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Peter are you shooting after astronomical dark? before that time over exposing the sky is easy to do but after astronomical dark not so easy. btw at 90mm you should be able to expose for around 5sec without star trails.
Closing the aperture (a larger f number) will increase possible sharpness and reduce lens distortion of stars. Shooting with a camera on tripod is my normal method and stars in the middle of the field of view can be round while lens distortion makes them fan shaped in the corners. reducing aperture this effect is reduced.
light pollution and even over exposed sky can be dealt with in processing, but if your initial test shots look over exposed look at reducing ISO or increasing f stop (google "exposure triangle"). For shooting galaxies or nebulae you want longer exposure times rather than more and shorter exposures so you can collect as many photons as possible and have as much signal to work with in the shots.
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