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Collimation woes... laser vs cap!!


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Ok I haven't taken my scope out in a few months because its way out of collimation and I have been too busy to mess with it. Now the star gazing bug has bitten me bad and Im ready to get out under the stars again. The past weekend I got all my equipment out and unboxed my new laser collimater and sure enough the red dot was about an inch (2.54 cm) to the left of the center mark on the mirror. So I realligned the secondary to make the laser line up on the center mark of the primary mirror and then adjusted the primary to hit the bulleyes of the collimator. To my knowledge when that happens my scope should be in perfect alignment. So to test this theory I popped in my collimation cap and to my horror nothing looked lined up right. The secondary mirror looked "ok" but I could not see all of my primary mirror, I only saw two of the three mirror clips. So I collimated everything according to the cap. Collimating looked well, everyhing was in concentric circles and appeared to be great. I pop the laser back in and like some grim nightmare it was back to about an inch off. I dont understand why. Everyone talks about how collimationg takes seconds but I have fought, cried, cussed, and wept like a child for the past two days and can't resolve the issue. Me personally I believe the laser is Rubbish and I should probably throw it out of the house. I keep going back to it because in the back of my mind I know its a "laser" it "should" present a straight line and that if that line hits the center mark of everything.... then it should be collimated. After I collimated the scope using the laser (everthing through the cap still looked wrong) I took my scope out for a star test, and I couldn't achieve a clear and crisp focus using my stock 25mm EP. I reeled the focuser all the way out and the star become lopsided and when I reeled the focuser all the way in the star did become a fairly concentric circle.

My gear -

Orion XT8 (203 mm by 1200 mm focal length f/5.9)

Orion LaserMate Collimater

Stock collimation cap

Any help or advice would be greatly appreciated. Personally I think the laser is a paper weight and I should trust my eyes and the cap but I would like some other opinions.

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I'm no expert, but maybe check your laser. Can you rest it on two V shapes, like 4 nails knocked into a piece of wood that allows the laser to rotate freely. Aim the laser at a wall, say 3 meters away and rotate the laser on the cradle you've made. If the red dot moves around as you turn the laser, it itself is out of Collimation. Generally, trust your eyes

Barry

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I'm with Barry, it certainly does sound like the laser itself isn't collimated. I bought a cheap laser collimator because it was user collimatable as I would never trust a factory to get this vital calibration correct for me.

There is much debate over the usefulness of lasers for this task but a properly collimated laser with an accurately centre marked primary mirror is a joy to use!

Sent from my iPhone from somewhere dark .....

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Another thing to look at is how well the laser collimator sits in the focuser. There only needs to be a tiny

movement and it will throw the beam out just like a misaligned laser. I use a colli cap in the daylight and then star test when out side. :smiley:

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My cheap laser can wiggle a bit inside the focuser, this means the red dot can hit the primary a good inch of the centre point, but actually the scope is collimated ok. so to is the laser. the problem lies in the amount of play between the focuser and the laser itself. now I just trust my eyes. :)

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You need to use the cap first of all to ensure that the primary mirror is aligned under the focusser tube and in the correct position. Initial collamation is best done using a cap/cheshire. I would only use the laser after that. The laser can appear to have the scope collimated whereas in fact it might not be. Check out Astrobaby's guide to collimation.

Simon

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You need to use the cap first of all to ensure that the primary mirror is aligned under the focusser tube and in the correct position. Initial collamation is best done using a cap/cheshire. I would only use the laser after that. The laser can appear to have the scope collimated whereas in fact it might not be. Check out Astrobaby's guide to collimation.

