[Space Walker]

Learnt from an astrophotography book, focal length 20mm or under, the explosure time must not exceed 35 seconds at 10-20 degree towards sky otherwise stars cannot keep as dot shape because of the self rotation of the earth. I am using an APS-C DSLR and plan to buy a 14mm/F2.8 lens (x1.5=21mm) for this purpose. I am worrying that aperture 2.8 may not be able to have enough explosure in half minute to capture the weak stars light, or even require F4 as the effect may be better? Did any brothers have experience to share?

Also, please share comments on below three 14mm/2.8 lens.
Nikon
Sigma
Tamron

Thank you in advance!