Collimating my 12" Meade LB

[midjam]

Hi,

I'm having some difficutly trying to collimate my 12" Meade LB, I have used a few resources online and kind of have it except I'm going a little OCD on one particular aspect.

It seems that the secondary mirror is either too close or far too from the focuser as when I look into it, the primary mirror does not fill the intire secondary, as in the primary mirror clips are not right to the edge of the secondary I can see the housing that holds the mirror cell at the base of the OTA. hope that makes sense.

The batteries on my camera are gone so I'm unable to take a photo to show what I'm talking about so, I knocked up this quick diagram. BTW, I checked the spider vanes and it's centered.

Thanks

[knobby]

Is this with a Cheshire or film cannister? Is the focuser racked in or out?

[midjam]

I made a collimation cap using an old fabric softner lid which was exactly 2" and had a nice dot right in the centre of it, heated a screwdriver and punched a hole through. Don't have enough cash for a cheshire just yet

The focuser is all the way out, it's a little better all the way in but, still the primary does not fill the whole secondary.

[knobby]

Honestly I'm no expert but maybe the hole is too small?

[midjam]

Thanks, could be however it looks the same without it just harder to tell. I'm a bit of a perfectionist so, maybe I need to adopt a that'll do atitude until I can get a cheshire

[Etoille]

Although I cannot be sure its the same issue, this sounds very much like a similar problem I had with my Skywatcher Flextube from the factory. I could collimate the secondary with the primary and get the laser spot bang in the centre (I was using a laser collimator), try as hard as I might, I couldn't get the textbook view with everything including the primary clips perfectly centered from the edges of the outer circle as per the classic pictures. the scope seemed to work OK as it was but it still bothered me. I tried carefully adjusting the secondary (dropping it onto the primary would be a catastrpophic mistake - therefore keep the scope horizontal for this!) up and down along its axis and rotating left/right and also checking the tension on the spider vanes but there seemed to be no adjustment that could place the clips where they should be - evently spaced from the sides of the outer circle. Eventually following some advice I adjusted the focusser by carefull tweaking of the adjustment grub screws. This paid off as eventually I did get that textbook view with everything properly centered.

Without a photo though, it is difficult to tell exactly where the problem lies and I would check everything else first but it could be that although the secondary is in perfect aligment with the primary, and the secondary is perfectly positioned in relation to the tube and focuser (i.e. in the center of the tube and at the appropriate position down from the vanes, the focusser itself might not be quite fully in alignment with the secondary, i.e. not being perfectly at right angles to the tube/secondary, either because its not correctly seated on the tube or simply requires adjustment in relation to its base plate.

If you can post a pic at some point it might help others to diagnose the problem. Do you not have a camera on your phone?

[midjam]

Ok thanks, I did notice earlier that everything seemed to be aligned (apart from the primary filling the secondary) but, I could see the side of the focuser tube.

Maybe the focuser could do with adjusting or a shim.

I'll get some batteries for my camera and take some photos as it's pretty hard to explain

[NGC 1502]

I really don't think you have a problem here. From what you have said, it just seems that the secondary mirror is just a bit larger than is absolutely necessary, no big deal.

Some obsess in having a minimum size secondary to get the best contrast that an over size one is said to reduce , but the effect is minimal, I wouldn't worry myself.

If it were my scope, I'd just collimate it and enjoy the great views that a 12" LB can give.

Regards, Ed.

[midjam]

OK I have a photo but, it's not great quality.

It all seems to line up and looks like the diagram in the manual: http://www.meade.com...lightbridge.pdf

However, the focuser tube seems to be angled a little towards the top end of the tube (away from primary) as I can see a little of the outside of it.

[Daniel-K]

dont forget in a fast scope it the secondary has to be offset too. swamp thing will be about soon he will tell you more

[midjam]

I have seen in the manual that the focuser will seem to be positioned more to the left of this image and the secondary will appear egg shaped. But the focuser in the reflection seems to be slightly angled towards the left of this image. I'm having some problems describing my problem, does this make sense

[midjam]

Here's another diagram, I could try and shim the focuser a little on the left of this image to try and square it up but, would that cause other collimation problems?

[umadog]

As mentioned above, your primary shouldn't fill all of the secondary. So that's fine.

You're more or less collimated, but it looks to me as though your primary tilt is wrong based upon the position of the ring binder. This is what you're aiming for (second image): http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Board=reflectors&Number=3033065 I'm also seeing what looks like the edge of the tube (i.e. the OTA) along the bottom edge of the primary mirror.

My suspicion is that you're tilted your mirrors an awful lot when you should have been making other adjustments (e.g. secondary rotation and spider). This may be why you're seeing the OTA in the primary mirror and the outside focuser edge. I would suggest you start from scratch. Go back and make the three adjustment screws at the back of the primary of equal length. Do the same to the set-screws controlling secondary tilt. Then work through the procedure in order (well described here: http://www.propermotion.com/jwreed/ATM/Collimate/Chesire.htm). If you do it out of order, you can end up tilting things too much. Follow the instructions in the first link for positioning the secondary in the focuser. Only then go on to finalise the secondary tilt and primary tilt. If you secondary isn't correctly positioned and you attempt to make up for it by radically tilting it, then you may see the sorts of things you show.

The offsetting of the secondary referred to above isn't something you really need to worry about. You will automatically get the required offset if you follow the collimation instructions. If you're interested: the offset is achieved by tilting the optical train rather than re-positioning the secondary. Here's why: http://www.cloudynights.com/item.php?item_id=2677 (see "Uni-directional offset" image).

[midjam]

Thanks, I really appreciate the time you put to writing this. I did check that the primary mirror adjustment screws were at equal length and think I have setup the secondary tilt screws properly, It does sound like it might be that though.

I'll go through the links you have given me and re-check all the screws and start again tomorrow. I'm guessing that a cheshire will help

[jamespels]

I'll pass on the best bit of advice I have ever been given on the subject.... read this: http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

Astrobaby's guide is possibly the definitive guide to collimation and will help you through pretty much any issue. The offsets in your photo looks like it could be in collimation for a fast scope but I am not an expert - all is explained in AB's guide.

Hope this helps,

James.

[umadog]

Glad it helped. Keep in mind tha it's a truss Dob so you'll be tweaking the collimation every time you use it in the field. You'll soon get good and fast at doing it. Nevertheless, you will find it helpful to do what you have been doing: setting aside time during the day to collimate. Don't be at all concerned about decollimating everything and starting again. Each time you do that you'll get faster and also more accurate. The issues you've been having will work themselves out as you read more and practice. Once your secondary is correctly positioned in the focuser (that's best done during the day) it will not need adjusting often. The secondary and primary tilt adjustments, on the other hand, are the ones that need tweaking on a nightly basis.

[midjam]

I reset everything and started again. I have noticed, what I thought was the outside edge of the focuser is infact light

If I have the focuser all the way out I'm getting light coming through the right hand side of it, however if I roll it all the way in the light goes and it all looks square. I feel a little stupid now

The focuser still looks a little too far over to the left as in the photo above, would this be a primary or secondary adjustment? Should the donut be in the center of the focuser or secondary circle?

[umadog]

This new interface is mangling links. Go here: http://www.physiol.ox.ac.uk/~raac/collimationLinks.shtml Second link down. You'll see what it should look like in detail.

[midjam]

Thanks for all the help guys I did a star test last night and all seems fine, still need to buy myself a cheshire think it will come in handy