Should I buy samyang 14mm when I already have 17-40L?

Hello
Ive been wondering if should I get the samyang 14mm f/2.8 for astronomy photography.
I already have the canon 17-40L and its kinda soft at f/4 at the left corner, and it doesnt have wide aperture.

Should I get the samyang 14mm? Will it be sharper at the corners than the 17-40 at 17mm at f/4?
There have been few times that the 17-40 wasnt wide enough for me, maybe I should try the sigma 15mm f/2.8 fisheye? Is it sharp wide open?
I already had a samyang 8mm, really liked it and sold it due to upgrading to 5DC.

The advantages of the Samayang are obvious in that for untracked imaging you would be able to gather much more light with the same exposure duration. If you are tracking (Astrotrac or piggyback on a telescope on an equatorial mount) it shouldn't matter very much and you can get away with the f/4.

I have both lenses. The biggest problem I found with the 17-40 (besides the max apeture) was the ridiculous ammount of chroma. Very happy with the Samyang the few times I have used it; after I got a good copy that is.

doidinho wrote in post #15935935
after I got a good copy that is.

Curious, did you have an issue that the lens needed to be returned?

The Samyang 14mm is very sharp in the corners. The main problem is the huge degree of moustache-type distortion.

How much do you shoot at ultra-wide angles? If you do a lot of shooting at those focal lengths, it may be worth buying the Nikon 14-24mm instead, and use it with an EOS mount adapter.

the jimmy wrote in post #15936075
Curious, did you have an issue that the lens needed to be returned?

I did, the first one I bought had some sort of issue with the focal plane not being parallel to the front of the lens.

Second copy was spot on thought. I did a bunch of research before buying the lens and QC deffinately appeared to be an issue. The good reviews really overshadow the bad though.