How frustrating it can be to get a faint object centred in the viewfinder, I find it very hard to see them and have had many attempts and trial exposures to get the camera aligned on them.
Please read the words of caution at the end :)
This idea helps simplify the process.
The additional items required are a DSLR Shoe mount microphone bracket, and a laser that will be snug fit in it.
The first step is to fit the microphone mount to the camera, slip the laser into the holder and get it aligned.
I found mounting the camera on a tripod and pointing at a distant object like my local church spire, then adjust the laser to point at the middle of the frame, my sample image above has the laser pointing a bit low. Tighten the adjusting screw to lock the bracket in position then remove the laser.
Above shows the laser mounted on the hot shoe and mounted on the Barn Door tracker.
When starting a session, my method is to polar align the barn door mount while the laser is fitted at the hinge point, I use rubber bands so its easy to slip it out after alignment. Then move the pointer to the top of the camera, you can check the laser is still aligned by turning it on and the beam should end in the middle of your frame
Step 2: Start Your Session
It should be a simple matter to loosen the camera ball joint, point the laser towards your chosen target and lock in position, you can then look through binoculars after locking the camera and then turn on the laser to ensure you are on the faint object.
Take a single exposure to confirm your alignment and you should be good to go.
Hopefully it will save you lots of frustration getting set up.
A FEW WORDS OF CAUTION.
Green lasers are very bright and easily seen, NEVER EVER point them at aircraft, or any other moving object in the air or ground. The time on should be short blips to test where the laser is landing there is no need to hold it on for seconds at a time. If you hit an aircraft with one you run the risk of a very heavy fine or worse.
Some people suggest pointing the laser through the viewfinder and the beam comes out the lens, this will work but its possible to damage your sensor or coatings if you are using a powerful laser.
I find the laser stops working if kept out in the cold, when you are done put it in your pocket in case you want to use it again.
Laser Pointer . The one I used
Microphone Mount. Can be bought from many sources but here is one.
Step 3: Additional Info
The barn door tracker in the image is from a previous instructable.