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If you can get a 300mm lens running at f/2.8, that would be great. You won't have to go higher than that. Remember, your 300mm lens will be the equivalent of a 480mm lens on a full frame camera. However, I would also look at wider angle lenses as well. A good 15-35/50mm lens at F/2.8 and even a 50mm prime lens would be good. The Canon 50mm, f/1.8 prime lens is good for astropics, and it's pretty cheap. If you can afford it, grab the Sigma 15-35mm f/2.8 ART lens for Canon cameras. It'll do wonders for both your ordinary pics and your astropics. You can even go cheaper and get good quality lenses from Samyang and Rokinon. A friend of mine uses these lenses to great effect. Especially the Samyang 16 and 24mm lenses. For a tracker, if you really want to make one, then go ahead. But, I'd just fork out for either a Skywatcher Star Adventurer or an iOptron Skytracker Pro.
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