I'm yet to pull mine to pieces, but I'm sure the centre screw holds the secondary mirror assembly in place, ie. unscrew it all the way and the secondary mirror plus its holder would fall away. The three "collimating screws" just butt against the secondary mirror holder and so can tilt it slightly in different directions. Obviously, to push one hard in, the other two have to relax out a little, so you tweak the three of them a little bit in turn, until you get to the correct secondary mirror angle with all three screws fairly firm against the holder.
My experience is that, when the three screws are loose, the secondary holder will rotate with the main screw (so the mirror begins to face away from the focusser. If you reach in and hold the mirror holder (watch you don't touch the mirror surface, screwing the screw moves the mirrorholder closer to or away from the primary. Now, for a Bintel GSO newtonian, unless something has happened to it since it left Bintel, I would expect this up/down position to be fairly accurate. Unless someone has screwed the centre screw, it's hard to see how that will have altered. Maybe if the focusser has been replaced??
Looking through a sight tube (Cheshire, collimating cap, 35mm film cannister with hole in bottom etc), it's fairly easy to locate the secondary mirror - particularly the rotation around the centre screw. Just hold the mirror holder and turn it while watching, until the secondary mirror looks like a circle. Now, there is a lot of stuff about secondary mirror offsets - search these threads and you'll find some discussion. Any detailed article on collimating a newtonian scope will go over this matter for you. My understanding is, if through the sight tube you see a round secondary mirror well centred in the view, you are close enough.
Now the laser collimator. You do have to get it collimated in itself. That means, as you rotate it on a suitable homemade jig, the dot stays within a few mm radius circle on a wall 2-3 m away. They usually have the back of the little laser assembly held firm, maybe in a rubber O-ring, but the front of the laser assembly normally has three grub screws holding it in place. Are they under some sort of cover or trim? Once you can get at those, you can make slight adjustments, loosen one a 1/4 turn, tighten another a 1/4 turn and check to see if improved or not.
Once that is done, try to get the laser to be a snug fit in the focusser adapter or barlow or barlow adapter. I read a recommendation of wrapping one layer of "contact" (yes, for the kid's schoolbooks) and I found a nice flowerly bit and put it on. Perfect, very little slop now. I put my laser in and tighten up the set screw (brass compression ring design) and it seems very snug and well set in position.
Re the barlowed laser - get a piece of white card and punch a small hole in it - up to hole punch size. After having collimated secondary and primary mirrors as best as you can with the direct laser, now introduce the barlow and hold the card down the inside of the scope between the focusser and the secondary mirrror such that the laser beam passes through the hole. Presuming it is illuminating the ring binder centre spot well, you'll see the shadow of the ring binder on the card. If I have collimated with my laser, that shadow is exactly centred, on the white card, around the laser beam every time. I think I'm lucky - my setup and laser have been well behaved.
Let us know how it goes.
