Laser collimation woes - can't be normal

[justbecause]

Hello there

I received my laser collimator today (one of those $30 ebay ones) and it did not quite work as I wanted.

I would adjust everything just so and then take the collimator out - when I would then put it in again right away the laser would be off on both the primary and in the collimator target.

In the event of user error, here is what I did:

1. Insert collimator (already turned on to the lowest setting) with the target facing straight down
2. Screw it in tightly with no play whatsoever
3. Adjust secondary mirror with small allen key until laser is in the circle on the primary
4. Adjust primary until laser disappears in the target hole
5. Take collimator out

Then, as mentioned, after re-inserting the laser was way out of the circle on the primary. Then I adjusted it again, removed it, reinserted, and it was off again!

What am I missing?

Thanks!

[cometcatcher (Kevin)]

Sounds just like my ebay laser collimator. Your laser collimator might need collimating.

Slip your laser collimator in the focuser tube but don't tighten and rotate it around. If the dot rotates in a big circle the laser itself is off.

I could never collimate my laser properly so I just rotate it around the circle it makes as center. Then I use a Cheshire eyepiece to align the main mirror.

[justbecause]

The telescope in question is a Skywatcher 130 Dob with a rotating focuser tube (that is how the focus is adjusted), so I get two results:

If I tighten the laser in place and rotate the focuser tube with the laser, the laser is pretty much steady on the primary.

If I do not tighten the laser and turn it around loosely within the tube, then the laser dot is not steady on the primary - but I would not really expect that due to the 'wobble' from not being tightened.

It's a bit annoying because obviously I purchased the damn thing to get it just so, now there's no way of telling what makes it better or worse!

How drastic are the differences between having it spot on or slightly outside the centre ring in practice?

Should I get a star out of focus and see whether the rings are concentric as a guide? Is that a good test to determine whether the scope needs collimation?

Cheers!

[justbecause]

So it's pretty clear that whatever way I tighten it leads to a different result.

If I screw it tight, open the screws, screw it tight again, the laser falls to a different spot (without rotating the laser itself in the tube or even so much as touching it).

That means my eyepieces (which I never tighten as much as this laser) have the same inaccuracy in their actual placement.

Should I get a solid and sturdy similar result every time I insert the laser as long as the mirrors haven't moved or is the only important thing that IF the laser falls into the centre of the primary, the laser also centres on the target of the laser?

[barx1963 (Malcolm)]

Hi
There are a couple of things that spring to mind. The focuser is often a weak point with less expensive scopes, but if the laser point is not moving when you rotate the focuser, that eliminates one possibility. The other is how snug is the fit of the collimator in the focuser before tightening the screw? It ideally should be fairly snug, if it moves around a bit that is a likely cause.
My old laser that was a less expensive one had a bit of play, when I upgraded to my Howie Glatter model, it was much snugger and the difference in repeatability was very apparent.
Also does the focuser have a compression ring ( a little piece of metal, usually brass that distributes the tension of the screw around the eyepiece) or does the screw contact the side of the eyepiece/collimator barrel directly?

Last comment, did the scope come with any instructions regarding collimation?

Malcolm

[CJ (Chris)]

Hi
Those helical focusers have a fair bit of slop in them which might account for your varying results with a laser. You could try using some plumbers ptfe tape to tighten up the fit but it's probably not really necessary for visual use. I found my 130 dob, the Heritage model, pretty good optically, given the price and the design philosophy. That focuser was a bit annoying though.

[justbecause]

The collimator is not very snug in the focuser, it certainly has some wiggle, so however the screws are tightened will determine how the laser sits. That would be the cause then. There is no compression ring unfortunately, making things worse I suppose.

The collimation instructions mention that one should makeshift a collimation cap if one does not have a collimation tool, and the rest is on mirror adjustment etc. - but it's all standard.

Apart from the wiggle, can I still assume that if a laser shot straight into the centre of the primary comes out into the target of the collimator, the procedure is successful?

I.e., when I then reinsert the laser, re-adjust secondary and primary mirrors to fit again, am I just re-aligning them to the angle of the laser beam but the net result is the same, i.e., centring the mirrors successfully both times but in different positions?

I am German and this inaccuracy kills me inside

Originally Posted by barx1963

Hi
There are a couple of things that spring to mind. The focuser is often a weak point with less expensive scopes, but if the laser point is not moving when you rotate the focuser, that eliminates one possibility. The other is how snug is the fit of the collimator in the focuser before tightening the screw? It ideally should be fairly snug, if it moves around a bit that is a likely cause.
My old laser that was a less expensive one had a bit of play, when I upgraded to my Howie Glatter model, it was much snugger and the difference in repeatability was very apparent.
Also does the focuser have a compression ring ( a little piece of metal, usually brass that distributes the tension of the screw around the eyepiece) or does the screw contact the side of the eyepiece/collimator barrel directly?

Last comment, did the scope come with any instructions regarding collimation?



Malcolm

It's the same scope, the Heritage - I've actually screwed the focuser all the way in so as to eliminate the slop for collimation purposes...

I bought that scope for its portability and would like to take it to dark sites and then collimate on-site prior to using it as the suspension in my car is probably not going to do me any favours when taking it somewhere in the Hinterland.

