Another collimation question (laser oddness?)

[Ben Ritchie]

So far i've been using a cheshire eyepiece to collimate my Skywatcher 300P dob, it's what i'm familiar with, but I also got myself a Baader laser from FLO along with the dob - the idea being that the laser would be easier to check collimation when out in the dark. The problem is that the laser and the cheshire don't agree...

The 'scope has a compression-ring Moonlight focuser, so both the cheshire and laser are solidly held in place with no 'slop'. According to the cheshire i'm very close to collimation, only a touch out, and a quick star-test looked pretty good - I didn't do a detailed test and the 'scope has been moved back inside since then, so it could have shifted a little ... but, I think, not a lot. So i'm inclined to believe the cheshire. However, the dot from the laser is about an inch outside the centre-dot on the primary, and as I rotate the laser the dot traces a circle around the centre of the primary, keeping at roughly the same distance from the centre the whole time. It never goes near the centre of the mirror.

My instant reaction is to wonder if the laser itself is well out of collimation, as the cheshire would indicate that the secondary is correctly aligned. But I thought that the Baader lasers were supposed to be pre-collimated? So can an expert in the dark art of collimation suggest what's going on!?

[kniclander]

beware the long dark road of collimation obsession.....

if you use a laser to collimate the pirmary, the secondary must be exactly centred in the primary first otherwise the primary collimation will be well off (whether or not the laser is collimated). that's why I think the cheshire is geenrally preferred for the primary.

if your laser describes a circle when you turn it, it sounds like the collimation of you laser is off. However, an inch off the centre is really a hell of a long way though so are you absolutely sure your secondary is properly centred according to your cheshire/cllimation cap? (but methinks that wouldn't make the dot describe a big circle so maybe your laser is a really long way off?)

I followed Astrobaby's instructions (not sure how I insert the link but I am sure you can find them...) which were really good.

ps - have you considered whether your centre spot is really centred...NOOOOOOOOOOOOOOOO

[glennbech]

I've just taken leaps ahed in the process of collimation but still feel it's a bit of a struggle. My pain is to get the secondary "round and centered" and that I'm not happy with the adjustment screws for the Explorer 200P that I'm otherwise pretty pleased with.

My process has boiled down to a combanation of the cheshire and laser.

1) I check that the secondary is centered over the primary and is tilted so that it is "round" in shape when looking at it with the cheshire. I hope I don't have to do this often, it was a pain in the you-know-what the last time I did it.

2) I check that the secondary is well aligned by using the laser collimator to center the spot on the primary.

3) I switch back to the cheshire when adjusting the primary

My experience is that errors indtroduced by flexibilitues in the the drawtube / EP Holder etc and 2 meters of travel makes the return beam pretty useless.

So far I've found it a bit diffucult to see through that little pinhole when the primary is well aligned, but Im sure I get it pretty close :-)

[steppenwolf]

and as I rotate the laser the dot traces a circle around the centre of the primary, keeping at roughly the same distance from the centre the whole time. It never goes near the centre of the mirror.

It sounds to me as though the laser collimator is out of collimation - sadly all too common! I have made a collimating jig for my own laser collimator but the odd shape of the Baader version makes it difficult to self test.

[Nostromo]

Sometimes even the best lasers get uncollimated due to vibration. I'd just check the collimation of the laser to itself by putting it into something V-shaped and rotating it - that's a quick way to check. Most collimators can be re-collimated but that means taking them apart and it's a bit of a pain.

Nick

[Nostromo]

hmm do the Baader ones have collimation screws? Nick

[Kai]

They do but there are factory sealed.

[steppenwolf]

Ritchie, these two recent threads may be of some interest:-

First one

Second one

[Nostromo]

They do but there are factory sealed.

Well, I rekon once you have determined that your laser is out you might as well give it a shot IMHO. By factroy sealed do you mean the screws are bonded in or something? If so that may be a hassle. Nick

[acey]

Ben - if the Cheshire says it's collimated and you're happy with the star test then the job's done. Ditch the laser.

[DIYASTRO]

I have two laser collimation units, one is Revelation the other is Meade. Not seen or used Baader. The problem is that the ones that I have contain a simple cheap and nasty little pen type laser. This is what is held in place by the three screws. This means that you can alter the position of the very thin cylinder which is the laser pen. The screw which switches the laser on simply pushes against the button of the pen to operate. The main problem is that the little circuit board which has the laser diode attached to it moves when you apply pressure via the screw / switch.

So obtaining what we are all looking for, which is an accurate laser collimator which remains set to centre and pin point accuracy is never going to happen.

I have changed mine to separate laser diode which is much more robust and came from a laser spirit level!

Try this.. Place your laser collimator on vee block 3 meters away from projection surface (wall) rotate and mark the position of laser dot, or the circle it inscribes. Switch it of and turn the screw to turn it on again, repeat process. I bet you get a different position and size of circle

[Nostromo]

Yep these pen lasers are to be avoided, I discovered the same thing. You need a proper switch at the end. So having a concentric laser is important, but the other thing is the diameter of the laser barrel. It needs to be exactly the same as the eyepiece, and no manufacturers are able to get this the same on both the EPs, adapters and lasers due to machining tolerances. Also you can't rotate your 1.25 to 2" EP adapter once collimation is achieved because they put pressure on one side of whatevers insterted into them - and this throws everything out. I have seen so called comparison tests where lasers were compared within a 1.25-2" adapter, which is a bit of a waste of time Also if you collimate an SCT with a diagonal in it the same applies, you can either collimate with the diagonal in or take it out but your'e introducing more flex into the train... Nick