Collimation tools (split from Lightbridge Updates)

[norm]

Hi Astronut,

The scope sounds very impressive. I am a newbie and just wanted to know what tools you will be using to collimate the scope. I went into Astro-Opticals and they basically said to me don't bother with a cheshire eyepiece or site tube and go straight for a laser collimator. I allways had the impression these were the basic tools to use and master?

Just wanted to know your thoughts or anybody elses. He also said something about putting a mark on the cover of the primary mirror so when the laser projects back up you don't get all those reflections.

Apologies now if I haven't clarified myself here .

[janoskiss (Steve H)]

I would not listen to most dealers about the type of collimation tools you need. I was told a similar thing about the laser by staff at a scope shop in Melbourne too. I was also told that once the scope is collimated, I should only ever need to make small adjustments on the secondary, yes the secondary! the small mirror near the focuser! I was told that when star testing I should be adjusting the three screws behind the secondary in the centre of the spider.

I am not kidding!! The guy showed me this in person and pointed out the adjustment screws for the secondary as the ones to tweak while star testing. He also told me not to touch the primary adjustment screws because that one stays put in transport, but the secondary goes out really easily. In case you don't know, this is all incorrect, back to front. See any good source on collimation, e.g., www.skyandtelescope.com. With this experience and their over inflated prices, they have lost me for good.

Cheshire/sighttube is the first collimation tool to get, and the only one you need. But a laser can be helpful in aligning the secondary. If you want one of those return beam lasers, they are on special at bintel for $79.

[janoskiss (Steve H)]

I've split this discussion from astronut's Lightbridge thread: http://www.iceinspace.com.au/forum/s...ead.php?t=8814

[astronut (John)]

Hi Norm, I was told the same thing by a very experienced astronomer, cheshire is the way to go

[cjmarsh81]

I had read this in reviews on the internet too. That is why I found collimation so confusing at first, I thought how in the hell do you adjust those three screws on the secondary in pitch blackness? It is only recently that I found a review that said the secondary is done pretty much once a year and the primary is adjusted each time you use it. This makes a lot more sense to me.

P.S. I have a chesire/sighttube and I find it is all I need. I have read too many reports on the laser collimators being inaccurate I have not wasted my time on them.

[Vermin (Tom)]

A laser can be a handy tool, but can be considerably better with a few adaptors. I use,

1) a basic laser beam to square the focuser in the secondary cage

2) a holographic grid attachment on the laser to centre the secondary in the focuser and to rotate the mirror to face to the focuser

3) the basic laser beam to aim the secondary at the primary

4) A barlow attachment on the laser to aim the primary at the secondary.

When star testing after aligning with these tools, the primary does not have to be tweaked at all.

[cjmarsh81]

Maybe when I get into astrophotography I will get that piccy with my collimation. Until then, if it looks good through the EP it will do me.

[vespine]

So I went and got a 12" Lightbridge yesterday.. To cut a long story short, Astro Optical shipped one from Sydney for me, supposedly because they were going to collimate it for me... Yes: he didn't want to sell me the telescope that was in the shop because he wanted me to get a collimated scope and he'd sold out of the tools to do it himself .... He was getting the scope collimated, packed away and shipped from Sydney to Melbourne so that I could get a collimated scope, I'm not making it up. He also said the laser is the way to go, but after this experience I'm even more convinced that the cheshire is the way to go (I even called bintel, no cheshires there either )

Anyway so after putting it together, to me it looks out. The bits are not lining up. To see the reflection of my eye right on the centre dot of the primary I have to move my eye towards the edge of the draw tube and then the reflection of the primary is far from centred in the secondary... But really, I got no idea and I've been reading about "off set" and finding it a little confusing so I thought I should at least do a star check or something before I touch anything, just in case I'm mistaken..

So I wait until dusk and just have to first point the scope at Orion's nebula, and even tho it wasn't even really dark yet I was stunned as to how NOT faint it is!! After I stopped gasping, I did a star test and sure enough it is definitely in need of some adjusting, thinking it would probably take me half an hour to do the 1st time and not even kidding myself that it's something I could attempt in the dark, the clouds came over so that I wouldn't keep torturing myself with a view that was now obviously impossible to bring to perfect focus and overly comafied..... Going to have fun trying to collimate it tonight.... without any tools...

