I would recommend against it. These dirt cheap teles are even recommended against for daytime use: they tend to have low contrast, no auto focus, dim view through viewfinder and to be quite soft. Now, do you really want to use these for the least forgiving of all subjects? Reproducing the image of stars is the hardest test for any lens. Most professional lenses need to be stopped down half to one stop to get sharp stars in the whole image. A dirt cheap 50mm f/1.8 has to be stopped down to f/4 before being acceptably sharp (The same goes for the f/1.4 version - which is usable at f/2.8). That's two full stops. If you have to do this with your 500mm f/8, you suddenly got a 500mm f/16. Your dirt cheap 62.5mm aperture becomes a significantly less cheap 31mm aperture... Imaging Deep Sky at f/8 ain't too much fun: long exposure times and all the mess coming along with it: critical polar alignment, tracking, ... But doing it at f/16 is just hellish.
My advice: save yourself from a lot of frustration and keep your money. Invest it in a less cheap but decent lens:
50mm f/1.8 or f/1.4*
100mm f/2.8 or f/2
70-200mm f/2.8* or f/4 (performance nears prime performance) (Canon - not Sigma which often suffers from aligment problems)
(* lenses I use myself. I also have the 10-22 f/3.5-4.5, 17-40 f/4 and 300 f/2.8. I use the first 2 hardly for astrophotography and the last is way too expensive to be included in that list...)
Ok, the last ones are rather expensive... Most of these can be found on the used market for 60-70% of the new price. These are all stunning lenses and they'll give you a lot of pleasure in return and not only for astrophotography but also for daytime use and retain their value pretty well when you want to sell them again.