Not all laser collimaters are collimated themselves. Check colimation of your laser before collimating anything else with it. I have an American made laser, and it came collimated and guaranteed beyond question by the dealer. He bought up the entire supply when the manufacturer ceased pruduction.
Spin the laser in the EP holder with the thumbscrew slightly tightened. The laser dot should stay in one spot instead of making a circular motion when spun around. The thumbscrew may be cocking the laser sideways. Hold the laser tight against the stop with the thumbscrew loose, then see if the spot shifts when the thumbscrew is tightened. I suggest buying a Baader Click-Lock EP Holder which guarantees centering on lockup. Rack and Pinion focusers are themselves often causes of image shift, as well as floppy mirrors in old SCT's and MCT's.
You must ALWAYS be centered when using a laser collimator, or you will always show miscollimation every time you check it. You must have standard reference positions, such as a R&P focuser solidly against its inner stop and solidly locked, mirror focuser against its stop and locked, EP always in the same position on star tests, etc. A Crayford focuser with Click-Lock EP holder is a zero tolerance system for aligning EP's, binoviewers, lasers, etc. Mount the Click-Lock with the nosepiece and thus the Click-Lock is the same orientation ALWAYS. I set the Baader emblem on the 2" nosepiece next to my JMI thumbscrew. Unless you go through such persnickety setup procedures, YOU ARE NOT DOING A PROPER COLLIMATION. Period. R&P focusers typically show collimation shift every time you slew the scope to a much different orientation. They are not a stabilized focuser design, and are usually cocked over when you lock them up. My JMI NGF Crayford focuser took care of that issue.
The weight of a long and heavy optical chain shows some decentering at high mags on star tests when hanging horizontally and bent by gravity. Binoviewer owners know this one, and ignore it unless serious. Any miscollimation below the diffraction limit is no concern to me. As long as I get good Airy disks on nights of good seeing, all is well.
For those who use a backup helical fine focuser like I do, at high mags you will see a star do a circular motion as the helical focuser is spun around, as the two thumbscrews decenter the EP to one side slightly. Do not have a rotating helical focuser or EP holder with thumbscrews in the optical path if you can help it. It's better to hold the laser against the stop while checking collimation than cocking the laser with a thumbscrew. You should spin the laser in the holder after laser collimation adjustment and see that any circular movement is concenctric with the mirror donut. You can then adjust out the miscollimation in your laser if possible. There will always be a slight circular motion if you spin a perfectly collimated laser in an off-center helical focuser EP holder. I know this is being persnickety, but little tolerances add up in a long chain of variables. Airliners often end up being several inches longer or shorter than the last one through the assembly line due to very slight tolerance shifts during assembly that add up.
Do NOT keep collimating your SYSTEM to a changing variable in the system. Get a routine down with solid reference points and guaranteed centering of ALL components, or you are like a puppy dog chasing its tail and never reaching the goal. I am shocked at star parties at just how miscollimated most scopes really are. My used LOMO 150 was fairly well miscollimated when I bought it (the collimation screws told a tale of a ham fisted previous owner, so I replaced them with better screws), but my new LOMO 102MN had the best collimation I have ever seen in a new scope, even better than my Stellarvue NHII, and the LOMO had been hauled around in the dealer's van for quite some time, and was a month shy of being 7 years old when I bought it, and had traveled halfway around the world though various shippers to get to me. The LOMO 102MN spinning finder scope is FAR more complicated than a simple Newtonian spider that does not move. I doubt I will ever touch the collimation unless severely shocked by a hard fall. Also, my Meade SN8 now holds collimation VERY well, so most of the Newtonian collimation problems are probably induced or allowed by their owners or built in at the factory, I would assume, from what I have seen so far. My LOMO 150 collimation has not shifted one iota since I took care of previous ham fisted dinkerings. I have a padded JMI hardcase for my SN8 (and a Pelican case for my LOMO 150), but I doubt many Newtonian owners buy hardcases for their water heater sized OTA's, if such things exist for them. My SN8 collimation was star test checked the other night, and had not changed one iota since I installed the JMI Crayford focuser over three months ago. With the junky Meade plastic focuser, it changed every night and on almost every slew to a new target.
There is also a wonderful product called Lock-Tite that can prevent collimation screws from backing out. Use it if necessary. A drop of Super Glue can work wonders as well. If miscollimation keeps popping up, you have movement going on somewhere, unless you stress your gear so hard that you are always bending things. Lock up your SCT mirrors, and use a microfocuser, Crayford, etc., for fine adjustments. My LOMO 150 has no mirror shift when focusing, but Meade SCT owners are rarely so fortunate. On Newtonians, the spiders are shifting in their mounting holes or other such nuisances caused by sloppy build tolerances or undersized and overstressed components. The spider is shifted over one way when you collimate, then after a bumby ride to a observing site with the spider hanging the other way against gravity, it migrates over across the mounting hole and stops where it contacts the opposite side of the hole, and things such as that.
I found locking compound on my LOMO 102MN adjustment screws throughout the scope. Do you think those crafty Russians might be on to something here? Might the builder of some of the world's best professional observatory scopes and military optics know a thing or two about maintaining collimation in a demanding environment? The scope seems built as if to go to war (the dovetail is machined steel with safety stop groove, not extruded aluminum with no provision for a safety stop as on many Chinese scopes, on a 6# OTA no less!) and if necessary it certainly would, judging by past Russian appropriations of technical gear when under foreign attack. Most amateur scopes seem built as if to sit in a showroom and nothing more. An Orion XT8 Intelliscope Dob has a street price that the LOMO 102MN optical tube alone sold for when new. You get what you pay for. I took care of MANY issues with my Meade SN8, and you probably will with your 12" Dob as well. Were it not for the excellent matched USA glass in the scope, I would have dumped the entire SN8 LXD75 outfit. Good luck with your Dob!
Phil
P.S. My star test has to be quite a bit off at high mag before I ever see it in a focused image when observing. Misfocusing, or sorry cheap Chinese EP's, have been a bigger problem with me than miscollimation. Also, cooling down an MCT, SNT or MNT shows a much bigger improvement than correcting a slight miscollimation. My Star Splitter EP's, for instance, will clearly show an Airy disk across the entire FOV, on both colors of Albireo, even in the fast f/5.5 LOMO 102MN. Such EP's are good for your star testing of your collimation. If you get Airy disks across the entire FOV, I would say LEAVE THAT SCOPE COLLIMATION ALONE!!!
