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The extremly metal-poor star forming galaxy IZw18 is a key laboratory for galactic and stellar evolution studies. Deep HST V- and I-band imaging has revealed an underlying population comma and determined its distance through observations of cepheids and the tip of the red giant branch. It will soon be targeted by JWST-NIRCAM and MIRI to provide a complete census of evolved red stars and young stellar objects. However comma deep imaging in the blue and ultraviolet which is where the young massive stars are the brightest and are best studied is lacking comma which we here propose to remedy. By deep far UV to blue imaging in 4 filters a census of the hot star population in IZw18 and the extinction law at very low metallicity would be obtained. With the exquisite spatial resolution of HST and using the HeII 4686 narrow-band filter comma we can comma for the first time comma identify what sources are responsible for the HeII emission including Wolf-Rayet stars comma but also hard ionizing sources comma such as rapid rotators comma binary stripped stars comma or accreting compact objects comma and provide constraints on stellar evolutionary theories at low metallicity. The observations would also allow a much improved determination of the recent star formation history comma and a chararacterisation of the luminous red stars that JWST will see.