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Thread: Collimate sct

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    Mullinator's Avatar
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    Smile Collimate sct



    How to use a laser to collimate an sct? Tried online but no real explanation that I can understand.

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    Default Re: Collimate sct

    Welcome to the forum.

    I have never heard of using a laser on an SCT.

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    Default Re: Collimate sct

    I have used the laser on my C11 when it was WAY out of collimation.....Put my hand over the aperture and see a laser dot 3 inch away from secondary...WOW that's out!! lol

    I take the diagonal off the eyepiece so its straight through to the secondary mirror.
    Point the scope away from anyone/animal/thing that could be affected.
    Put something in front of the scope aperture, like a sheet of paper or white cloth in front of the scope, shine the laser through the eyepiece and see where on the paper the laser falls. If it does not fall on the paper then the "target" on the laser at the eyepiece end. All should be OK
    If the laser appears on the paper...adjust the screws at the front of the scope on the secondary mirror. Adjust the screws VERY slowly. Keeping the scope pointing horizontal in case you unscrew the screws too far and the mirror falls out and lands on the primary mirror.....tears will follow!! As one adjusts the screws one will see that one screw makes the laser move one direction while another screw will make the laser move another direction. Try getting the laser to the centre where it will appear on the laser collimater target. When happy, wait until dark then use a bright star to improve collimation more.
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    Default Re: Collimate sct

    Hi and welcome to the forum.

    I tried collimating my C11 with a laser collimator and made things worse! Later I found out the collimator itself was not collimated (it was a cheap one). Then I discovered the Duncan mask and have never used a laser again. The Duncan mask is really easy to make from an old pizza box top (make sure there's no grease on it). It is also intuitive to use and much faster than using the laser (once you get the hang of it). Once you use the Duncan mask always follow up with a star test to make sure everything is copacetic. Here's a link on how to fabricate one. Duncan mask.

    Cheers,
    JT
    Last edited by jaetea; 02-02-2017 at 05:45 PM.
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    Default Re: Collimate sct

    You live in Canada. All you do is point at Polaris and do a star collimation. Lasers are not used for SCT collimation. There are expensive lasers specifically designed for SCTs, but they require a follow up star test, so really, what's the point.
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    Default Re: Collimate sct

    Mullinator, to AF

    Dave, I'm curious as to how you are keeping the laser square in the diagonal? By the very nature of the hyperbolic secondary and the spherical primary any position that's not square and dead centered, I would have thought would amplify the apparent error in not holding the laser square.

    Are you using a laser normally used with collimating Newtonians? If so which or otherwise?

    I have heard of using a laser with an SCT, but these are specialized bits of gear, just for SCT's. Newt lasers can't be used due to the error most have from either the laser's own poor collimation & construction and error win the diagonal of the SCT. Being so specialized, I've stuck with adjusting my SCT collimation just with star testing. The alignment error has never been so gross as your 3" would lead us to consider. The lasers used for SCTs normally have three beams as a single beam will never appeared centered unless dead centered. The triangular arrangement of the three beams is what the scope is collimated to. The same approach on a Newt can't be used with an SCT . This is why I'm very curious for more details!

    Even with a Newt, unless the laser is properly centered in the focuser, and the secondary is accurately collimated to begin with, the laser won't be giving you a collimation guide! This is not a rebuke. I am really interested to know and understand what you are doing

    Mullinator, not knowing your experience, I should warn you that while lasers are a good tool, they are not fool proof, and often create a false sense of security if the user does not know how to handle them & their gear properly. Cheap lasers often require collimation themselves!!! And regardless of the laser you use, unless you know the ins and outs of properly manipulating your scope, you actually WON'T be collimating your scope.

    Don't underestimate the accuracy of eyeballing the collimation of your SCT with a med/high power eyepiece. It is extremely accurate and a laser won't necessarily improve things.

    If you are experienced, then my apology for repeating what you are familiar with. All the same, thanks for your question as it has pricked my attention with Daves reply!

    Alex
    Last edited by mental4astro; 02-02-2017 at 06:25 PM.

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    Default Re: Collimate sct

    Quote Originally Posted by mental4astro View Post

    Dave, I'm curious as to how you are keeping the laser square in the diagonal? By the very nature of the hyperbolic secondary and the spherical primary any position that's not square and dead centered, I would have thought would amplify the apparent error in not holding the laser square.

    Alex
    No, Alex. Remove the diagonal. Have the laser pointing directly up inside the SCT towards the secondary. This is only to get the secondary mirror back to some sort of alignment. Once the laser is shining back onto the target on the laser colimator, then do a Star test and collimate with the bright star.

    I did this when my secondary mirror was way off due to not paying attention when turning the three screws and losing track I could not get it back inline and collimated easily. It was so far out I could not even see a star. I found using the laser collimator from my 8" Newton, I could see where the light was headed and made it so much easier to adjust the three screws on the secondary.
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    Default Re: Collimate sct

    I would just get some "Bob's Knobs" and collimate on a nice bright star simply by hand and eye. I like using my Canon T5i inserted into the tube with the screen flipped around and magnified so I can see the defocused star and which way to adjust the secondary. Just like they say in the movie Contact...Small moves Sparks! You'll get there.
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    Default Re: Collimate sct

    Thanks mate! Makes sense now. What you had done was first was stuff up things with your scope, used the laser to achieve a better rough collimation, but still did a star test to get things spot on! Mate, you really must have stuffed things up in the first place! Good to see that you were able to fix it. Your idea to use the laser this way?

    Alex.
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    Default Re: Collimate sct

    Quote Originally Posted by jaetea View Post
    Hi and welcome to the forum.

    I tried collimating my C11 with a laser collimator and made things worse! Later I found out the collimator itself was not collimated (it was a cheap one). Then I discovered the Duncan mask and have never used a laser again. The Duncan mask is really easy to make from an old pizza box top (make sure there's no grease on it). It is also intuitive to use and much faster than using the laser (once you get the hang of it). Once you use the Duncan mask always follow up with a star test to make sure everything is copacetic. Here's a link on how to fabricate one. Duncan mask.

    Cheers,
    JT
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