It depends on the science you're doing as even these small effects add up, there's a project within the LSST science team (which a college is working on) to red

It depends on the science you're doing as even these small effects add up, there's a project within the LSST science team (which a college is working on) to reduce this scattered light (search for "low surface brightness"), where there's a whole lot of work around modelling and understanding what effect the telescope system on the idealised single point that is a star. There are projects (dragonfly and huntsman are the ones I know of) which avoid using mirrors and instead use lenses (which have their own issues) to reduce this scattered light.