Is telephoto zoom lens really good for taking photos with DSLR cameras?

I'm thinking of buying a f8-16 telephoto zoom lens with 2x teleconverter for my Nikon d5100. How good will it be for wild-life photography? Can it also be used for astrophotography?

Added (1). What I have in mind is Rokinon 650-1300 f/8-16 Telephoto zoom lens with 2x teleconverter 650-2600.probable made in USA.

I think you need to see what lens it is you are getting because an f8 lens at 16mm does not make any sense. You need a long lens. 200mm at a minimum plus your converter.

Sounds like one of those cheap chinese things. It's better than nothing

Those super-long telephoto lenses are total junk. Just get a Nikon 55-300 VR.

F/8-16? Do you realize how slow that is? The teleconverter will make it twice as slow.
No lens is made in the USA. And the Rokinon is probably made in China.
I'd pass on these.

Those preset lenses are awful. Too slow to use and even slower with a teleconverter. Even with a tripod and shooting in bright sunlight you will need long exposures and most of the pictures will be blurry.

Buy a real Nikkor lens, or if that is too expensive, having a look at the Tamron 70-300mm. It's a lot cheaper, and almost as good.

I recommend going without the teleconverter to begin with. They are harder to use, need longer exposures and a more stable tripod and degrade image quality. See how you get on with the 300mm lens and if it just isn't enough, even after cropping, have a look at a teleconverter.