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Confused by laser collimator - beam is not centered on the secondary


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Hello SGL!

After a long break, I'm back on SGL. The monsoons have started retreating after several months of uninterrupted cloudy skies, and it's starting to get clear. I recently acquired a Sky-Watcher 8 inch collapsible dobsonian, and a GSO laser collimator. I collimated the scope with the laser collimator, and mostly, it seems to be fine. The laser beam hits the center of the center spot on the primary, and the return beam hits the exact center of the collimator, as expected. I tried observing a few objects, and the views are extremely sharp, and the telescope performs very well. 

However, I noticed that the laser beam doesn't hit the center of the secondary mirror. It seems to be offset quite a bit. I tried adjusting the tilt and rotation of the secondary, and also tried ajusting the spider vanes (as much as the adjusting screws allow), but this doesn't seem to help much. The only possibility I see is that the focuser itself is skewed, and is not perpendicular to the primary's optical axis. To repeat, the collimation seems spot on, and the views are extremely good, but this offset is bothering me. 

post-42486-0-01100600-1446274898_thumb.j

I would like to understand if this is normal, and if it'll affect the telescope performance in any way. ( I'm very happy with the performance right now, but it's kinda bothering me that the laser is not at the center spot on  :p )

PS: Rotating the collimator in the focuser doesn't change the position of this beam. It seems to be always at this offset spot, no matter how the collimator is rotated.

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Is there any play in the collimator as it sits in the focuser? In other words, can you wobble it?

There's a small amount of play only when the fastening screws are completely loose, and in spite of that, the laser spot doesn't move much from that position.

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Unfortunately, no. This is the only collimation tool I have. My friend has a Howie Glatter. I'll try to get access to that the next time I meet him. Maybe I can verify with that once. I'll also try to get my hands on a Cheshire, though I haven't seen any for sale in India, and imported ones tend to be quite expensive! Thank you!

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Overall then, I would just enjoy the good images you get.  The worst that can be happening if you are getting good images is you may be losing a little of the light from the primary, as the secondary is a flat surface it will reflect the same quality from most of its area.

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Overall then, I would just enjoy the good images you get.  The worst that can be happening if you are getting good images is you may be losing a little of the light from the primary, as the secondary is a flat surface it will reflect the same quality from most of its area.

Well, I guess that's what I'll do then, until I find some elusive DSO that could profit from the little extra light that might be getting lost. Thanks a lot for your help, Simon!  :smiley:

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My pleasure.  I am sure though that others will be far more helpful than me...once they climb out of their pits, especially if they enjoyed good weather last night.

Haha!  :grin: I'm still waiting for reasonably good weather. The forecast indicates that it might be a month away!  :(

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From your photograph, it's impossible to know what, if anything, is wrong. The reason being that parallax-error is also a factor as we don't know where the camera was located when it took the picture you posted. If you're getting nice and sharp views through the telescope, I would not be concerned and just do a star-test to finish checking the collimation. A star-test is the real and true test of collimation.

As for offset problems turning out not to be a problem, read Astro_Baby's collimation-guide. It has offset well covered in it's images used to illustrate the guide. Here's a link:

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

Glad your Monsoon-Season is finished,

Dave

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To start out I would want to make sure the laser beam is collimated to the axis of the laser collimator. Many aren't!!! This can be done by making a V block in which to place and rotate the collimator. There are usually (well hidden) adjustment screws on collimators which allow you to adjust the laser's direction so that as the collimator is rotated the beam remains stationary on a distant point. Nevertheless the fact that your laser beam remains stationary on the secondary mirror when rotated suggests the laser is at least roughly collimated to itself.

Secondly I would want to make sure the secondary was central in the tube. Astrobaby explains how to do that by measuring and adjusting the spider in the link provided above.

