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  #1  
Old 03-08-2018, 04:29 PM
Granada
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Collimation

I use a laser collimator to collimate my reflector's primary and secondary mirrors which is easy enough. However if I rotate the laser in the eyepiece holder (say quarter of a turn) the laser dot leaves the centre of the circle on the primary mirror.


I do as a test because obviously the eyepieces I put into the holder won't always be rotated the same way. Therefore my question is whether the misalignment that I observe with my laser (when rotated) will affect the overall collimation? Or should the telescope be collimated in such a way that even when you turn the laser in the eyepiece holder, the red dot doesn't leave the primary mirror's circle?



Cheers
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  #2  
Old 03-08-2018, 05:31 PM
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doppler (Rick)
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Many laser collimators need collimating themselves, so the laser dot should be steady in one spot when being rotated if its collimation is good.
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  #3  
Old 03-08-2018, 09:48 PM
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mental4astro (Alexander)
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Granada,

Like Rick says, some laser collimators need to be collimated themselves!

Have a read through the following IIS thread I started about how to check your laser and how to collimate one of these:

Tricking up your GSO laser collimator

One thing that a laser won't do is actually tune the secondary mirror! A laser will not help with the centering of the secondary in the focuser, and won't check if the secondary is square with the focuser. This last point alone will mean that if you rely solely on the laser to "collimate" the secondary, if the secondary is not square then whatever you think your scope is collimated, it will actually be a false reading.

Many people think that "primary" and "secondary" also relate to the importance of each mirror. Not only is this incorrect, but the truth is the secondary mirror needs to be sorted out FIRST before the primary. And this means not just that the reflected laser beam is on the centre mark, but that it is also properly collimated in relation to the focuser and in the optical train. Only when the secondary is properly sorted in all its parameters, only then are you ready to deal with the primary.

I too fell into the trap of solely relying on a dumb laser for all my collimation protocol. And as time went on because the secondary was not being properly treated, the image was progressively getting worse and worse, despite all my efforts with the laser. It was only because I had a gut-full of a s.h.i.t.t.y image, and having exhausted everything I could do with the laser, for some reason I decided to check what was going on with the secondary. To my amazement, the secondary mirror was totally WAAAYYY off! Way out of center, not square (rotated) and off to one side. I was stunned because I thought I was doing everything correctly, but NO ONE had ever mentioned to me that a laser is only a tool to use AFTER the secondary is sorted, and that the laser won't deal with it, not even the bloody astro retailer I bought the blasted laser from.

The humble Cheshire eyepiece is the best tool to use to deal with the secondary. A Cheshire eyepiece will sort out the entire collimation process of a Newtonian, though a laser can be more convenient to deal with the primary. Once you have dealt with the secondary with the Cheshire, then you can use the laser to first fine tune the centering of the beam in the centre spot, and then you can tweak the primary.

So, you have to do two things, get your hands on a Cheshire eyepiece, and verify that your laser is properly tuned too which is very easy to do.

Alex.
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  #4  
Old 05-08-2018, 09:48 AM
Granada
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Great, thank you for the advice!
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  #5  
Old 05-08-2018, 12:24 PM
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ChrisV (Chris)
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I have collimated my laser, but still have a similar problem to you!

I think there's a bit of play in how my collimator fits into the scope. Its a 1.25" collimator and I have to put it through a 1.25-2" adaptor for my newt. Some of the adaptors I have make it better than others. Or maybe my focuser is slightly tilted?

I also have an orion cheshire collimator. Took a while to get the hang of, but I feel that it does a better job.
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  #6  
Old 05-08-2018, 10:51 PM
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floyd_2 (Dean)
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I don't think this answers the query re laser collimators, but I've always used the sight tube, Cheshire and autocollimator eyepieces for precise collimation. Just like Alex said, you need to get the secondary right before you do much else. You should also check your focuser to make sure that it's perpendicular to the light path - shim it if you have to. Interestingly I don't see many autocollimator eyepieces on the market anymore. I'm not sure why, as that was what I used for fine tuning every time...I wouldn't be without one if I had another reflector.
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  #7  
Old 06-08-2018, 08:37 AM
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Quote:
Originally Posted by floyd_2 View Post
Interestingly I don't see many autocollimator eyepieces on the market anymore. I'm not sure why, as that was what I used for fine tuning every time...I wouldn't be without one if I had another reflector.
Yep, too many people see "laser" and think its the right tool for the job when it isn't. So they sell instead.
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  #8  
Old 06-08-2018, 09:03 AM
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mental4astro (Alexander)
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If the laser leaves the centre of the centre-spot, there's a few reasons for this.

1, The laser is not collimated. The piece I wrote discusses how to rectify this.

2, Play in the drawtube of the focuser. Any play while you make adjustments to the laser/Cheshire will have the collimation/laser beam wonder off away. Some focusers can control the amount of play through a locking screw or a tension control screw. Other focusers need to have shims attached to the draw tube to reduce or eliminate play. This play will change the position of the laser/Cheshire/eyepiece/camera every time you make adjustments to the focus or tweak the piece in the focuser.

3, Poorly fitting laser/collimator. Sadly these tools can be poorly made so that there is too much play between the tool and the focuser fitting. One solution is to wrap some tape around the fitting surface of the laser/Cheshire barrel to tighten the fit. Care needs to be taken here not to make the fit too tight and not to over tighten the set screws that secure the piece in the focuser so not to damage the tape.

