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Terminus: a Versatile Simulator for Space-Based Telescopes

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Terminus: a Versatile Simulator for Space-Based Telescopes Draft version January 26, 2021 Typeset using LATEX twocolumn style in AASTeX62 Terminus: A Versatile Simulator for Space-based Telescopes Billy Edwards1, 2 and Ian Stotesbury1 1Blue Skies Space Ltd., 69 Wilson Street, London, EC2A 2BB, UK 2Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK ABSTRACT Space-based telescopes offer unparalleled opportunities for characterising exoplanets, Solar System bodies and stellar objects. However, observatories in low Earth orbits (e.g. Hubble, CHEOPS, Twinkle and an ever increasing number of cubesats) cannot always be continuously pointed at a target due to Earth obscuration. For exoplanet observations consisting of transit, or eclipse, spectroscopy this causes gaps in the light curve, which reduces the information content and can diminish the science return of the observation. Terminus, a time-domain simulator, has been developed to model the occurrence of these gaps to predict the potential impact on future observations. The simulator is capable of radiometrically modelling exoplanet observations as well as producing light curves and spectra. Here, Terminus is baselined on the Twinkle mission but the model can be adapted for any space-based telescope and is especially applicable to those in a low-Earth orbit. Terminus also has the capability to model observations of other targets such as asteroids or brown dwarfs. 1. INTRODUCTION was launched in December 2019, and Twinkle (Edwards To date, several thousand extra-solar planets have et al. 2019d) will operate from a low Earth orbit and as been discovered. With many of these now being de- such will have to contend with Earth obscuration. tected around bright stars, and with many more to The orbit will cause gaps in some of the observations come from missions such as the Transiting Exoplanet obtained by these missions which will impact their in- Survey Satellite (TESS, Ricker et al.(2014); Barclay formation content due to parts of the transit light curve et al.(2018)), the characterisation of these worlds has being missed, decreasing the precision of the recovered begun and will accelerate over the next decade. Ground- transit parameters. Additionally, the thermal environ- based instruments have detected absorption and emis- ment of a low Earth orbit and the breaks in observing sion lines in exoplanet atmospheres via high resolution can lead to recurring systematic trends such as ramps in spectra (e.g. Hoeijmakers et al. 2018; Ehrenreich et al. the recorded flux due to thermal breathing of the tele- 2020)) while the Hubble and Spitzer space telescopes scope and detector persistence. Such gaps and system- have used lower resolution spectroscopy or photometry atics are experienced in all exoplanet observations with to probe the chemical abundances and thermal proper- Hubble (e.g. Deming et al. 2013; Kreidberg et al. 2014). ties of tens of planets (e.g. Sing et al. 2016; Iyer et al. It should be noted, however, that Hubble is situated in 2016; Tsiaras et al. 2018; Garhart et al. 2020). an equatorial orbit which is significantly different to the In the coming years several missions, some of which sun-synchronous orbits of CHEOPS and Twinkle. Sun- arXiv:2101.10317v1 [astro-ph.EP] 25 Jan 2021 are specifically designed for exoplanet research, will be synchronous orbits allow for certain areas of sky, specif- launched to provide further characterisation. While the ically those in the anti-sun direction, to be observed for James Webb Space Telescope (JWST, Greene et al. longer periods without interruption. Additionally, the (2016)) and Ariel (Tinetti et al. 2018) will be located thermal environment is more stable due to the smaller at L2, observatories such as the CHaracterising ExO- variations in the spacecraft-Earth-Sun geometry. Previ- Planets Satellite (CHEOPS, Benz et al.(2020)), which ous missions to have operated in sun-synchronous orbits include the Convection, Rotation and planetary Tran- sits (CoRoT, Bord´eet al.(2003)), Akari (Murakami Corresponding author: Billy Edwards et al. 2007) and WISE/NEOWISE (Wright et al. 2010; [email protected] Mainzer et al. 2014). Due to it's Earth-trailing orbit, 2 Edwards & Stotesbury Spitzer (Werner et al. 2004) did not experience gaps in and the simulators developed for the CHEOPS and Col- its observations. orado Ultraviolet Transit Experiment (CUTE) missions When designing future instrumentation, understand- (Futyan et al. 2020; Sreejith et al. 2019). While the ing the expected performance for the envisioned science complexity of these types of tools can be hugely advan- cases is paramount. Static models, often referred to as tageous in understanding intricate effects it can also be radiometric or sensitivity models, are suitable for study- their biggest weakness; such sophisticated models re- ing the instrument performance over a wide parameter quire a great deal of time to develop and run as well as space (i.e. for many different targets) as they are gen- an excellent understanding of all parts of the instrument erally quick to run and require relatively minimal infor- design. They can therefore only be applied to highly re- mation about the instrumentation. Radiometric models fined designs and run for a small number of cases. The are a useful way to understand the capabilities of upcom- solution to the issue of complexity versus efficiency is to ing exoplanet observatories and have been widely used. use both types of models. For Ariel, ExoSim is used to The ESA Radiometric Model (ERM, Puig et al.