Pls help newbie trying to collimate 10" LB :( ...

[mbaddah]

Hi all

After having recently purchasing a new 10" LB, and never collimated before, I thought why not play around with the screws on secondary and primary, mis-collimate it (if that's a word), then try collimate from scratch to try and master it.

Several hours later I still can't get the primary mirror exactly aligned with the secondary :(

It looks something like the picture attached. I have turned the secondary screws to tilt it and the refelction of the primary seems to stay hidden in the corner.

I've also tried loosen the main centre screw on secondary and move it up/down but to no avail.

I'm using a Orion Chessire/Sightube collimating piece. Your help is greatly appreciated thankyou.

[erick]

Yes, you now know the meaning of "mis-collimate"! It's a real word. :sadeyes:

Did it all start off pretty well in collimation? If so, you will be able to get it back there.

I'm sure that it must be the tilt of your secondary. So that reflection of the primary is moving around all over the place as you adjust the three secondary collimation screws? Surely, then keep fiddling. If that doesn't get you there, loosen the three off and grab hold of that secondary mirror holder and move it about by hand while watching the primary reflection through the focusser. Once it is roughly centred, hold it and screw the three collimating screws back against the mirror holder. If you cannot get it centred, something has gone mechanically wrong. Check the focusser and secondary area carefully, is my suggestion then.

I vaguely recall a thread about a bent central bolt on the secondary of a lightbridge some months ago??

[erick]

Here it is:-

http://www.iceinspace.com.au/forum/showthread.php?t=25533&highlight=bent

OK, it was one of the three screws that was bent.

[mbaddah]

I will try again tonight thanks for that. I have had a quick look at the secondary screws they seem fine (not bent), I had replaced the original knobs with a set of bobs knobs. Will let you know how it goes tonight, cheers :)

[astronut]

Collimation can be a scary thing when you first start out.
Here is some help, go to www.andysshotglass.com (http://www.andysshotglass.com) there is a very good video that will help you through it.:thumbsup:

[rmcpb]

Before you start playing with the three alignment screws you need to centre the mirror in the eyepiece with the centre bolt otherwise all your adjustments will be to no avail.

[mbaddah]

Thanks guys for the prompt response.

How can i accurately know if the secondary mirror is centered 100% in the focuser? Cheers.

[erick]

Centering the secondary mirror - you'll know just looking at it. Not towards the spider, not towards the primary mirror, but centred. And also looking round, not elliptical.

Use Andys' video. Look on the site for "Collimating a Newtonian..." It is a 3.7MByte down load, but a great movie. (One point - in one photo he is talking about adjusting the primary mirror tilt, but he has his fingers on the primary mirror locking screw.)

Do you have a collimating cap, or a sight tube (often combined with Cheshire, eg. Orion's)? If not, get a 35mm film cannister and drill a 1mm hole in the centre of the bottom. This will fit where a 1.25" eyepiece will and will centre your eye in the focusser (put it in from under the adapter and nip it up firm - don't want it falling out into the OTA. Not the best solution, but it will do. A long sight tube is better (eg. the Orion) since you can adjust it so the view of the secondary mirror just fits inside the circular end of the tube making it easier to get it well centred (and correctly rotated to a fully circular view and not elliptical).

[mbaddah]

I have the Orion Chessire Sightube purchased from Bintel ($70). I sometimes find it bit difficult to look through due to the small peephole and cross in centre, or is that just me :(

[erick]

The hole is fine for me but the crosshairs drive me crazy - never in focus for me. But crosshairs are there for a good reason - for final alignment. But you can use them as a guide on the secondary mirror position. But I suggest that you ignore them and concentrate initially on the edge of the secondary mirror. Perhaps put a piece of white paper on the opposite wall of the OTA so the mirror and mirror holder stand out distinctly. Objective is to get that secondary nicely centred in the view you have of the end of the sight tube, and looking completely round. Relevant adjustments are the central screw, the three collimating screws (though they are primarily for another purpose) and the nuts positioning the spider (hopefully the spider is well centred from the factory, but you never know.)

[Kokatha man]

Hi mbaddah - yeah, anyone with a sense of adventure who actually wants to "know/understand" the "guts" of collimating will do what you did: plenty won't and they'll never really get to first base with collimating: don't panic though, it'll work out!

