AstroPhotography Tool (APT)
The focussing aid is really neat. Within APT you display a live view from the camera, which can integrate several successive frames to increase brightness. The focussing dialogue can then be dragged until it locks onto one of the brighter stars and puts a red circle around it, as shown. In the dialogue a brightness profile of the star is drawn and the width of the profile is shown numerically. The lens is then focussed to minimise the width (strictly, FWHM = full width at half maximum of the profile).
Another nice feature of APT is that it shows the temperature of the detector chip after each exposure, so you can check whether the thermal noise is likely to be increasing.
There may be similar tools for camera makes other than Canon - search online.