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Laser collimation not all its cracked up to be...


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I have been using a laser collimator to adjust my 8" Newt and after an initial panic that I had not done it correctly I managed to lull myself into a false sense of security.

I had a go at some planetary imaging recently and could not get a sharp image for love nor money, which made me a bit suspect. So while I was at Astrofest I picked up a standard Orion Cheshire eyepiece and had the shock of my life when I got home and took my first look... It was way out, one whole side of the primary was obscured.

I am now of the opinion that laser collimators have their place, but only for those quick pre-session tweaks.

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The fact is that laser collimators are way out. They get sold as an idiot proof solution to something some folk struggle with so beginners buy them and use them thinking they're doing the job and they're not. Collimating a laster collimator is a second level of hell if you struggle with cheshire collimation and newbies will not be able to.

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The fact is that laser collimators are way out. They get sold as an idiot proof solution to something some folk struggle with so beginners buy them and use them thinking they're doing the job and they're not. Collimating a laster collimator is a second level of hell if you struggle with cheshire collimation and newbies will not be able to.

I have actually checked and mine is not bad at all. The problem is when the secondary moves, the primary can usually be moved to compensate and appear correct from the path of the beam, but in fact the secondary is not centred correctly on the primary. At least that is my conclusion :D

Edited by palebluedot
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Depends on the collomator used. A lot require collimation and setting up before use. Others can have their alignment offset by the eyepiece thumb screw to secure it. I use the Hotech which has a self centering fitting and works a treat.

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So are you saying that if the collimator is properly collimated (and centred in the eyepiece adapter), the beam hits the centre of the primary, comes back and hits the centre of the screen then it is impossible for the mirrors to be out?

It's impossible. As you said, as long as the face of the secondary is along the correct 45° angle to the focuser and the primary then the laser will come back up the line.

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Yes... The idea of the collimator is to square up the axis. If your focuser is slightly off square having a self centring laser will thus collimate the centre of the optical path correctly. Using a visual collimator IMO has more chances of miss-alignment as you have to place the centre of your eye in the same centre of the axis and there is more chances of human "best guess".

The collimation of a laser has been well documented, as is the affects of the thumb screws pushing a laser beam which is why Hotech use the self-centering method. I then check my collomation using the star ring method on an out of focus star... it's never been out.

One thing for sure is that collimation should be done regularly, and before any session, even in a permanent set up in an observatory. Lots of things can make the optical axis shift slightly.

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The OP's problem is not with his laser collimator but rather with understanding what laser collimators are used for. Collimation is 50% tools and 50% knowledge. You need both.

The OP mentioned that the secondary mirror did not seem to be positioned correctly under the focuser and a noticeable portion of the primary mirror was obscured. Bear in mind that a laser beam hits a very small part of the secondary mirror. Since that beam does not interact with the secondary mirror edge, how can we expect a single laser beam to tell us if the secondary mirror is positioned correctly under the focuser? It does not. The secondary mirror could be squire, triangle, or irregular shape yet the laser beam will tell us nothing about the secondary mirror edge.

See attachment. In every frame, the laser beam will hit the primary mirror center and retraces it path to the source yet the secondary mirror seems to wander around. Only one frame captures an optimal placement for the secondary mirror.

To properly position the secondary mirror, you will need to use a sight-tube – as the OP did. The sight-tube does reference the secondary mirror edge for optical placement.

In summary, do not expect the laser collimator to perform an alignment it was not meant for it to do.

P.S. Laser collimators can be used to position the secondary mirror but you will need to use a special holographic attachment as shown in the second attachment.

Jason

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Hmmm, we seem to have a difference of opinion. I am inclined to believe Jason as it reflects my observations (no pun intended) Can anyone else confirm this?

(not the bit at me being ignorant :D )

Edited by palebluedot
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You can have the secondary turned and the laser will still show it as collimated - been there and felt the frustration.

I use a cheshire to get the secondary in the correct position and tilt of the secondary and then use a laser to fine tune the tilt and then the primary. I always do a final check with the cheshire.

Edited by Starflyer
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Dont want to get too involved in your discussion. My tuppence worth is that you should centre the secondary first and then use the laser to get close, then maybe use the cheshire to get it spot on.

Mind you, I've discovered the barlowed laser technique and its very good. Especially if you dont have someone to help you at the other end of the scope. I find it a real pain to get to the primary collimation knobs and going back to the eyepiece. The barlowed laser technique gets over this.

Barts

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I am inclined to believe Jason as it reflects my observations (no pun intended) Can anyone else confirm this?

You should be inclined to believe Jason because he actually knows what he is talking about.... :):D

I see a lot of Newtonians that have rotated and skewed secondary mirrors, and the culprit is pretty much always using a laser as the only tool for checking collimation. Even looking down the focuser will give you an idea if the secondary is centred and presenting as a circle rather than an oval. A sight tube properly positioned in the focuser makes it a good deal easier to see any errors.

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It would be better if lasers were sold as tools for rapid primary mirror collimation, rather than "laser collimators", which implies all aspects of telescope collimation.