That central bolt/screw on the secondary doesn't loosen/tighten as such (unless you go beyond the limits of its' intended adjusting - but you would have known when you fiddled around to mis-collimate the scope, if that had happened.)

Presuming you didn't (despite what I said in my opening sentence) actually go to the masochist's stage of buggerising around with the spider vane adjusters on the scope - then the most likely real problem is your playing with that central screw/bolt.

That's the gizmo that moves the secondary mirror up and down the tube, it's spring-loaded and that's why I said in normal situations you don't refer to its' adjustments as "tightening" or "loosening."

Before I go any further - I presume that you have the instruction sheet that came with the Orion sight tube/collimator?

My first suggestion is to back off the 3 screws set at 120 degrees apart on the sec back - if you need to move the sec down the tube/truss towards the primary this is not necessary but if you need to go "up" away from the primary you'll need to do it - so do it anyways!

Then by just looking straight down the focusser tube (no Orion in but trying to centre your head/eyeball over the open tube) see if the sec mirror surface is fully visible: if it aint adjust that central sec bolt/screw to push or pull it into a roughly central position relative to the bottom end of the focusser tibe.

When you've got a rough set on this drop the Orion in, and looking through its sight hole refine the process - only this time you will be seeing the bottom end of the Orion tool as the reference circle that you need to get the secondary mirror centralized in, as opposed to the bottom end of the focusser tube when you did the preceding "rough" set (and you won't be worrying now about your head/eyeball position.)

A word of caution - if you find that you needed to move the sec "up" by screwing that central bolt/screw clockwise, just keep checking that there is still space underneath the 3 setscrews: cos the sec holder will impinge upon them if you don't make sure there is space for the mirror and holder to move "backwards" - and then you might "bend" one of them etc.

Finally, when you reckon that the sec mirror looks centralized in the bottom end of the Orion, and still with a tiny bit of slack in the aforementioned 3 screws, reach in with one hand, being carefull not to touch the actual sec mirror surface, and physically rotate the sec until, still peering down the Orion, the oval sec mirror looks as if it is circular (which of course it aint, but looking down the Orion you should be able to twist/turn the sec with one hand untill it appears perfectly concentric within the Orion's bottom end.)

Then (and it gets like the octopus analogy, especially if you haven't installed "bob's knobs") pinch up the 3 secondary screws just enough such that you can take your hand away from holding the sec mirror and it should stay in the position you held it in.

Then it's time to adjust the secondary's tilt using those 3 screws as per general instructions. I find doing this inside with the ol' sheet of A4 lightly taped to the inside of the tube opposite the focusser, and with the open end of scope pointing at a white wall, and a light shining onto the cutaway of the Orion tool facilitates the whole shebang.

Cheers, Darryl.

[mark3d]

i managed to collimate mine fairly well using a laser collimator by trial and error (and cursing lol) but i have since found this article:

http://gmpexpress.net/~tomhole/blaser.pdf

it seems the best combination is a sight tube for secondary alignment, followed by a laser for primary alignment.

i think i will buy a sight tube at some stage as i know the alignment is not perfect, but currently i have pinpoint stars so im not stressing too much :-)

[mbaddah]

Firstly thankyou guys for all your help. I applied all your advice and its definitely looking alot better than before! :D

I found loosening one screw completely and playing with 2 adjusting screws only before tightening the 3rd screw greatly helped the collimation,is that right?

I have attached 3 pics of what it currently looks like so far. The first is straight down the Orion Collimator (was difficult to shoot), the second and third I removed the collimator and looked straight down the eyepiece (shot at slightly different angles).

From memory the primary mirror was centred correctly, is there anything else left to be done? Thankyou.

[mark3d]

the interior black circle seems to line up well with the outer white donut so i think the secondary is fairly well aligned. i think the fact it is all off-centre means the primary needs tweaking?

i am in the same boat so dont take this advice too seriously :P

i have a similar crescent to you, between the big white outer donut and the interior black circle. i dont know what that relates to (less than 100% perfect secondary alignment?) or how much of a problem it is.

[centauri]

When performing the centering of the secondary mirror with the focuser place a white piece of card or paper between the secondary and the primary.
This helps us beginners in the ranks of collimation not to get confused with primary reflections.

