Hello Loky!
from me as well.
Starting your observing with 20x90s would be a bit challenging due to the high (relatively) magnification. If you could complement those with a pair of 7x-10x and 50-60mm aperture, it may make your life a bit easier. I find that I often need to go back to my 12x60 when I am also using 20x80 or 30x100 in order to reorient myself when I am looking for targets.
But the 20x90s should be a good performer for the night sky. I'm happy with all three pairs I have bought from AOE, accepting that they are the budget end of the market.
[EDIT:- For example, if one wants top end binoculars, here is what one pays:-
http://www.iceinspace.com.au/forum/s...ad.php?t=21345 ]
As Mike says, you'll need a good support for the 20x90s. I can manage my 20x80s on a Velbon tripod in the $100-120 range. The 20x90s might be a bit too heavy - you can only try. Don't be surprised if you find you really need a $300 tripod to hold them properly!
Do read the articles Mike has pointed you to. Search for "binoculars" and "parallelogram" mounts threads on the forum.
Good luck.
Eric