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laser collimator question


Whippy

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Before I pick one up, I'm slightly curious about laser collimators..

As I understand it, when you're adjusting your secondary mirror you should be directing the beam into the middle of the primary right? No problems with my skyliner as it's got a black dot there to show you. But on my TAL there isn't a dot, so how would you know if you've got it smack bang in the middle? Guesswork?

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no. I think you can make a dot, but I've always found this idea strange. It must involve removing the mirror from the tube altogether, which would even mean removing the 2ndary and spider. I wouldn't try that!

there must be a way if you take advantage of the simple fact that the mirror is curved, so when level, the middle of the mirror would be the lowest point. A liquid with low surface tension (i.e. not water) would collect right in the middle.

Would this be accurate enough, as long as you ensure the mirror is level?

Just an idea :?

Andrew

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Hm, interesting idea. The only liquid I can think of is Mercury, and I really don't fancy playing with that (pointless trivia: the phrase 'mad as a hatter' comes from when hat-makers used to use Mercury to brush down velvet, over time it's absorbed into the blood stream...). Even without playing with dangerous substances, it's a bit of a faff every time you want to collimate isn't it?

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Having a centrally marked mirror is a huge help in collimation, whether you use a laser, collimator, a cheshire collimator or just an alignment tube.

I used a laser collimator on my F6 Dobsonian but I'm not so sure it was a huge step forward from using a cheshire or aligning by sight using an old 4mm eyepiece with the lenses removed. I seemed to get better results with the latter method actually - I suspect that my laser collimator was itself a little out of alignment which can make big difference when you try and use it !.

John

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Good article! But I think I'll pass on actually taking the mirror out of the tube as to be honest, I'm such a clumsy so and so! I think I'll stick with my cheapy old collimation cap.

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I have also heard tales of laser collimators being out of alignment.

Apparently the way to check is to position them in "backwards" so that the laser is pointing at a wall then turn the laser slowly (360 degrees) in the eyepiece holder. If the dot stays still on the wall its ok, if it goes round in a circle then the laser is out of alignment.

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Bill, many laser collimators have collimation adjustment themselves. I always found it amusing that I had to collimate my collimator before i collimated my newt with my collimator...... :D

I'm sure I've seen an article on the web somewhere by a guy who built a special frame to put his laser collimator in to hold it dead level while he collimated it, and ..... you've guessed it !... that frame needed adjustment or "collimation" before use so he had to collimate the collimator frame then collimate the laser collimator then collimate his scope.

... or just use an old 35mm film cannister with a hole in it :D.

John

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Bill, many laser collimators have collimation adjustment themselves. I always found it amusing that I had to collimate my collimator before i collimated my newt with my collimator...... :D

LOL!! :lol::D :D

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I marked my primary the other day, it's really easy to do and it's the best thing that i've ever done to my scope, saw the cassini division for the first time after collimating using the cheshire and the center spot.

I used the paper circle folded twice method with a paper reinforcement ring.

Martin

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I did an article some time ago about collimating a Newtonian. It too involved placing a black target dot at the centre of the primary mirror.

Even though you get that dot bang on the centre of the disc, it does not necessarily mean that you have got it on the optical centre.

A mirror is a series of concentric zones of differing focal lengths, ie the parabaloid. Sometimes the figure of revolution does not coincide exactly with the physical centre of the mirror. Therefore it is always advisable to to a star test after collimation, to check for slight coma. It may not be there, but then again it might

Ron. :D

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I'm sure I've seen an article on the web somewhere by a guy who built a special frame to put his laser collimator in to hold it dead level while he collimated it, and ..... you've guessed it !... that frame needed adjustment or "collimation" before use so he had to collimate the collimator frame then collimate the laser collimator then collimate his scope.

So you have to collimate the collimator that collimates the collimator that collimates the collimator!!! :shock:

or have I missed a step? My brain hurts! :D

I'll stick to the Celestron colli-mate that came with my scope!

Bill£

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I'm sure I've seen an article on the web somewhere by a guy who built a special frame to put his laser collimator in to hold it dead level while he collimated it, and ..... you've guessed it !... that frame needed adjustment or "collimation" before use so he had to collimate the collimator frame then collimate the laser collimator then collimate his scope.

So you have to collimate the collimator that collimates the collimator that collimates the collimator!!! :shock:

or have I missed a step? My brain hurts! :D

I'll stick to the Celestron colli-mate that came with my scope!

Bill£

But how would you collimate the first in the long chain of collimaters :lol:

:D:lol::D

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I wouldn't. I'de machine it out solid aluminum like they should be in the first place! The Meade laser collimator that I had was £50 and a nice machined piece of metal. However upon unscrewing it unside is a 99p laser pointer with a keyring hole! I jest not. This is why most of them have to be collimated to align this laser pointer inside the 1.25" nosepiece adapter. If they made them as one piece and factory aligned them then there wouldn't be this problem.

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Talking of laser collimaters...

I have added the Baader laser collimater to FLO's stock (should arrive Monday). I dithered for a while as I found it difficult to take it seriously (it has a key-chain!) but I have been hearing good things so thought I'd give it a go.

Would anyone like to review it?

I'll post it out free-of-charge and collect it after you have finished.

No obligations, speak as you find :D

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When I got a laser collimator I used those punch hole repair thingies to put a centre spot on the mirror.

Now collimation is so much easier especially as you often find you don't have a lot of time to do it before a session.

However, the laser collimator on its own is not enough to ensure proper collimation. Apart from the reasons cited, many focusers exhibit image shift which doesn't help.

I say get that mirror out and centre mark it. you'll be glad you did.

Geoff

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Been using them for years in XRay and Gamma Ray Detectors.

Bill£

As in Gamma Ray Bursts, wondering out loud? I suspect that these are not twopenny things that one could mount on ones shed pointed up linked to a flashing beacon type thing. I bet they cost thousands and are not very sensitive and other bad stuff.

Any chance of enlightening those of us who have just woken up please Bill?

Captain Chaos

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