[mark3d]

great tip, thanks a lot

[erick]

If you get a bit of white card and cut and bend it the right way, in some scopes you can clip it in place with a "bulldog clip" onto one arm of the spider. I could do this with my 8". It's proving to be a bit more complicated with the 12" - I have to redesign my cardboard (Don't drop the clip down onto the primary :scared:) Also useful is a piece of paper/card on the inside of the OTA, opposite the focusser, behind the secondary. Perhaps a different colour so the reflection of the first piece in the secondary stands out against it. A really clever design (I must get onto it) would provide both these backing cards in one piece clipped onto the spider.

[mbaddah]

Tonight when I got home I played around with the pimary mirror a little. The first shot is straight down the focuser while the second is through the Orion collimator.

How do you reckon it's looking so far? Does the secondary look like its centered correctly?

Cheers and many thanks for your help.

[Kokatha man]

Mbaddah brudda: you look well on your ways to being inducted into The Collimation Hall of Fame - a little tweak here and a tiny twitch there: before you know it you'll be responding to newbs who're asking how to collimate their scopes - congrats!

ps - as you approach collimator commander status you have to do the mandatory star-test examination for perfecto diffracto rings.

[mbaddah]

Thankyou for the words of encouragement :thumbsup:

So it needs bit more tweaking?

[Kokatha man]

Well, it's pretty darn close: after a while you can start becoming really obsessive about "spot-on" - I'd recommend seeing if you can do a star test if you feel like it mbaddah. You'll need a pretty good "seeing" night and I'd suggest start with about 100 mags or so (the rule is double etc that, but I found that helped me get the gist first up.)

Get a star (not a dim one) focussed in your scope and then take it out of focus either side of focus; observing how the star appears to expand into a soft out of focus "disk." If there isn't too much turbulence etc in the atmosphere you should, on carefull obsevation, notice that there should/will be a dark central spot in this disk, which, as you go much further out of focus, develops into an image of your spider assembly. But well before this, in fact at about the time you see the tiny central spot, you'll see a number of "diffraction rings" like tiny black/dark circles (hopefully concentrically) surrounding this central dark spot.

I don't seem to get past seeing 5 of these concentric within each other rings (like an archery or rifle target around the central spot) but they should be, along with the central spot, centred and evenly concentric within your out-of-focus star disk. Bear in mind that these dark rings will be very close to each other and may take some discerning at first. If they're not uniformly concentric then they indicate more collimation tweaking.

Once you get the hang of seeing them, bung up the magnification for a more reliable indicator - but remember, if it's a lousy night you'll most probably have buckley's chance of seeing them! I'm sure there's links on IIS to star-test images and most manuals have something about them in them.

Cheers, Darryl.

[mbaddah]

Since last night was one of the first clear nights we've had all week, decided to take the scope out and test out how she purrs...

Everything looked tack sharp, with one slight problem. If I move head little bit, I can see a ghosting effect. For example when looking at Saturn, i'd have to have my eye dead on the centre to see it as one, otherwise i'd see two saturns moving around each other!

Is this due to collimation or some other factor? Thanks guys.

[wavelandscott]

I'd suggest that you start a seperate thread for this question in the eyepiece section below...I think that you will get more responses there than buried at the bottom of a collimation thread.

I would suspect an eyepiece for ghosts but, it could be a number of other things such as:

Some eyepieces are more sensitive to eye placement than others
Internal relection within the eyepiece (for a number of reasons)
Stray light from a steetlight or other
It might be the reflection off of your own eye too...

What eyepieces were you using?

[mbaddah]

Thanks mate I've started a new thread on the matter (located here http://www.iceinspace.com.au/forum/showthread.php?p=302376#post302376)
Cheers.

[mark3d]

got the hang of it! after some experimentation i would really suggest a laser collimator and a barlow! you just tweak the secondary until the laser is centred on the primary, then just tweak the primary until the laser lines up in the collimator eyepeice. doing a star test proved its that easy. a set of bob's knobs and it will be a piece of cake!

[mark3d]

having said that it would be really good to fabricate a small disc with a hole in the centre to make the barlowed laser a fine point, as outlined in the S&T article.

that is a bit of a problem though because if that is not precise the collimation will always be out. the one i made out of cardboard was rubbish.

[mark3d]

having said that it would be really good to fabricate a small disc with a hole in the centre to make the barlowed laser a fine point, as outlined in the S&T article.

that is a bit of a problem though because if that is not precise the collimation will always be out (and my collimator doesnt take threads so id need to make a 2" one for the barlow).