Question: In some appratus for the Young's double slit experiment a collimating lens is used infront of the double slits, meaning plane wavefronts hit the double slits. But why do we need/want plane wavefronts hitting the slit in the first place, won't spherical wavefronts work just as well? Also if we did not have plane wavefronts would we still get Fraunhofer diffraction (I assume we will since we will still be viewing the image in the image plane).

Answer: 
Plane wavefronts ensure that the field at the apertures are in phase and coherent and thus the interference pattern is produced (see Fig.[1](https://i.stack.imgur.com/jDRIx.png)).

Non planar input to an aperture equals a case where the point oscillators at the apertures are not in phase and depending on the coherence length of the incoming field, might not be coherent (see Fig.[1](https://i.stack.imgur.com/jDRIx.png)). Thus, the interference pattern will change, position will shift and visibility will decrease. 

Read for example 

[Lasse-Petteri Leppänen et. al., New J. Phys. 16 113059, (2014)](http://dx.doi.org/10.1088/1367-2630/16/11/113059)

[E. Hecht, “Optics.,” Addison Wesley, 2002](http://books.google.fi/books/about/Optics.html?id=7aG6QgAACAAJ&redir_esc=y)
![Line of coherent point sources[3]](https://i.stack.imgur.com/jDRIx.png)
[Line of coherent point sources](http://books.google.fi/books/about/Optics.html?id=7aG6QgAACAAJ&redir_esc=y)
It depends on the coherence length of the light source. If the wavefronts are curved, and it’s difficult to ensure that light generated at the same time is impinging on the slits at the same time — you need a relatively precise alignment in both the centre of the lens and the angle.
If you collimate a beam, the wavefronts are planar, so it’s less sensitive to a movement of the centre of the lens.