4, Crappy set screw design of the focuser! This is a real hidden trap. If the focuser has two or more set screws to hold the eyepiece in place, then this is a MAJOR problem for any collimation attempts!!! If you release and tighten two or more set screws, YOU WILL NEVER RECENTRE THE LASER, CHESHIRE, EYEPIECE OR CAMERA to the exact same spot EVER! This will always put the laser/cheshire/eyepiece/camera in a new position every time you make a change. The focuser below is an example of a crappy focuser set screw design with two set screws. And it doesn't matter if your focuser has the set screws push directly onto the piece or uses a brass compression ring, two or more screws are always bad news. You need to only use ONE set screw at all times, otherwise you will forever be fighting collimation issues visually and photographically, and in the collimation process.

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5, Primary mirror shifting in its cell. This you will really only see when the scope is altered in position from vertical to horizontal, and even rotation. This means that there are issues with the mirror cell: springs are too weak (VERY common problem and mass production scopes all have undersized springs - just to save a few cents), too much play in the cell mechanism, mirror is loose in cell, locking screws not fixed (careful when fixing the locking screws as these WILL ALTER THE COLLIMATION! You need to fix the locking screws with the laser on to see that collimation is not altered), cell is loose in the OTA, cell/OTA is poorly designed/fabricated.

6, Secondary mirror shifting. Like with the primary mirror, this is only noticed when the OTA is moved about.

These are the only ways in which I can determine as to why a laser beam/collimation will actively shift.

If the secondary mirror is not properly set, ie: not square with the focuser, but held securely in the holder, the laser will not go for a wonder. It can't if everything else is fixed. But it will mean that the scope is not properly collimated.

A focuser that is not square also won't see the laser beam wonder off, but it will affect the collimation because it is not properly set in the optical train. Can be tricky to verify if your focuser is not square, though it is not difficult to rectify, but it won't alter the position of the laser beam or collimation if it is not square.

If you see the laser beam go for a wonder, you need to go step by step, one at a time to eliminate any of the variables. And different scopes will have different idiosyncrasies.

Alex.

Last edited by mental4astro; 06-08-2018 at 09:33 AM.
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  #9  
Old 06-08-2018, 09:18 AM
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mental4astro (Alexander)
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Quote:
Originally Posted by sil View Post
Yep, too many people see "laser" and think its the right tool for the job when it isn't.
I bought my GSO laser from an astro retailer here in Oz. They told me nothing about how to use it. I was very inexperienced and naive, and even if the retailer did tell me how to use it, I certainly would not have understood how involved the collimation process is (to start with), and I may even have questioned if the laser was faulty, or my scope was faulty. Nor does the retailer have time to explain this. It's a crappy situation really.

Now, collimating a scope is not hard. It is only hard at the start when you are trying to get your head around the concept. Just remember that you are just ALIGNING the optics, that's it, pure and simple. The biggest hurdle with collimation is the bloody word "collimation"

In a way it is a good thing that one's collimating prowess takes this long winded way. Good because it progresses with one's experience, understanding and proficiency at using all this gear. In the end, tweaking one's scope takes just moments, and it becomes second nature & a part of the setting up process, and a way of getting in-tune with your scope, as these moments spent touching up the alignment at the start of the night will mean that your scope is then giving you the very best performance it can. And this is to your benefit.

Alex.
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  #10  
Old 06-08-2018, 09:49 AM
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Quote:
Originally Posted by mental4astro View Post
The humble Cheshire eyepiece is the best tool to use to deal with the secondary. A Cheshire eyepiece will sort out the entire collimation process of a Newtonian, though a laser can be more convenient to deal with the primary. Once you have dealt with the secondary with the Cheshire, then you can use the laser to first fine tune the centering of the beam in the centre spot, and then you can tweak the primary.

So, you have to do two things, get your hands on a Cheshire eyepiece, and verify that your laser is properly tuned too which is very easy to do.

Alex.
This is the route I have followed Alex and I like the cheshire. The only trouble is I am long sighted. So the cross-hairs of the collimator are out of focus without glasses, but with glasses on it hard to look through the collimator.

Maybe I need better glasses. And apologies if I'm railroading this thread.
And that's an interesting comment about two or more set screws. I never thought about that. Hmm, makes you wonder - the premium focusers such as moonlites have multiple set screws ...
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  #11  
Old 06-08-2018, 10:08 AM
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mental4astro (Alexander)
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I too am long sighted... Getting worse too for my sketching <sigh>

There's a few Cheshire options.

1, the worst is the short Cheshire eyepiece. Puts the cross hairs too close to your eye to properly see. The cross hairs don't need to be sharply focused in your vision, but not a hopeless blob. Also the short Cheshire doesn't reach into the OTA to help with the centering of the secondary mirror as it leaves too much space between the outline of the secondary and the inside rim of the Cheshire. Yes they work, but they are the least reliable.

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2, Long Cheshire - easier to see the cross-hairs in it. I use one of these, and while the cross-hairs are not sharply focused, they are sharp enough so I can see past them for the process. It's longer reach also helps better frame the outline of the secondary mirror inside the Cheshire.

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3, "Auto-collimator" style Cheshire. A slightly different take on the cross-hair Cheshire, and in some ways a bit easier to use for overall collimation. Once your secondary's position in the focuser has been optimized (once determined you won't need to muck around with this again), these auto-collimators are pee easy to use. You might find one of these easier to use if you are having trouble with the cross-hairs.

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I have a 1.25" and a 2" long-tube Cheshires (some focusers are 2", others 1.25"), a couple of 2" auto-collimators and a 2"/1.25" laser. If I can't figure out the collimation with my various Newts, then I'm giving up!

Alex.
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