I have been trying to use my T3i for nightsky photography but having issues with focusing the stars etc. The LCD screen is useless for this purpose and with the stars been so small I am struggling to focus them with viewfinder. Anyone have experience getting around this problem?
I'm assuming you're using the kit lenses? Unfortunately those don't have focus meters on them. If you have a lens that does have a meter, simply turn it to infinity. If it doesn't then the best method is to chimp it. Focus on something very distant, like the horizon. Then turn it a nudge past that (or try it there). Take a picture, view it, zoom in on some stars and check the focus, nudge the focus ring. Repeat.
Make sure you give yourself about twenty five or thirty seconds @ ISO 400 to build up enough light build up, and use a tripod and remote shutter control. Any longer exposure will give you egg shaped stars or star trails. Local light pollution may have a great deal of detrimental effect on your luck, and exposure settings.
The key is to focus on something that is equivalent to infinite distance. The moon, a planet, a bright star are all far enough away to be at infinite distance.
Dim astro fields will be a bit of a challenge to aim at. I find the 10x viewfinder magnification helps to find and aim the camera. At 200mm, your exposure time will be limited to about 3 seconds before star trails appear, and you won't capture many stars in the image. At 50mm you will have about 4 times as long to expose before trails appear.
Longer exposures will require you to compensate for the rotation of the earth. I've built a simple 'barn-door tracker' that moves the camera slowly in the opposite direction. You can find directions on the internet to make a manual one inexpensively.
At 200mm, I start to experience sky glow in the image from local light pollution at about 30-60 seconds (depending on the direction the camera is pointed - away from the light pollution, or towards it). My best success has been to make about eight 20-second exposures and then stack the pictures. There are some free software packages that allow image stacking.
With these steps I have captured about 13,000 stars in one image when pointed at the Pleiades at 200mm, in addition to showing a little nebulosity around the main stars.