Morphology and photometry of comet C/2019 Y4 (ATLAS) from STEREO-A HI-1

We report on comet C/2019 Y4 (ATLAS) during it transit of NASA/STEREO-A's Heliospheric Imager-1 (HI-1) field of view from 2020 May 25.9 until 2020 June 6.8. These observations encompassed the comet's perihelion passage on 2020 May 31.0 and occurred at a different viewing geometry than from Earth since STEREO-A is in a trailing Earth-like orbit, with a ~70 & deg STEREO-Sun-Earth separation angle at the time of observations.