Author Topic: Collimation question: What would cause this?  (Read 100 times)

bijstentetal

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Re: Collimation question: What would cause this?
« Reply #15 on: January 25, 2018, 11:46:33 PM »
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OK--first, your second post shows the primary mirror center spot moving. What actually moves is the reflection of the secondary mirror.
I corrected this with an edit. Hope this is now more accurate.

rennlispuring

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Re: Collimation question: What would cause this?
« Reply #16 on: January 26, 2018, 12:23:22 AM »
Getting there...

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cromsotejbi

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Re: Collimation question: What would cause this?
« Reply #17 on: January 30, 2018, 10:01:23 AM »
Thanks Vic, tomorrow I will conquer this beast.

I take it that the aim is to get the yellow and red crosshairs aligned and the PM center dot under it?

pelotwollgar

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Re: Collimation question: What would cause this?
« Reply #18 on: January 30, 2018, 02:02:47 PM »
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I take it that the aim is to get the yellow and red crosshairs aligned and the PM center dot under it?

Actually, yellow, red and blue. When red (primary mirror) is concentric, the center spot (by definition) will be centered!

tirafarpa

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Re: Collimation question: What would cause this?
« Reply #19 on: January 30, 2018, 03:00:00 PM »
I know what my problem is now but I do not know why I have it or how I can correct for it.

If you take the image in #17 you will note that my clips are all relatively where they should be. The PM spot is toward the PM relative to the SM.

As mentioned before, the moment I start correcting for that with the obvious bolt choice on the PM (the one below the top clip) the PM tilts the reflection towards the top and in so doing moves the clip and the mirror edge off in that direction as well. The best way I can describe it is if you view the following image. With everything set up ready for PM adjustments, there is an image plane in which the PM and the three clips reside. You see this in the image at #17. Now when you correct the PM to bring the PM dot up towards the donut, you tilt the whole image towards the top. This results in the top clip now falling outside that illuminated circle. It creates a clearly cut off edge in the view.



I tried to adjust as per the recommendations by rather loosening the other two screws than tighten the one under the disappearing clip. This helps to bring the PM dot closer but I still cannot get it completely centered without that edge dropping off.

I tried various SM heights up and down the tube. This seems to have no impact. I tried centering on the SM dot rather than concentric circles, no impact. Try shimming the focuser, no impact. Every single adjustment results in you getting an almost exact same final view regarding the clipping shown.

What could result in you running out of adjustment space like this? Is it one of the mechanical elements not aligned correctly to start with? Some people suggested that I need to tilt the focuser (like the Moonlight allows). I did this manually on my stock focuser and I could not see the problem resolving.

I am wondering if this could be a side effect of an oversized SM? Or maybe this is what you get when your PM is not 100% centered?

I took the attached 120" single shot of M8. I left the final PM collimation out of the adjustments but ended up with the top just starting to get eclipsed. You can see the effect of this in the stars on the left side of the image. At least I now know what caused my stars in that edge to be so bluntly cut off.

For all my sins I ended up replacing the focuser up side down so tomorrow I will need to fix that. Will give it one more try then to try and get that cutting off sorted. Any suggestions are welcome as always.


Mark Dominguez

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Re: Collimation question: What would cause this?
« Reply #20 on: January 31, 2018, 06:12:11 AM »
I'm wondering what would happen if the primary mirror's center marker were a few millimeters away from the center?
Has that ever been checked?

Micheal Luther

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Re: Collimation question: What would cause this?
« Reply #21 on: January 31, 2018, 12:57:27 PM »
Don, I did check it twice now. Once with the Catseye overlay and today measuring it. Relative to the 200mm mirror it looks right visually.

If that could be a cause then I wonder if the non centering of the mirror would also cause it. That effectively place the center dot off the optical center as well. I measured the one side of my mirror to be slightly less than 0.5mm off compared to the rest. Not sure if that small error can translate into what I am seeing. Maybe over 1000mm it becomes a factor?

Cesar Lawhorn

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Re: Collimation question: What would cause this?
« Reply #22 on: February 02, 2018, 05:48:39 PM »
No, the mirror being off-center would affect its relationship to the corrector, but would be compensated for in collimation.
Especially with such a tiny error. 0.5mm on one side translates to 0.25mm off center. Negligible.

ontoolhaiworl

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Re: Collimation question: What would cause this?
« Reply #23 on: February 03, 2018, 12:30:25 AM »
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.... Now when you correct the PM to bring the PM dot up towards the donut, you tilt the whole image towards the top. This results in the top clip now falling outside that illuminated circle. It creates a clearly cut off edge in the view...

When you correct the primary mirror tilt adjustment, the primary mirror shouldn't move relative to the secondary mirror--the clips and the primary mirror center spot (in light blue circles) should remain fixed and the secondary mirror reflection (inside the green circle) should move into alignment with the primary mirror center spot.

The image in post #15 clearly shows the secondary mirror surface is large enough to contain the entire primary mirror reflection. The only possibility I can imagine is that the front aperture (the cell surrounding the meniscus) is the light "cutoff". If you shine a light down the tube, you should still be able to see the mirror clip(s) in the shadow surrounding the brighter center of the primary mirror.

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Ronnie Walsh

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Re: Collimation question: What would cause this?
« Reply #24 on: February 09, 2018, 05:10:10 AM »
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Quote

.... Now when you correct the PM to bring the PM dot up towards the donut, you tilt the whole image towards the top. This results in the top clip now falling outside that illuminated circle. It creates a clearly cut off edge in the view...

When you correct the primary mirror tilt adjustment, the primary mirror shouldn't move relative to the secondary mirror--the clips and the primary mirror center spot (in light blue circles) should remain fixed and the secondary mirror reflection (inside the green circle) should move into alignment with the primary mirror center spot.

The image in post #15 clearly shows the secondary mirror surface is large enough to contain the entire primary mirror reflection. The only possibility I can imagine is that the front aperture (the cell surrounding the meniscus) is the light "cutoff". If you shine a light down the tube, you should still be able to see the mirror clip(s) in the shadow surrounding the brighter center of the primary mirror.
Let me keep that first paragraph and the image in mind when I work on this again tomorrow. To me, it looked like the blue circles were moving when I adjusted the PM. But I am very confused at this stage so I will have a fresh look again.

I indeed can see the clip that moves out of the bright area in that shadow area you refer to (after adjusting the PM). It sits there around 1mm off the edge of the illuminated circle if I correct for PM alignment.

If the smaller (190mm) meniscus cause a light cut off that is exaggerated on one side, what would be the cause of that? Is the fact that only one side keeps getting cut off maybe indicating that either the PM or the meniscus is out of alignment somewhere? I could not replicate this cut off on any of the other clips so far. Just that top one that keeps coming up.