Dave I would consider a 50mm not ideal for astro work unless you are going to track your shots. Its bright and fast but a bit too zoomed in to be useful unless the camera is being tracked.
A wider field lens say 14mm is more useful. The Samyang 14mm seems to get good reviews and good bang for your buck but it is more expensive than the 50mm F1.8. 50mm F1.4, I am not sure what they cost.
Greg.
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