I like to use captures of the Milky Way to assess the quality of a lens. Though I am by no means an expert in Astro Photography, there are a few point that might be of interest to you: - Most lenses don't have an infinity stop. It is therefore mandatory to establish the correct infinity setting for your lens by focusing on a far away subject during day light. - The maximum exposure time to prevent appearance of star trails depends on the quality of your lens. If you get blurry star images, the trails will be hidden in the blur. The direction in which you shoot is also a factor. For example, the Polar Star will not exhibit trails even at long exposures. - The longer the exposure, the more stars you will see. - If you only see few stars at edges of your picture, your lens exhibits significant corner softness. - The Milky Way has a reddish hue when photographed, quite different to its appearance to the naked eye, since we don't see colors in low light. - The best results you will get using a good wide angle lens at a large aperture and high iso. If you use noise reduction, the weaker stars will disappear. - As an example I attach a capture taken with D700, Zeiss 25/2 @2.0, iso 3200, tripod, mirror up. Exposure was 8s (you can verify this by counting the flashes of the anti-collision lights of the plane; the frequency of the flashing in 1/s).