The EXoplanet Climate Infrared TElescope - EXCITE
Authors: Pascale Enzo , Butler Nat , Nagler Peter , Netterfield Calvin B. , Tucker Gregory , EXCITE collaboration the 
Published: July 2021

EXCITE is a balloon-borne near-infrared spectrometer designed to observe from 0.6 to 4 micron and to perform phase-resolved spectroscopy of hot Jupiters during a Long-Duration Balloon (LDB) flight from Antarctica in 2024.&#160; These spectral measurements probe varying depths in exoplanets atmospheres thus contributing to our understanding into atmospheric physics, chemistry and circulation.&#160;&#160;EXCITE uses a commercially available 0.5 m diameter telescope, coupled to a cooled spectrometer, and pointed with high accuracy and stability using the successful Balloon Imaging Testbed (BIT) pointing platform.&#160; The combination of these elements results in a unique instrument for exoplanet atmospheric characterization.&#160;EXCITE will measure spectroscopic phase curves of bright, short-period extrasolar giant planets over full orbital. Hot Jupiters provide an ideal laboratory for understanding atmospheric dynamics and the resulting phase-resolved spectroscopy maps the temperature profile and chemical composition of the planet as a function of planetary longitude. The wavelength range covers the peak in the planet&#8217;s spectral energy distribution&#160; and H2, CO2 , CO, CH4 , TiO and VO spectral features. These data, combined with state-of-the-art 3D general circulation models (GCMs), will be used to study the atmospheric dynamics and chemistry in these strongly-irradiated planets. This&#160; will allow us to refine these models and improve their predictive power. Ultimately, the spectroscopic phase curves obtained from EXCITE can be used to study the links between the atmospheric properties of hot&#160;Jupiters and their formation, bulk properties, orbital dynamics and environment. The LDB flight of EXCITE will fulfill a critical need as the first dedicated instrument for exoplanet atmospheric characterization in the current decade.EXCITE will use mostly off-the-shelf components. A schematic of the optics layout is shown in the diagram below (credit L. Mugnai). Ambient temperature optics include the telescope, which has a diameter of 0.5 m. One dichroic filter (D1) transmits wavelengths shorter than 1 micron and reflects longer wavelengths. The transmitted light is used to feed a fine pointing photometric camera (FGS) that provides the telescope attitude error. Infrared light propagates through the cold optics (< 120K) inside a long duration cryostat and it is dispersed by a prism.&#160; Light is further split into two channels. Channel 1, covering the&#160; 1 to 2.5 micron region of the electromagnetic spectrum, and Channel 2 from 2.5 to 4 micron. A single, cold field-stop (slit) is placed at the prime focus to limit radiative backgrounds.The two spectrometric channels are designed to achieve a spectral resolving power larger than 50.&#160; Both spectrometers are imaged onto a single Teledyne H1RG detector , read through the ASIC for Control And Digitization of Imagers for Astronomy (ACADIA) detector controller that was developed for the Nancy Grace Roman Space Telescope (NGRST).. Detector and cold optics are operated at cryogenic temperatures using two mechanical cryocoolers. EXCITE will use a pointing system similar to that previously flown on Super-BIT. The achieved stability of the line-of-sight is better than 100 milli-arcsec.&#160;In this presentation I will review the instrument design and the status of the project which schedules a test flight from North America in 2023, and a science LDB flight in December 2024.&#160;