Complex sentence: At most oppositions, Hygiea has a magnitude that is four magnitudes dimmer than Vesta's, and observing it will require at least a 100 mm (4-inch) telescope, while at a perihelic opposition, it may be observable with 10x50 binoculars.
Split into simpler sentences: At most oppositions, Hygiea has a magnitude that is four magnitudes dimmer than Vesta's, and observing it will require at least a telescope. However, while at a perihelic opposition, it may be observable with 10x50 binoculars.