is ae chip version useless if I will only use the lens for astrophotography? (Sorry, realize this is not a “canon” question although I will be using it on a Canon 6d)
No need to say sorry as you are using a canon camera and a lot of us use older lenses from different companies. I have found that the chipped adapters some times cause problems but then again I don't own one. What I do have and use is the cheap chinese ones and they aren't chipped and they work fine with no problems on my canon 60D and my 1D mark3. The only minor thing is when the M42 mount lenses are mounted on the camera with the adapter, the aperture indicator mark is to the upper right of the top of the lens instead of directly on top but this is an advantage as the flash on the camera blocks the view when the indicator mark is directly on top. Hope this helps a bit
For astro-photography it probably will not add much value as you may have guessed. Focusing in live view will probably be your best bet, but comparing against the focus confirmation may be nice. I would still probably grab the chipped version because it will be nice to have aperture control in camera when you want it. I also believe it provides exif data which you may find to be a nice feature.
You need at least a 2.8 for shooting stars. The rokinon can't take standard filters so I don't use it for anything else. Also you can check this http://fixthephoto.com/blog/retouch-tips/how-to-edit-night-sky-photos-in-photoshop.html
