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I use a 10 mm chrome ball bearing as my artificial star. If it is sunny, I put it in the sun at the far end of my garden (about twenty metres away) against a dark background. By my calculations (possibly faulty!), the image of the Sun on the 10 mm ball bearing is about 22 microns across, and at a distance of 20 metres, this is a "point" of light less than a quarter of an arc-second across - well below the diffraction limit of my scope!
If it is cloudy (or at night), I set it up at the far end of my longest hallway (about 10 metres long), turn off the hallway lights, and shine an LED torch down the hall to illuminate the ball bearing.
Both scenarios give me a brilliant, clearly distinguishable "star" image to bring in and out of focus.
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