PSF nowcast using PASSATA simulations - Towards a PSF forecast Turchi A.a Agapito A.a Masciadri E.a Beltramo-Martin O.b Milli J.d Plantet C.a Rossi

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PSF nowcast using PASSATA simulations - Towards a PSF

forecast

Turchi, A.a, Agapito, A.a, Masciadri, E.a, Beltramo-Martin, O.b, Milli, J.d, Plantet, C.a, Rossi,

F.a, Pinna, E.a, Sauvage, J.F.c, Neichel, B.b, and Fusco, T.c

aINAF-Osservatorio Astroﬁsico di Arcetri, L.go Enrico Fermi 5, Firenze, Italy

bLAM-Laboratoire d’Astrophysique de Marseille, UMR 7326, 13388, Marseille, France

cONERA, B.P. 72, F-92322 Chatillon, France

dIPAG-Institut de Plan´etologie et d’Astrophysique de Grenoble, 414, Rue de la Piscine, 38400

St-Martin d’H`eres, France

ABSTRACT

Characterizing the PSF of adaptive optics instruments is of paramount importance both for instrument design

and observation planning/optimization. Simulation software, such as PASSATA, have been successfully utilized

for PSF characterization in instrument design, which make use of standardized atmospheric turbulence proﬁles

to produce PSFs that represent the typical instrument performance. In this contribution we study the feasibility

of using such tool for nowcast application (present-time forecast), such as the characterization of an on-sky

measured PSF in real observations. Speciﬁcally we will analyze the performance of the simulation software in

characterizing the real-time PSF of two diﬀerent state-of-the-art SCAO adaptive optics instruments: SOUL at

the LBT, and SAXO at the VLT. The study will make use of on-sky measurements of the atmospheric turbulence

and compare the results of the simulations to the measured PSF ﬁgures of merit (namely the FHWM and the

Strehl Ratio) retrieved from the instrument telemetry in real observations. Our main goal in this phase is to

quantify the level of uncertainly of the AO simulations in reproducing real on-sky observed PSFs with an end-

to-end code (PASSATA). In a successive phase we intend to use a faster analytical code (TIPTOP). This work

is part of a wider study which aims to use simulation tools joint to atmospheric turbulence forecasts performed

nightly to forecast in advance the PSF and support science operations of ground-based telescopes facilities. The

‘PSF forecast’ option might therefore be added to ALTA Center or the operational forecast system that will be

implemented soon at ESO.

Keywords: simulations,adaptive optics,forecast,atmosphere,optical turbulence

1. INTRODUCTION

Atmospheric optical turbulence (OT) is the main cause for the degradation in performance of the observations

from ground-based telescopes. The deformation induced by OT on the incoming wavefront from space can be

roughly summarized in the spreading of the point spread function (PSF), which describes the response of the

telescope optics to a point light source. The PSF quality is inﬂuenced by many parameters, however a detailed

description of these eﬀects is out of the scopes of the present paper. To get a summarized description. A top-class

telescope with an 8-meters diameter (D) primary mirror, in theory, could achieve an angular resolution ∼λ/D,

which equals to 0.13” in V band. The OT is typically measured with the seeing () parameter, which, on the best

observing sites, has a typical median value around 0.7” in V band (order of magnitude). The seeing represents

the spreading of the PSF and its value sets a hard limit to the angular resolution achievable from ground. This

conditions is typically referred as ”seeing limited”, and the main limiting factor at least until infrared bands (Q).

The Adaptive Optics (AO) aims to overcome this limitation and correct the deformations induced on the weave-

front by the OT. However the AO eﬃciency is aﬀected by the optical turbulence itself.

Most AO facilities may use diﬀerent operation modes that are able to perform diﬀerent kind of OT corrections,

depending on the needs of the scientiﬁc programs and the available atmospheric conditions. Single Conjugated

Send correspondence to Alessio Turchi - e-mail: alessio.turchi@inaf.it

arXiv:2210.11244v1 [astro-ph.IM] 20 Oct 2022

Adaptive Optics (SCAO) aims to obtain the best performances over a limited portion of the sky. If the target

is near to the guide star (on-axis case), the shape of the PSF depends only on the integrated turbulence pa-

rameters (i.e. seeing, τ0...), however in the oﬀ-axis case, where the guide star is separated from the target, the

PSF is impacted by the vertical structure of the turbulence (C2

Nproﬁle) and the wind speed. This is even more

important for the Wide Field AO case (WFAO), where the correction is applied to a large portion of the sky. In

any case the knowledge of the PSF attainable in speciﬁc conditions is fundamental to correctly match the AO

facility and the scientiﬁc program to the best atmospheric conditions.

