Collimation issue

[AlanK]

Hi folks

Help required re collimating my 12” dob please.

I have used a collimated laser collimator for years, and the scope appeared to be well collimated (the laser reflects back to its source).  But I have recently acquired a collimator cap and expected the view through it to show perfectly concentric circles. As the image through the cap shows this is far from the case…

I am at a loss to know what adjustments are required, but I would guess the secondary is out of position.  However the laser hits the centre of the primary and I am feeling hard of adjusting it as any change in angle will mean the laser will be well out of line with the centre of the primary.

Any thoughts would be appreciated (other than the primary mirror needs a good cleaning)

Thanks

Alan

[Sunshine]

Lasers can be out of alignment themselves, i found the same issue when fiddling with a laser and my lightbridge, a laser worked well for a rough alignment.

I found that using the collimation cap before a session, then i only needed a very minor adjustment in the field using a star test which is always the ultimate test of collimation.

[Buzzard75]

Looks like the secondary might be out of alignment with the focuser. That's not something you can check with a laser and has to be done visually with a collimation cap, Cheshire or similar. I don't have the link handy for Astrobaby's guide, but I'm sure someone will be along shortly with it.

[N3ptune]

http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

 

[wookie1965]

I used astrobaby`s and this to do my 200p.

http://www.schlatter.org/Dad/Astronomy/collimate.htm

[Tomatobro]

 

This is what it should look like (from AB's collimation guide)

[AlanK]

Thanks for the input.  I'm sure the laser is correctly collimated so presumably the height of the secondary might be the problem. As near as I can tell the secondary is centred in the tube. I'll allocate a half day sometime to try and get sorted.

im happy enough with the view of DSOs, doubles etc. But I have always been disappointed with planets. From what I read, short focal length light buckets require accurate collomation for decent views of planets.

[Jason D]

Hello Alan,

Imagine yourself standing in front of a wall-mirror. Imagine you are wearing a pair of special glasses that emit a laser beam. Imagine you position your head to directly look at your reflection with the laser beam emitted from your glasses reflecting back to its source. But something does not look rightt!!!! Your reflection does not seem to be centered with respect to the mirror frame. Why!!!! Because the laser beam does not interact with the mirror frame to tell you whether the mirror frame is centered with respect to the laser or not. Your friend comes along and slides the wall-mirror until your reflection is centered. While your friend is moving the mirror, the laser beam continues to be reflected back to its source. 

The moral of the above example is that "conventional" laser collimators can't be used to center the secondary mirror under the focuser because they do not interact with the secondary mirror edge. There are endless positions of the secondary mirror that will allow the laser beam hit the primary mirror center and retraces its path to the source but only in one position the secondary mirror will appear centered.  A sight-tube is the best tool to use to center the secondary mirror.

Do not get me wrong.  I am not implying that the laser collimators are useless. On the contrary, they are good tools  to use for collimation. Let e clarify, Collimation consists of three independent alignments:

1- Positioning the secondary mirror for optimal field illumination -- achieved when the secondary mirror appear centered under the focuser

2- Fine adjusting the secondary to eliminate focal planar tilt -- achieved when the laser beam hits the primary mirror center

3- Adjusting the primary mirror to eliminate coma -- achieved then the laser beam retraces its path back to the source

"Conventional" laser collimators do a great job with #2 and #3 alignments but a poor job with #1 alignment. Interestingly, #1 alignment is the least important.

Looking at your photo, it is clear that the primary mirror reflection is not aligned with the secondary mirror edge. If the photo was taken after completing alignment with the laser collimator then it is clear that the secondary mirror is not well-positioned under the focuser -- it is not centered/rounded under the focuser.

Jason

[Moonshane]

Great analogy Jason.

[AlanK]

Thanks Jason. So my understanding is that the mirror frame needs to be moved, but not the plane/angle of the mirror? Otherwise the beam from the laser glasses will be reflected away from the target.. So I will try moving the secondary up or down and see what happens. Weather forecast for today is only poor so will give it a go later.

Alan

[Jason D]

Hello Alan,

Refer to the following old post. It includes suggested steps to help you with your alignment:

 

Once you complete the proper laser collimator steps (laser hits primary center then retraces its path back to the source), you are guaranteed to have the primary mirror reflection centered with respect to the focuser edge -- no guarantees with respect to the secondary mirror edge . Think of the secondary mirror as a window to the primary mirror reflection (or the frame of the wall-mirror I mentioned in my last example). Your job is to move the secondary mirror (the wall-mirror frame) in such a way until it is centered with the primary mirror reflections. The proper steps are in the post I referenced.

Bear in mind that once proper collimation is met including a well-centered/rounded secondary mirror under the focuer, the secondary mirror silhouette (aka secondary mirror shadow) will appear shifted towards the primary mirror. In addition, ignore the reflections of your spider vanes. The vertical ones will appear slightly shifted towards the primary mirror. Here is a photo of what you should see (I took this one for my well-collimated scope)

Jason

 

 

 

 

[AlanK]

Well I spent most of the morning trying to collimate using the cap.

The initial lining up of the secondary ended like this -

 

And the final collimation of the primary looks like that (sorry about the state of the primary)

I dont know why the focusser tube will not line up central but as it is all reflected back on the secondary I assume there will be no light loss. (maybe the tube is not quite square or the primary mirror is not exactly 45 degrees - I dont know).

IN any case I will try a star test the next time a clear night appears. It seems the fine weather has now broken and we are back to normal - cloud rain and fog)

Thanks for all the advice.  Clear skies to you all whereever you are

Alan

[Jason D]

Collimation looks good.

To further perfect positioning the secondary mirror, I would give it a slight (and I mean a slight) counter-clock twist looking down the tube then recollimate only with the laser collimator. If this step will cause some headache then leave collimation as is and enjoy the views after star test confirmation. Again, the step I mentioned in this paragraph is a nice-to-have step -- not a required-to-have step.

Jason

[AlanK]

Hi Jason

just been out and tried a star test. Seeing is very poor with a lot of high level cloud around, but think the alignment is pretty good - dark spot looks central in the blurry moving image.  It will be good enough for my purposes I think (visual obs only). Probably need to get my specs upgraded now!

Thanks for your assistance

Alan