Purpose of VenSpec-U
VenSpec-U is a dual channel (low resolution, hereafter LR: 190-380 nm; and high resolution, hereafter HR: 210-240 nm) UV spectral imager designed to monitor cloud top abundances of volcanic sulphured gases (SO, SO2) as well as UV contrasts through spectral analysis of backscattered sunlight on the day side of Venus. The entrance objective is shared by both channels and consists in two lenses forming the image of the field of view (FOV) onto a common plane including two slits. Both slits (LR and HR) are parallel, and their corresponding FOV are slightly shifted in their narrow direction, equivalent to 9° (corresponding to 30 km at the cloud top level of observation). Each slit image is then spectrally dispersed by their respective toroidal holographic grating, and is formed on a shared E2V Capella CMOS detector. Therefore, the narrow-slit axis of the detector contains the spectral information, whereas the long-slit axis contains the spatial information along the 22.5° FOV of each slit. Binning on the spatial axis is performed on the CMOS, enabling a SNR ~ 100 for both channels (resulting in a coarser spatial resolution on the more dispersed HR channel). The remaining spatial direction is provided through orbital scrolling, an observational strategy known as pushbroom. Observations can be conducted in a strict nadir geometry (null emission angle), or in near-nadir geometry (emission angle < 30°).