This is a fast, cheap lens.  Although cheap and very lightweight it is a powerful tool for wide-field astrophotography.  It is known as the Plastic Fantastic (or Nifty Fifty) for a reason.

The lens used in combination with an APS format Canon DSLR is suitable for wide-field targets up to 25 degrees wide.

 

Wide-open, at f/1.8, it exhibits noticeable aberration in the center of field.  However, by stopping down the aperture to f2.8 there is major improvement.   I shoot this lens at f4 when I'm not too fussed about round stars.  When I want round stars across the field I'll stop it down to f/6.3.  Works good for me.

 

Automatic focusing will not work at night time on stars.  Focus the lens during the day and mark the focus barrel so you can check the focus quickly at night time.

 

I've used the 50mm lens to take lots of images (see examples below) and I can say that this lens is excellent value for money at just US$100.

 

Its big brother, the Canon EF 200mm f2.8 L USM II is also good value for money. 

 

Canon's Plastic Fantastic - The Nifty Fifty

The Canon EF 50mm f1.8 II at around US$100.00 is great value for money
The sticky tape holds a slip of ruled paper and the focuser grip is marked
with a red ' V ' which lets me focus to infinity quickly at night in the darkness.

Some images taken with this lens include:

Milky Way - Mosaic of Southern Cross, Pointers and Eta Carina (Ultrawide image)

Milky Way - Table of Scorpius

Milky Way - Southern Cross and Pointers

Milky Way - Sagittarius (M8 to M7)

Milky Way - M16-M17 Region

Small Magellenic Cloud & NGC104 - 1:1 closeup

The field of view on 50mm lens makes it suitable for imaging large chunks
of the Milky Way or of whole constellations like this this image which was
awarded an honorable mention at the 2009 Malin Awards.