I think you mean the "secondary" mirror. :) Also, it would be easier to do this with a sight-tube than with a Cheshire, because the sight-tube better frames the secondary. Assuming the secondary is rounded and centred (which it really should be--that step doesn't need doing at all often) then there's no reason to do an initial alignment with a Cheshire. Just go ahead and do the whole thing with a laser if you have one. A laser is easier to read than a sight-tube for the secondary tilt alignment. A barlowed laser and a cheshire are equally accurate and equally easy to read (providing it's a good Cheshire) for the primary tilt. The laser being mis-collimated is not a worry for the barlowed laser approach. You can easily and instantly check the collimation of the laser rotating it in the focuser. If the beam doesn't move with respect to the centre spot then your laser is aligned just fine. Yes, a mis-collimated laser is a problem but it only takes 2 seconds to check that it's ok. If you do this check then you don't have to worry about the laser providing misleading information.

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once you get used to a cheshire, this can be used for all of the aspects of collimation. you sometimes need to use an extension on the cheshire to get it far enough out to see the whole secondary but that's easy enough.

'my' steps to cheshire collimation are as follows - you may need to look in more detail at 1) and 2) if imaging. to do a full collimation you cannot do the next step until the previous one is confirmed:

1) assume the focuser is square to the tube (it usually is and if not by a degree or so this can be adjusted with the secondary tilt - see below)

2) check that the spider vanes opposite each other are the same length - ideally they should all be the same length but in my tubes the tube is not round so the ones opposite each other are the same but not the same as the other pair. this should need checking only once when you buy the scope unless you change the shape of the tube with rough handling.

3) ensure the secondary (not what is reflected in the secondary) is central to the drawtube (i.e. the bottom of the Cheshire) and round. this checks that the secondary is far enough down the tube and rotated in the right direction. movement up and down the tube is via the centre bolt which also needs to be reasonably loose when turning it if required. if the secondary is correct up and down the tube but not across the tube, you may need to adjust the spider vanes a little (which should always be at 90 degrees to each other), or it may be that you have overly tilted your secondary in previous attempts to collimate?). this needs to be checked every now and again but may never need adjustment.

4) then use the cheshire to get the fuzzy cross hairs at the bottom centred on the reflected ring on the primary. this is your secondary tilt. check this each time you observe but will rarely move.

5) then use the cheshire to centre the reflected black dot (this is the hole you are looking down) in the donut. aim for a slim even ring of white around the dot and inside the donut. this needs checking a slight tweak every time you observe.

this method ensures you are collimated. I don't like star tests personally but they can be useful if you have good skies and are accurate in centering e.g. polaris in the field of an eyepiece. this latter point is critical.

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I basically do what Moonshane describes. I do steps 4 and 5 whenever I observe. Steps 2 and 3 are generally "set and forget" so I only do these rarely. In fact, on my Dobs the spider vane length is virtually non-adjustable anyway.

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So to test this theory I popped in my collimation cap and to my horror nothing looked lined up right. The secondary mirror looked "ok" but I could not see all of my primary mirror, I only saw two of the three mirror clips. So I collimated everything according to the cap. Collimating looked well, everyhing was in concentric circles and appeared to be great. I pop the laser back in and like some grim nightmare it was back to about an inch off. I dont understand why.

What you have described is a common issue reported by many who start collimating with both a laser collimator and a collimation cap. Think about it: The laser beam hits a tiny area on the secondary mirror and does not interact with the secondary mirror edge. Why would you expect a single laser beam to center/round the secondary mirror under the focuser?

The secondary mirror alignment consists of two completely different and independent alignments. This is the cause of confusion. The single laser beam takes care of only a single alignment – not both.

Read the last post on the following thread

http://www.cloudynig.../o/all/fpart/21

In case the above thread is inaccessible, I will copy its content into this post:

First, the following procedure is based on a laser collimator and a collimation cap combination which is common with beginners. If you own a sight-tube/cheshire combo tool then the following procedure will not apply.

In addition to the collimation cap that comes with many mass produced reflectors, many beginners purchase a laser collimator and follow the proper steps to find out they can’t see all the primary mirror clips via the secondary mirror. By proper steps I meant aligning the secondary mirror by redirecting the laser beam to the primary center then aligning the primary mirror by redirecting the laser beam back to its source. When beginners run into this issue they wonder if the problem is with the quality of their laser collimators. Then they realign the secondary mirror using the collimation cap to bring all of the primary clips to view. Now the laser beam no longer hits the primary mirror center. At this point frustration builds and no matter what they do, they just can’t reconcile between the collimation cap and the laser collimator.

