LOL Why did I do it.

[Doc]

Had a very annoying and head scratching two hours last night.

For some stupid reason I thought I would start from scratch and readjust my primary mirror as the locking scres had reached their end.

I wish I hadn't.

Because of this my laser collimator and eyecap were giving different results. So I started to adjust my secondary and this lead to me slackiening off my secondary.

I was swearing by now as nothing lined up. I couldn't even get my mirror clips into view.

Anyway to cut along story short, two hours later all is lined up and secondary sits further back and took scope out last night and it's great could even split the double double.

But what a panic.

[Psychobilly]

Because most of us are incessant fiddlers ... and just can't resist it...

Glad you got it back better than before...

Peter...

[Keithp]

I've probably got that to come yet, was a little apprehensive about modifying the secondary with a washer between the adjusters, but ploughed on carefully and got it all lined up. I must admit resetting the primary back sounds like a good idea. In theory would that mean the primary should be squared up when set fully back on its adjusters?

[Mikea]

You did it because there was this niggling doubt....maybe if I......could just.........just a minor adjustment..............it would all be better.........then.......aarrgghh........... what's happened?!!

I hav only escaped a similar fate because I couldn't find the correct allen key (or whatever it is, I can't see it properly) to fiddle with my secondary mirror.

Anyway - you achieved the desired result - you made it better!!

Well done!

[JohnC]

Know the feeling Mick.

Did the same once with an 8" SNT. Never did get it back to how it was.

I always have to fight the urge to meddle!!

[glowjet]

"If it aint broke don`t fix it", but I'm afraid we are all guilty from time to time, me included. Trying to improve what can`t be improved. Now where did I put that nerve medicine the Doctor prescribed?.

At least Doc you have now had the experience, and will not be afraid to do it again.

John.

[Astro_Baby]

Just think you much you learned about it all. It wont be so scary next time and now you KNOW its all ok.

[albedo0.39]

Good to hear you are back on track Mick. (phew...) . Interesting comment from Astro Baby ...because as I'm still quite new to all this stuff, weeks ago, I actually deliberately mis-adjusted everything several times in a row and started out from scratch with collimation.

I twisted the mirror on it's axis, had the main screw wound in and out, and of course had all sorts of mess in the primary. The result is that now, I am really confident about "what to do when...." but only because I'd made perhaps 10 attempts to get it right.

BTW.. My collimation was so good the other evening that when I had a 9mm, and defocused on Arcturus I thought "what on earth are those very tight concentric rings doing around that black hole.."

Yes.. I had an almost perfect airy disc and hadn't even realised. Talk about pleased.

If I were you (on a night you don't plan observing of course) I'd be tempted to get out the cheshire and have a go at mis-alinging everything and starting from scratch. Certainly worked for me !

[Doc]

Good to hear you are back on track Mick. (phew...) . Interesting comment from Astro Baby ...because as I'm still quite new to all this stuff, weeks ago, I actually deliberately mis-adjusted everything several times in a row and started out from scratch with collimation.

I twisted the mirror on it's axis, had the main screw wound in and out, and of course had all sorts of mess in the primary. The result is that now, I am really confident about "what to do when...." but only because I'd made perhaps 10 attempts to get it right.

BTW.. My collimation was so good the other evening that when I had a 9mm, and defocused on Arcturus I thought "what on earth are those very tight concentric rings doing around that black hole.."

Yes.. I had an almost perfect airy disc and hadn't even realised. Talk about pleased.

If I were you (on a night you don't plan observing of course) I'd be tempted to get out the cheshire and have a go at mis-alinging everything and starting from scratch. Certainly worked for me !

Thanks guys. I did learn a few things and I suppose thats the way it should be done.

Steve as you are the master collimator at Salisbury I will ley you adjust mine until I can see concentric rings or the airy disk as I've never seen them in my dob.

But my collimation is good as I split the double double but I still say it's a tad off.

[calibos]

Theres a couple of things to do once that never need to be done again unless one takes off the focuser or secondary to work on them.

Centering the spider. Now take your ruler and measure the distance from the screw hole in the mirror holder to the endring. It should be dead centre within the tube. If its not, adjust the spider vane screws till it is.

Ensuring the focuser is square to the tube(It probably is but its best to check after any dismantling and if one ever can't seem to get their collimation just right, one knows its nothing to do with a non square focuser.

A collimated laser is handy for this. Shim it with some tape around the barrel to ensure a tight square fit in the 2"-1.25" adapter.

Remove your secondary mirror and screw a short length of threaded rod into the mirror holder. Turn the laser on. Make sure the laser dot hits the threaded rod. If it doesn't, shim the focuser or if it has shimming screws, use those to adjust the angle of the focuser till the laser dot hits the threaded rod.

Remove the threaded rod. Now the laser dot will hit the far side of the OTA opposite the focuser. Take a ruler and measure the distance from the dot to your tube endring. Now measure the distance from the laser collimators apeture to the endring. The distance should be the same. If its not then again, adjust the focuser till it is.

Centering the secondary under the focuser. Get your sight tube and possibly shim this with tape too to ensure a tight well centred fit in the focuser drawtube. Affix a sheet of paper underneath the secondary, blocking the primary from view. This is to remove distracting reflections from the primary and make it easier to see when you have the secondary rotated to show a nice circle. A brown paper bag is good for this as the colour makes it easier to see for the next step. Get some graph paper and tape it to the far side of the tube opposite the focuser.

