Hubble Space Telescope (HST)

The Hubble Space Telescope provides ultraviolet, visible, and near-infrared transit and eclipse spectroscopy and established many early detections of atoms, aerosols, and atmospheric escape.

The Advanced Camera for Surveys is an imaging instrument on the Hubble Space Telescope, designed with separate wide-field, high-resolution and ultraviolet channels. Filters select spectral bands; dispersing elements also provide low-resolution spectroscopy. The different channels have different detectors and optical paths, so the instrument name alone does not specify the relevant wavelength coverage.^[ref]

For an exoplanet transit, a sequence of dispersed images can be divided into spectral channels, each yielding its own light curve. Comparing fitted exoplanet transit depths constructs an exoplanet transmission spectrum. Detector response and pointing variations must be calibrated along with the planetary signal. Historical observations should be interpreted using the channel and observing mode actually employed.

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.