Originally Posted by CJ

Hi
Those helical focusers have a fair bit of slop in them which might account for your varying results with a laser. You could try using some plumbers ptfe tape to tighten up the fit but it's probably not really necessary for visual use. I found my 130 dob, the Heritage model, pretty good optically, given the price and the design philosophy. That focuser was a bit annoying though.

[barx1963 (Malcolm)]

Ok, my guess is that it is the slop of the collimator in the focuser that is the main issue. Personally, I would just use a simple collimation cap to start of with. I have one around here somewhere that I could lend to you and also a semi decent laser that may have a better fit. Let me know and I can pop them in the mail to you to test out.
One important principle with collimation is that a laser is not the be all and end all. I use one to quickly check that everything is still aligned but regularly do a full collimation with sight tube and cheshire to ensure it is all still good.
Also, please remember ( and this will grate against your Germanic personality!!) that while collimation is necessary, perfect collimation is often not and spending hours getting it just right is often a waste of valuable observing time. Get it as good as you can, then enjoy using your scope!!

Malcolm

[CJ (Chris)]

Originally Posted by justbecause

It's the same scope, the Heritage - I've actually screwed the focuser all the way in so as to eliminate the slop for collimation purposes...

Good thinking. You've done this sort of thing before!

[justbecause]

That's a very kind offer Malcolm but I would be happy to just purchase the right gear and not cause you any effort - are there any good go-to online shops in Oz?

I've done another check on the laser and yes it is actually out of alignment and drawing a circle, however, it does not appear to have any collimation screws I can see.

It's basically a throwaway at this point but I wonder whether the following workaround makes sense to get a decent alignment nonetheless:

1. Twist the laser around in/with the focal tube to circle the laser around its actual intended centre point just as cometcatcher mentioned to determine a real centre point and get this imagined centre point onto the primary mirror circle (the laser almost traces the black circle itself) -> this should align the secondary mirror

2. Leaving the secondary mirror alone now after it has been adjusted to achieve the above, fudge the laser within the focus tube in such a way that it actually shines into the centre of the primary mirror circle (as a stationary beam this time) and then adjust the primary mirror screws until the return beam hits the target on the laser -> this should align the primary mirror

Is this sound logic?

Originally Posted by barx1963

Ok, my guess is that it is the slop of the collimator in the focuser that is the main issue. Personally, I would just use a simple collimation cap to start of with. I have one around here somewhere that I could lend to you and also a semi decent laser that may have a better fit. Let me know and I can pop them in the mail to you to test out.
One important principle with collimation is that a laser is not the be all and end all. I use one to quickly check that everything is still aligned but regularly do a full collimation with sight tube and cheshire to ensure it is all still good.
Also, please remember ( and this will grate against your Germanic personality!!) that while collimation is necessary, perfect collimation is often not and spending hours getting it just right is often a waste of valuable observing time. Get it as good as you can, then enjoy using your scope!!

Malcolm

[luka]

Those cheap laser collimators are almost guaranteed out of collimation. Most likely it will have 3 small rubber plugs that you can dig out to get to the collimation screws.



There is a nice guide how to get them collimated here, with a simple "collimation device"

[ChrisV (Chris)]

I also got an ebay cheapie laser collimator. It was not collimated. Dig around under labels etc. The screws for mine were recessed and covered in some sort of plastic. I dug out the plastic and collimated it (using a similar nail and block of wood precision tool as in the link below). It was fiddly ... but finally i got to collimating my scope.

But as someone said - dont agonise over it. I also wasted a lot of time over this when I could have been out there looking at the wonders.

Some day I'll get the proper tools for Collimating.

[doppler (Rick)]

The best tool to get is one of these, if your primary is centre spotted it will get you 99% collimated.
I too have one of those ebay lasers lol it gets it close but the Cheshire is better.

http://www.ebay.com.au/itm/142458779558

[gaseous (Patrick)]

Originally Posted by ChrisV

But as someone said - dont agonise over it. I also wasted a lot of time over this when I could have been out there looking at the wonders.

I couldn't agree more with what Chris has said. I sympathize with your desire for perfection, but you need to know how to pick your battles, and absolutely perfect collimation is a battle you're unlikely to win on a regular basis, and certainly (in my opinion at least) not necessary to give you viewing pleasure, particularly with an ebay laser collimator (which is my chosen tool). If your stars are oval shapes then you're probably being a little too lax, but I think only the most skilled and experienced observer would perhaps be able to tell the difference between a scope with 100% accurate collimation and one that's very close but not quite perfect. Luka's link on how to collimate your collimator is very good, although in my experience collimating your collimator is much harder than collimating the scope itself.

[justbecause]

Thanks for the helpful responses - I will get a cheshire eyepiece (was just loving the nighttime capabilities of the laser because I expect to collimate when arriving somewhere dark after a bumpy ride) and follow the high precision collimator collimation method once I found the time to get the ingredients.

The fun never ends!

[barx1963 (Malcolm)]

You will probably find that a bumpy ride will not affect it that much, the mirror in these scopes is not huge so not much weight to be thrown around.

Malcolm

[justbecause]

All the better!

Must've done something right - or the conditions have suddenly improved drastically compared to the past two weeks - I've just had a look at Saturn from the roof top of my building around 8:30pm at 217x and it was crisper than I've ever seen through that scope, Cassini division clear as day which I previously had not even seen! Very happy now.

Originally Posted by barx1963

You will probably find that a bumpy ride will not affect it that much, the mirror in these scopes is not huge so not much weight to be thrown around.

Malcolm