I think if I still had the problem you have I might suspect the focusing tube's axis wasn't aligned to the central optical axis of the telescope. However I strongly recommend you wait for comments from more experienced collimators before adjusting focusing tube alignment. There are methods for doing this, including carefully measuring and marking the telescope tube directly opposite the focuser. I must say I have never done this. But as I say the more experienced here could give better advice than I can.

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From your photograph, it's impossible to know what, if anything, is wrong. The reason being that parallax-error is also a factor as we don't know where the camera was located when it took the picture you posted. If you're getting nice and sharp views through the telescope, I would not be concerned and just do a star-test to finish checking the collimation. A star-test is the real and true test of collimation.

As for offset problems turning out not to be a problem, read Astro_Baby's collimation-guide. It has offset well covered in it's images used to illustrate the guide. Here's a link:

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

Glad your Monsoon-Season is finished,

Dave

Thank you for that link, Dave! Yes, I'll try a star test once it gets clear. It's still mostly cloudy these days.

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From my over half a century "experience", if you are happy with the performance then leave well alone. OCD about such things is not the way to enjoy astronomy.  :smiley:

OCD - obsessive collimation disorder. ;-)

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To start out I would want to make sure the laser beam is collimated to the axis of the laser collimator. Many aren't!!! This can be done by making a V block in which to place and rotate the collimator. There are usually (well hidden) adjustment screws on collimators which allow you to adjust the laser's direction so that as the collimator is rotated the beam remains stationary on a distant point. Nevertheless the fact that your laser beam remains stationary on the secondary mirror when rotated suggests the laser is at least roughly collimated to itself.

Secondly I would want to make sure the secondary was central in the tube. Astrobaby explains how to do that by measuring and adjusting the spider in the link provided above.

I think if I still had the problem you have I might suspect the focusing tube's axis wasn't aligned to the central optical axis of the telescope. However I strongly recommend you wait for comments from more experienced collimators before adjusting focusing tube alignment. There are methods for doing this, including carefully measuring and marking the telescope tube directly opposite the focuser. I must say I have never done this. But as I say the more experienced here could give better advice than I can.

Yes, I saw that Astrobaby's collimation guide and measured the spider vanes. The sceondary appears to be centered well. I now strongly suspect that the focuser axis is not aligned. And as you suggested, I think I'll skip fiddling around it for now. I know a few more experienced astronomers in my local astronomy club. Maybe I'll ask one of them to help me with it the next time we meet. I'm a newbie, and I'm not confident enough to mess it with it right now :)

Thanks!

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From my over half a century "experience", if you are happy with the performance then leave well alone. OCD about such things is not the way to enjoy astronomy.  :smiley:

Thank you, Peter! I spent a few hours trying to collimate it unsuccessfully today, and I've started getting a back pain. I think I'll follow your advice and just enjoy the views for now :) Maybe some experienced folks from my local astronomy club can help me out at one of the star parties. 

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"However, I noticed that the laser beam doesn't hit the center of the secondary mirror .  ."

  It's suprising that no one has pointed out that in a properly collimated Newttonian, where the secondary is optically positioned on the focuser axis (the so called "offset" towards the primary), the focuser axis and the laser beam wiill not inrtersect the secondary mirror at its geometric center, but rather, skyward of the center.

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Yes. By 'skyward' I assume you mean to the left of centre of the secondary when looked at through the focuser. But it should lie on the centre line of the secondary. On the image above the spot appears to be somewhat above that centre line.

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Ouroboros said:  "By 'skyward' I assume you mean to the left of centre of the secondary when looked at through the focuser.  

But it should lie on the centre line of the secondary. On the image above the spot appears to be somewhat above that centre line." 

   In the picture, left is towards the open end of the telescope and towards the sky, so that is what I meant.  It looks like the outgoing and incoming laser beams are hitting the secondary just above (in the picture) the major axis of the secondary. Ideally, the beam impact should be on the major axis. They are hitting skyward of the minor axis, as they should. 

 It looks like the center of the secondary has been marked. That's not necessary or advisable, in my opinion.

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