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As an adjunct to Jeff's advice, tether the camera to the laptop, load up EOS Utility, and go into Remote Shooting Live View mode.
Place a bright star (like Sirius) in the centre of the frame, set the lens to AF (auto focus) and within the Live View controls, you can use the focus utility to nudge the focus in and out, with varying degrees of coarseness.
It depends on the lenses you're using; if it's the cheapy plastic kit lens, you will see loads of chromatic aberration. What you want to do is continue nudging the focus until you get rid of either the blue or the red fringing around the star. One of the two is going to be worse than the other, so, keep playing until you minimise the fringing.
Once you've reached an acceptable level of focus, make sure you set the lens back to MF (manual focus).
That should get you in good enough focus for widefields.
H
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