The knowledge (in advance) of the OT can ﬁnally help in planning telescope observations and tuning the AO

instruments for better performance [1,2].

Knowledge in advance of the atmospheric turbulence is possible only with a forecast model, which is also

able to provide the full C2

Nstratiﬁcation. Since such topic is of paramount importance and have such a deep

impact on ground-based observations, in recent years many telescopes, with top-class AO facilities, expressed

interest in having a forecast tool for OT. Speciﬁcally, the usage of dedicated atmospheric models to forecast

the atmospheric turbulence had a signiﬁcant growth in the last decade, with more and more large telescope

installations adopting such tools. Mauna Kea (KECK telescope) already had a forecast system for OT∗. The

Large Binocular Telescope (LBT) followed with the ALTA project†. The latter uses an advanced dedicated

system for forecasting OT using a high-accuracy mesoscale atmospheric model (Astro-Meso-NH, [4]), which has

been proven to provide reliable forecasts of atmospheric parameters [3] as well as the seeing [2] above the LBT

site. Now also the ESO’s Very Large Telescope (VLT) is planning to adopt a similar system during the course

of next year which could potentially have a huge impact on the astronomy community due to the importance of

the AO facilities at VLT and the huge scientiﬁc output of the telescope [5,6].

Recently it has been proven that, using an autoregression technique, it is possible to forecast the seeing with

performances (RMSE) at short time scale (1h) of the order of 0.1” [2,5] . Such a value is better than the intrinsic

uncertainty of measurements and this guarantees us to be able to achieve forecast of the optical turbulence with

a suﬃciently good accuracy.

In this paper we want to dedicate our attention on the possibility to forecast a few features of the PSF

obtained by AO systems. AO performance depends mainly on the OT and atmospheric conditions, the charac-

teristics of the observed target (magnitude M) and on the ﬁne tuning of the optical and control-loop parameters

to the matching conditions. Each AO instrument is diﬀerent and have diﬀerent speciﬁcations, however by joining

in succession, a forecast system of the atmospheric and turbulence conditions with an AO simulation software

that is able to simulate the performance of the optics and the control loop of the AO system, it is conceivable to

provide a prediction of the AO performance for each speciﬁc observed target and atmospheric condition. This

could provide an invaluable tool for the telescope operator that could carefully plan the observation to match

the best possible conditions and achieve the level of performance required, thus boosting the scientiﬁc output of

the telescope.

In the present paper we will limit ourselves to the study of the Full Width at Half Maximum (FWHM) and

Strehl Ration (SR), which are the principal numbers that characterize the goodness of the PSF corrected by the

AO and the attainable performance on an observed target. We might envisage in the future to include other

parameters such as the encircle energy or the contrast.

In the present paper we will provide an update to the results previously shown in [7], in order to characterize

and potentially evaluate the feasibility of such a forecast system for FWHM and SR for few of the best AO

systems currently available. SOUL at the LBT and SAXO (the AO instrument working with the planet ﬁnder

SPHERE) at the VLT. OT predictions can also provide detailed high-accuracy proﬁles for the stratiﬁcation of

the turbulence (C2

Nproﬁles), which could have a huge impact for oﬀ-axis predictions. However, since this is

a preliminary feasibility study, in the present paper, we will limit ourselves to the on-axis case which can be

typically characterized using only the integrated seeing parameter (the seeing is the integral of the C2

Nalong the

∗http://mkwc.ifa.hawaii.edu/

†http://alta.arcetri.inaf.it/

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PSFnowcastusingPASSATAsimulations-TowardsaPSFforecastTurchi,A.a,Agapito,A.a,Masciadri,E.a,Beltramo-Martin,O.b,Milli,J.d,Plantet,C.a,Rossi,F.a,Pinna,E.a,Sauvage,J.F.c,Neichel,B.b,andFusco,T.caINAF-OsservatorioAstrosicodiArcetri,L.goEnricoFermi5,Firenze,ItalybLAM-Laboratoired'AstrophysiquedeMarseille...

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PSF nowcast using PASSATA simulations - Towards a PSF forecast Turchi A.a Agapito A.a Masciadri E.a Beltramo-Martin O.b Milli J.d Plantet C.a Rossi

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