It's little wonder that newbies (and some oldies) get caught out.

I'll stick with a Cheshire!

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It would be better if lasers were sold as tools for rapid primary mirror collimation, rather than "laser collimators", which implies all aspects of telescope collimation.

I agree. Maybe we should start a campaign. I see FLO have a health warning on their site for laser collimators regarding focusers, maybe they could take the lead and extend their advice to include a bit about their ineffectiveness for secondary collimation.

I also have is on very good authority that Jason is the main man when it comes to collimation, sorry Jason if I was a little hesitant to take your advice.

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If you want to align your primary accurately using a laser you also need to look up barlowed laser collimation to overcome the inadequacies of your focuser.

Anyhow so far as self centering adaptors are concerned there's no real point unless all your EPs are self centering too - What's the point in placing your laser in the exact centre of the focuser and then placing your EPs offset?

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There are three collimation alignments. Two belongs to the secondary mirror and one to the primary mirror.

1) Optimizing the 100% illumination field location via secondary mirror coarse adjustment

2) Eliminating focal plane tilt errors via secondary mirror fine adjustment

3) Eliminating focal plane shift errors via the primary mirror fine adjustment (most important)

I will describe all three in this post.

There are two alignments for the secondary mirror.

The first secondary mirror alignment is somewhat “coarse” which calls for centering and rounding the secondary mirror under the focuser. This alignment is responsible for centering what is referred to as the “100% illumination field.” Typically, not every star in the FOV is at 100% potential brightness. Our goal is to have all stars around the FOV center to be at 100% potential brightness. The first two attachments illustrate what happens when the secondary mirror is not optimally positioned under the focuser. The illustrations are somewhat exaggerated to convey the point. The impact is more noticeable when imaging as opposed to visual observation. The next three attachments give more insight about the "100% illumination field" and how the secondary mirror placement impacts its location.

The second secondary mirror alignment is more “fine”. We try to redirect the focuser axis to strike the primary mirror center off the secondary mirror. This alignment is responsible for eliminating the tilt between the primary and eyepiece focal planes. If there is a tilt, the whole image at the eyepiece can’t come to focus simultaneously. The 6th attachment illustrates what happens when there is a tilt between the two focal planes. Again, the illustration is exaggerated to convey the points. The tilt impact is more noticeable when using a paracorr or when imaging. The 7th attachment gives more insight about the focal plane tilt error.

Finally, we need to adjust the primary mirror. In general, reflectors are somewhat tolerant to secondary mirror misalignments; however, the most critical alignment is the primary mirror. Even few millimeters misalignment is enough to significantly degrade the view especially for fast scopes. The best way to describe the view degradation due to misaligned primary mirror is "smearing". The formal name is "coma." The view will be smeared in some direction which will elongate stars and blur planets/DSOs. Refer to the 8th attachment.

The 9th attachment is an animation which illustrates "axial alignment" which is basically the 2nd and 3rd alignments covered above.

Laser collimators can handle the second and third alignments – not the first unless a holographic attachment is used.

Note: All images in this post are simulated images.

Jason

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Edited by Jason D
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  • 3 weeks later...

Great thread guys. I use a laser collimator but still get a poor image through the scope. I have coma the image is blurred to the right. However no matter what I try I cannot get the secondary lined up to show the reflection of the primary with all three clips visible. I have tried the film canister method to no avail. I have tried it with and without the lense fitted at the bottom of the draw tube. What is a 'Cheshire' will one of these help with secondary set up? The scope is TS 150mm 1400 newton.

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A Cheshire eyepiece is the traditional tool to help with collimation:

First Light Optics - Cheshire Collimating Eyepiece

No laser involved !

If you have a barlow lens you could try barlowed laser collimation. You need to get your secondary centered and tilted properly before you use the barlow though:

http://www.astromart.com/articles/article.asp?article_id=827

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Great thread guys. I use a laser collimator but still get a poor image through the scope. I have coma the image is blurred to the right. However no matter what I try I cannot get the secondary lined up to show the reflection of the primary with all three clips visible. I have tried the film canister method to no avail. I have tried it with and without the lense fitted at the bottom of the draw tube. What is a 'Cheshire' will one of these help with secondary set up? The scope is TS 150mm 1400 newton.

Hmmm... 150/1400 that's worrying. Has that got a spherical lens and a barlow in the focuser tube? If so it'll be a pig to collimate and will never be great.

The other thing to bear in mind is that the optical cente of a mirror isn't always the physical centre and center spots on mirrors aren't always central (Meade apparently hire a visually impaired man to apply them on the end of a long bendy stick in a darkened room).

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Hmmm... 150/1400 that's worrying. Has that got a spherical lens and a barlow in the focuser tube? If so it'll be a pig to collimate and will never be great....

Good spot - I'd missed that :)

That scope design does not work with a laser collimator as I believe that the lens at the bottom of the focuser tube messes up the laser :)

I suspect a collimation cap plus star testing is the best way with those.

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