sreesha So I can then safely buy a Nikon 135mm F2 lens?
Remember that at f/2, you may need a special filter, like some of the IDAS ones. Most interference filters are designed for a narrower ray angle.
In any case, you probably cannot use an f/2 camera lens at f/2 anyway. Probably more like f/3.5 to get a more aberration free optics. Stick to telescopes that are designed specifically for infinity focus.
That said, for very wide fields, I do use a Sigma 135/1.8 ART myself (with EF mount; but it is from Sigma's ART series, which are arguably better than Nikon lenses).
Askar has plenty of quite decent moderately short focal scopes; they should beat out a camera lens for astronomical use. I have their ACL200 (warning -- heavy as a real brick) and an FMA-230 (which I use as a guide scope; not at 230 mm focal length, but without the reducer, at 275mm). They also have a 135 and 180 in the FMA series.
The camera lenses just wastes a lot of glass so that they can accommodate focus at different distances. Telescope objectives just need to work at one focal distance. With that many glass elements, a f/2 lens is an f/2 geometrically, not in terms of light gathering. An f/2 lens is definitely not equivalent to a T/2 lens.
Even 200mm is plenty wide when coupled with a full frame CMOS camera. I dedicated a ASI6200 just for these shorter focal length lenses. For "real" DSO, you really don't need such a wide lens. There are only a few objects (like the California Nebula) that won't fit into the FOV of a 450mm focal length, with just an APS-C format sensor.
So, study the FOVs a bit before going out to buy a camera lens to use for astrophotography. It might just turn into something that gather dust on the shelf pretty quickly. It all depends on whether your interest is on large Milky Way expanses and dust clouds of our own galaxy, or on distant nebulas and galaxies.
Hint: when you stop the lenses down, which you will need to do to avoid horrible aberrations when the lenses are opened up, do not use the f/stop ring of the lenses (else you will get diffraction spikes from the corner of the iris blades). Use a series of filter step-down rings to produce a round entrance pupil.
My two cents: do not buy a camera lens on impulse (not even a Zeiss or Leitz). Study what the FOVs can achieve, and weigh their disadvantages when compared to an APO telescope that is designed just for infinity focus.
But its your money, perhaps it is worth paying to learn.