we finally will talk about new ideas to recover the actual velocity field and improve the precision by using the data coming out of the AO system