Now look through your sight tube. Rotate the secondary till you see a nice brown circle. Now one can adjust the centre screw while holding the mirror and trying to maintain that perfect rotation. One can see when one has it centred along the OTA axis by counting the squares of the graph paper. Once you are happy, then tighten up the 3 secondary tilt screws until you can see the same number of graph paper squares top and bottom, all the while trying to maintain the rotation (perfect circle brown reflection).

Now you have set all the things that only ever have to be done once or after you remove the secondary or focusser to work on them. (Square focuser, centered spider and centre secondary under focuser drawtube)

Henceforth, you should only ever need to make minute adjustments to secondary tilt. (No need for the brown paper bag or graphpaper)

In terms of the primary. On my XT12i where the collimation springs were good enough I learnt about something that made both collimation easier and more likely to hold well and require little to no adjustment from session to session.

(The following only applies if you didn't need your primary somewhat forward to enable some eyepiece or camera to come to focus)

Invariably when one uses all 3 collimation bolts, one ends up moving the primary further up the tube over time until like you found out, one had moved it so far up that the locking bolts no longer made contact with the cell. Not good in terms of not being able to use the locking bolts and not good in terms of the Springs being at their least efficient in their uncompressed state. The springs are there to help hold collimation which they wont unless they are compressed.

So, one unscrews the locking bolts till they almost come out. Tightens up the primary collimation bolts until they can be tightened no more. Picks a collimation bolt that they will never touch again (I pick the one on the same side of the OTA as the focuser. Easy to remember) Now leave this bolt fully tight. Collimate your secondary and then collimate the primary using only the other two collimation bolts. If you can get your primary perfectly collimated then jobs a goodun'. If you can't quite get it there using only the other 2 bolts, then loosen up that first unused bolt a little bit and try again using the other two. Once you find you can collimate the primary with just the two other bolts, then you never need to touch that first bolt ever again.

Then one tightens up the locking bolts till they barely touch the cell.

What this means is that:

1. Its easier and more intuitive to collimate with 2 bolts instead of 3.

2. Your mirror wont drift up the OTA over time/collimation sessions.

3. The springs are highly compressed and at their most efficient in holding the mirror. This means less likely to experience collimation drift during a session when move the scopes to different altitudes. If you do experience this, you'll know its not the mirror shifting but the truss poles shifting or LOTA endring shifting that LB's can be prone too. ie. You'll have ruled out mirror shift as a culprit from the getgo should you experience collimation drift during a session.

4. Following on from point 3. Your compressed springs will hold the collimation better between sessions as you bump and bang it while wheeling it out/assembling it.

One final point. I've heard it said that one can achieve hyperstable collimation by doing the above, but at the very end instead of just tightening the locking bolts till they barely touch the cell. Actually go further and tighten them all some more. You'll see you are putting the primary back out of collimation. Now actually collimate with the locking bolts. With the Collimation bolts pulling and the springs and locking bolts pushing, one is putting the thick metal lightbridge cell under tension. Apparently this makes for very stable collimation. I am sure I read of a small downside to this but I can't remember what it was. I'll have to look for that thread again and report back.

[Doc]

Wow fantastic write up.

This is what happened to me. I've been adjusting all three bolts so in time the mirror had risen up the Lower OTA. So I screwed all three down so springs were fully compressed and adjusted no more them maybe 1 turn. I admit I didn't leave one untouched but next time I will.

I think my success was mainly due to luck more then any scientific approach, so next time I'll follow your advice.

[calibos]

Just spotted an error in my post. Make sure you do the centering of the spider before doing the squaring of the focuser step. If you did it the other way around like I originally posted one would be possibly be adjusting the focuser to a non perfectly centered spider and then centering the spider which means the focuser would be out of wack again.

So centre spider first, then adjust focuser.

All that said, from the factory most scopes come with the spider properly centred and a properly square focuser. Its only after some disassembly of the scope to work on it that one would need to check these again.

[lguise]

One extra tip.. get upgraded springs. I have worked on a couple of LBs and the springs are too weak to support the cell.

Linton

[Beyond_Vision]

A great set of instructions Calibos. If you send me a PM with the edit included I will edit your post. I think it should also be made a sticky.

Regards

Kevin

[Doc]

One extra tip.. get upgraded springs. I have worked on a couple of LBs and the springs are too weak to support the cell.

Linton

Linton it's the 16" version so has the updated springs. As far as I'm aware only the 8" to 12" suffered from this.

[albedo0.39]

. The springs are highly compressed and at their most efficient in holding the mirror. This means less likely to experience collimation drift during a session when move the scopes to different altitudes. If you do experience this, you'll know its not the mirror shifting but the truss poles shifting or LOTA endring shifting that LB's can be prone too. ie. You'll have ruled out mirror shift as a culprit from the getgo should you experience collimation drift during a session.

4. Following on from point 3. Your compressed springs will hold the collimation better between sessions as you bump and bang it while wheeling it out/assembling it.

On the nail and great advice. I discovered this from the Yahoo group right from the very start about 8 weeks ago. Screw the primary adjusters all the way down, back off 1-1.5 turns and then adjust only 2 to collimate.

[adamsp123]

Don't do what I did and and have the 2 inch to 1.25 inch EP adaptor in at an angle on the focuser then put collimator into that there squint 1.25" EP holder, now that does play havoc with getting collimation!

[lguise]

Glad to hear the 16" has upgraded springs, I have worked on a couple of 12" versions to sort out collimation for friends and the springs were very poor. Ended up sourcing new ones from a local engineering firm.

Best wishes,

Linton