Somehow I pissed off all the sentry bots. When I first found the place I went in and one of the bots told me I didn't have access and had 30 secs to leave, so I left. When I went back later for the Brotherhood of Steel quest all the sentry bots were hostile and now I can't complete the quest. What should I do?

Come back between 10 and 5 game time when they are "open."

If that doesn't work, Stealth Boy up and haul ass to the third floor.

There are not hostile if you visit the place while it's open for public (10am-20pm?), if you are not interested in a visitor tour you could also kill them. There are just a relic from the REPCONN and have nothing to do with the quest.

I came back during the day and they are still hostile. What should I use to kill them? I do almost no damage to them. I tried using stealthboy's but the still come after me.

Once you have been in there and tagged by that sentry bot they seem to be hostile all the time.

So long as you don't attack them you should be able to sneak in using a stealth boy, leave any companions outside and don't go near the sentry bot!

Sneak around the tour route and lock-pick the door into the observatory if it is not open and go upstairs and steal the pass card to gain security access. You can also get onto one of the terminals and add you face to the security system so you don't get hassled.

Pulse Grenades...

Anything with EMP or Pulse in the name is golden... otherwise I find energy weapons work best against robots.