Hene or a say 5 mw green 532nm dpss comes in from the front. A greenee works well for this because they wink out quickly from back reflections. That pop out plug on the rear case is there for a reason, it lets the beam out.
Assume, for as long as you can, that no one in their right mind moved the front OC mirror.
Put the alignment laser along side the Coherent on a secure mount. Use two mm1 type mounts on posts as fold mirrors to get the height adjust and bore centering. You need the extra long path.
Make sure you have a sticker with a pinhole on the hene's output to see the return beam.
There is provision on most of them for a small amount of manual rock and scan built into the rear mirror mount, just don't stress the bellows too much by slamming the mount around.
You may see a very dim reflection of the plasma light from the rear mirror out the oc. Its very faint. Depends on the mirror set. Pros would probably try an autocollimator.
Extreme patience required. If you've never aligned a white light before, well, its not that easy at first. Don't be afraid to walk away and try again another day. Be sure the shutter is open, too.
I'd rock the rear plate vertically while scanning the horizontal screw, assuming the previous owner followed the book and never touched the Oc.
Qui habet Christos, habet Vitam!
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When I still could have...