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An interacting pair of spiral galaxies undergoing a galaxy merger. Their long tidal tails contain stars displaced by the encounter. They are also a nearby example of a starburst galaxy, with vigorous star formation accompanying the interaction.^[ref]

The Hubble Space Telescope image below combines observations from its Wide Field Camera 3 and Advanced Camera for Surveys. Bright young stars, emitting regions of the interstellar medium and obscuring interstellar dust reveal different aspects of the encounter. The close view emphasizes the interacting central regions; wider views show the long outer tidal tails.^[ref]

The Wide Field Camera 3 is an instrument on the Hubble Space Telescope with separate ultraviolet/visible and near-infrared channels. Its grisms disperse light onto the detector without an entrance slit, allowing a time series of stellar spectra during an exoplanet transit. The near-infrared channel uses two grisms covering different intervals; their combined coverage is broader than that of either alone.^[ref]

A spectral channel yields a light curve, and its fitted exoplanet transit depth contributes one interval of the exoplanet transmission spectrum. The resulting molecular bands can constrain water and the atmospheric scale height, subject to exoplanet clouds and reference-pressure degeneracy in transmission spectroscopy. Because the stellar photons dominate, stability and calibration are as important as collecting light.