(2015)) validate the outcomes of ArielRad for selected, represen- was used to simulate the performance of the ESA M3 tative targets. ArielRad is then used as the workhorse candidate EChO (Exoplanet Characterisation Observa- for modelling the capability of thousands of targets due tory, Tinetti et al.(2012)) and was subsequently used to its superior speed (Edwards et al. 2019b; Mugnai et al. for Ariel (Puig et al. 2018). A newer, python-based ver- 2020). sion, ArielRad, was recently developed (Mugnai et al. Here, we describe the Terminus tool which has been 2020) while PandExo has been created for simulating ex- developed to model transit (and eclipse) observations oplanet observations with Hubble and JWST (Batalha with Twinkle, to explore the impact of Earth obscu- et al. 2017) and the NIRSpec Exoplanet Exposure Time ration and allow for efficient scheduling methods to be Calculator (NEETC) was built specifically for modelling developed to minimise this impact. The simulator, how- transit and eclipse spectroscopy with JWST's NIRSpec ever, is not mission specific and could be adapted for instrument (Nielsen et al. 2016). These usually account other observatories, with a particular applicability for for efficiency of the optics and simple noise contribu- satellites in low Earth orbit. tions such as photon, dark current, readout and instru- The Twinkle Space Mission1 is a new, fast-track satel- ment/telescope emission. lite designed to begin science operations in 2024. It has More complex effects, such as jitter, stellar variability been conceived for providing faster access to spectro- and spots and correlated noise sources require models scopic data from exoplanet atmospheres and Solar Sys- which have a time-domain aspect. These tools usually tem bodies. Twinkle is equipped with a visible and in- also produce simulated detector images which can act frared spectrometer which simultaneously covers 0.5-4.5 as realistic data products for the mission, accounting µm with a resolving power of R∼20-70 across this range. for detector effects such as correlated noise between pix- Twinkle has been designed with a telescope aperture of els or inter- and intra-pixel variations. For example, 0.45 m. Twinkle's field of regard is a cone with an open- ExoSim is a numerical end-to-end simulator of transit ing angle of 40◦, centred on the anti-sun vector (Savini spectroscopy which is currently being utilised for the et al. 2016). Ariel mission (Pascale et al. 2015; Sarkar et al. 2016, Previously the ESA Radiometric Model (ERM, Puig 2017). The tool has been created to explore a variety et al.(2015); Puig et al.(2018)), which assumes full light of signal and noise issues that occur in, and may bias, curves are observed, has been used to model the capabil- transit spectroscopy observations, including instrument ities of Twinkle (see Edwards et al.(2019d)). Terminus systematics and the other effects previously mentioned. includes a radiometric model, built upon the concepts By producing realistic raw data products, the outputs of the ERM, but it has been upgraded to also have the can also be fed into data reduction pipelines to explore, capacity to simulate light curves. The code also con- and remove, potential biases within them as well as de- tains the ability to model the orbit of a spacecraft, thus velop new reduction and data correction methods. End- allowing for the availability of targets to be understood to-end simulators such as ExoSim are therefore powerful given solar, lunar and Earth exclusion angles. The capa- tools for understanding the capabilities of an instrument bility to model these gaps is not available in other tools design. Additional time-domain simulators of note in- such as ArielRad or ExoSim and is one of the driving clude ExoNoodle (Martin-Lagarde et al. 2019), which factors behind the creation of Terminus. Additionally, utilises MIRISim (Geers et al. 2019) to model time-series with the JWST MIRI instrument, Wayne which models 1 Hubble spatial scans of exoplanets (Varley et al. 2017) http://www.twinkle-spacemission.co.uk Terminus 3 the Twinkle mission will not be limited to exoplanet characterisation and will also observe solar system bod- ies, brown dwarfs and other astrophysical objects. As such, Terminus builds upon the work of Edwards et al. (2019a,c) and can be used to calculate the predicted data quality and observational periods for these objects, an- other feature which is not present in other similar codes.
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