In the above scenario, the straight laser beam only interacts with a tiny portion of the secondary mirror surface. It does not interact with the secondary mirror perimeter; therefore, how could a laser collimator which does not interact with the secondary mirror perimeter is expected to be capable of aligning the secondary mirror under the focuser!!!

Here is the catch. There are two alignments for the secondary mirror. Let me repeat this important statement: There are two different and independent alignments for the secondary mirror. The first is somewhat “coarse” which positions the secondary mirror under the focuser. The second is more “fine” which redirects the focuser axis (laser beam) to the primary mirror center. The first alignment is responsible for optimizing illumination whereas the second is meant to eliminate focal plane tilt. In layman’s terms, the first is responsible for optimally positioning the secondary mirror with respect to the star light cone reflection off the primary mirror and the second ensures the eyepiece lens and the primary mirror focal plane are parallel. Both alignments are independent. That is, you can align one without the other.

A straight laser beam can be used for the second alignment – not the first. This is the main source of confusion. Following the laser collimator proper steps will ensure the eyepiece lens and the primary mirror focal plane are parallel but it will not ensure the secondary mirror centered under the focuser which means it will not guarantee all primary mirror clips can be seen.

Here is how you can reconcile between a good laser collimator and a collimation cap:

1- Complete the alignment of both the secondary and the primary mirrors using the laser collimator following the proper steps. Completing this step will ensure the primary mirror reflection is centered under the focuser. Let me repeat: Completing this step will ensure the primary mirror reflection is centered under the focuser. Now you can use the primary mirror reflection as your reference to optimally position the secondary mirror under the focuser. You do not need to reference the focuser edge.

2- Insert the collimation cap and ensure the collimation cap pupil’s reflection is aligned with the primary mirror center spot. It should. If it is not then you have a more fundamental problem. Now your eye axis is aligned with the focuser axis. Maintain your eye position and evaluate how much of the primary mirror reflection you can see.

3- Think of the secondary mirror as a window to the primary mirror reflection. As I stated above, the primary mirror reflection is centered under the focuser after completing the proper laser collimation steps. Now figure out how best to move the window “secondary mirror” to bring the whole primary mirror reflection to view. Check the attached animations. In each frame, both secondary/primary mirrors were aligned with a laser collimator following proper steps. See how the primary mirror reflection remains still and centered regardless of the secondary movement.

4- To move the secondary mirror, you can use one or both of the following movements:

a. Use the center bolt to move the secondary up or down the OTA

b. You can rotate the secondary mirror

5- IMPORTANT: As you make the movements as outlined in step 4, ignore the primary mirror reflection. For example, if you decide to move the window “secondary mirror” closer to the primary mirror via the center bolt, make the movement without looking at any reflections. When you are done, go back to step 1. Do not evaluate the secondary position until you have completed the laser collimation steps since only completing the laser collimation steps will ensure the primary mirror reflection is centered under the focuser.

6- Repeat steps 1 to 5 until both the laser collimator and the collimation cap reconcile.

Note: Click on attachments to see animation

post-5330-0-48946500-1359908254_thumb.gi

post-17988-13387744378_thumb.gif

Jason

Edited by Jason D
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Another common issue when comparing laser collimator to collimation cap is the misalignment of the eye axis with the focuser axis. What might seem aligned by looking through the collimation cap will not agree with a quality laser collimator. That is because users will have the tendency to move their eye to see all primary clips.

Jason

post-17988-13387753774_thumb.jpg

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incidentally, I think your secondary needs to go up the tube a bit in the second animation :grin:

Thanks for catching that error. I uploaded the wrong GIF file. I just edited my post and uploaded the correct one.

Jason

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