[cjmarsh81]

If you have an old 35mm film canister you can drill a 1/16th hole in the center of the lid and cut the base off. This will give you a reasonable collimation tool if you have nothing else. Do a search on the internet about collimating. I would not even attempt it without some form of tool as you don't know if you are adjusting it in the right direction or not.

[cjmarsh81]

Try on of these

http://home.comcast.net/~astrohammer/tipscollimate.htm

http://www.schlatter.org/Dad/Astronomy/collimate.htm

http://web.telia.com/~u41105032/kolli/kolli.html

http://web.telia.com/~u41105032/myths/myths.htm

http://www.fpi-protostar.com/collim.htm

http://skyandtelescope.com/howto/sco...icle_787_1.asp

http://hometown.aol.com/billferris/startest2.html

[janoskiss (Steve H)]

Too much information CJ! There is enough pages there to make a book (or two). Are you a refractor dealer?

[vespine]

I'll definitely be trying to "film canister" trick, doesn't look like a star test will be likely tonight tho

[janoskiss (Steve H)]

One trick with the film canister is to stick something bright around the pinhole facing the secondary so you can see its reflection when you look through the pinhole.

[barees63]

Well, FWIW.. here's the sum of my experience so far..

First, that selection of web pages will definitely confuse you, many of them confused me with their contradictions..

The thing is you need to treat collimation as a series of steps..

What has worked for me is to:

1. initially get the secondary correct by taping some bright colored paper opposite the focuser and some white paper in the light path (I was going to say to block the tube but you know what I mean), this lets you see the secondary very clearly and you can then use a sight-tube (or cheshire combo) to get the secondary centered and rotated so it appears perfectly round.

2. Then use a properly collimated laser to tilt the secondary so the dot hits the exact middle of the primary centre spot..

3.Then use a cheshire to set the primary tilt.

The theory is only the last step needs to be done on a regular basis..

Hope this helps, probably just more information overload

Bruce

PS. Have a look at www.catseyecollimation.com

[Vermin (Tom)]

Originally Posted by barees63

The theory is only the last step needs to be done on a regular basis..

Because the Lightbridge is a truss tube scope, it is more likely that the secondary and primary will need adjusting if it is dismantled between sessions.

Numbering the truss tubes and putting them back in the same spots will minimise any secondary adjustment required.

If the truss poles are very similar or if the scope is not disassembled between sessions only the primary may need adjusting (after the initial alignment).

[Starkler (Geoff)]

Originally Posted by Vermin

Because the Lightbridge is a truss tube scope, it is more likely that the secondary and primary will need adjusting if it is dismantled between sessions.

Numbering the truss tubes and putting them back in the same spots will minimise any secondary adjustment required.

Yes indeed and I thought it very misleading to suggest that one can dismantle a truss dob, ship it interstate and have it magically in collimation when reassembled.

Yes you most likely will have to tweak both secondary and primary mirrors each time the scope is torn down and reassembled. Tube dobs only need a regular primary collimation tweak.

FWIW, if I owned a truss dob I would use both a laser and a cheshire, with the laser only for the convenience of easier secondary aiming.

[cjmarsh81]

Originally Posted by janoskiss

Are you a refractor dealer?

Nope. I just didn't want to fiddle with my scope until I knew what I was doing. I still don't, but I think my collimation is fairly good. I only tried Star testing a couple of days ago. hehe

[matt]

This is probably the nerdiest thing anyone's ever said but I actually enjoy collimating and star testing.

I think it's actually really enjoyable once you've got the hang of it.

I need help

[Starkler (Geoff)]

In the other thread I suggested that the first lightbridge mod should be blackening those white inner tube ends but I have changed my mind.

I reckon the first mod should be gluing a washer on the secondary holder for the collimation screws to bare against. You will be tweaking the secondary often and the holder is only soft plastic. The screws will quickly gouge holes that they will always want to slip into so a metal surface will be a lot more durable.

[cjmarsh81]

Originally Posted by matt

This is probably the nerdiest thing anyone's ever said but I actually enjoy collimating and star testing.

I think it's actually really enjoyable once you've got the hang of it.

I need help

Yes you are right. You need help. I hate collimating my scope. I can't get over the feeling that I will change something and not be able to put it back to working order. Optics to me is a bit of a black art at the moment, until I get a little better at it.