All you can do is give it a go. The 18mm closed down an f stop or two (to help reduce outer field curvature) should be alright for good 45 sec near the celestial equator and nearly a min near the pole and out about 30 deg each way.
The 200mm will be ok for moon shots, though the moon will be a bit small. Where the 200 will give you nice shots is when you frame the moon against some landscape feature, or for conjunction between the moon and planets.
