Author Topic: Collimation/star test question  (Read 125 times)

John Jefner

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Collimation/star test question
« on: January 09, 2018, 01:40:36 PM »
I recently got my first Newtonian, an Orion XT12g and I had it out for the second time a few nights ago. I did a brief star test last night and I'm not sure what to make of what I saw. Im a total noob when it comes to collimation so I was hoping someone could give me some input.

Temps where dropping fast all night and seeing was only ok, so this was more of a seeing/collimation test than a test of the optics. I collimted the scope using a Glatter laser and tublug (and a parallizer) and to my untrained eyes it looked about perfect.
When I pointed at Arcturus 160 and 230x I was able to focus it to a relitivly fine point but not as fine as what I saw the first time I had the scope out out. I noticed some hazy spikes protruding out from the point. Inside focus the "donut" looked quite unsymmetrical/lopsided with the secondary shadow off to one side. I beleive this is what you see when a scope is improperly collimated? The thing that threw me off was that outside focus the "donut" looked fairly symmetrical (I could see the primary mirror clips, is this normal?).
Does this sound like something was wrong? A couple things that came to mind where focuser alignment or maybe just tube currents? Any thoughts?

This is what I'm seeing in my tublug. All the off center rings kind of through me off but if I'm understanding correctly you all I need to look at when using the tublug is the reflection of the center marking donut on the primary. Does this look right?

Thanks!Attached Thumbnails




bardeperdi

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Re: Collimation/star test question
« Reply #1 on: January 10, 2018, 07:21:12 AM »
Quote
I recently got my first Newtonian, an Orion XT12g and I had it out for the second time a few nights ago. I did a brief star test last night and I'm not sure what to make of what I saw. Im a total noob when it comes to collimation so I was hoping someone could give me some input.

Temps where dropping fast all night and seeing was only ok, so this was more of a seeing/collimation test than a test of the optics. I collimted the scope using a Glatter laser and tublug (and a parallizer) and to my untrained eyes it looked about perfect.
When I pointed at Arcturus 160 and 230x I was able to focus it to a relitivly fine point but not as fine as what I saw the first time I had the scope out out. I noticed some hazy spikes protruding out from the point. Inside focus the "donut" looked quite unsymmetrical/lopsided with the secondary shadow off to one side. I beleive this is what you see when a scope is improperly collimated? The thing that threw me off was that outside focus the "donut" looked fairly symmetrical (I could see the primary mirror clips, is this normal?).
Does this sound like something was wrong? A couple things that came to mind where focuser alignment or maybe just tube currents? Any thoughts?

This is what I'm seeing in my tublug. All the off center rings kind of through me off but if I'm understanding correctly you all I need to look at when using the tublug is the reflection of the center marking donut on the primary. Does this look right?

Thanks!

Looks miscollimated to me. I think it may be an issue with the tilt of your secondary.

I don't use lasers because I can never be sure that they are orthogonal due to potential focuser fitment issues and the like. Instead I use a sight tube and a Cheshire.

Using the sight tube, I adjust the tilt of the secondary until the reflection of the edge of the primary is centered in the sight tube. That is all three retainers are visible at the edge of the mirror reflection.

Using the Cheshire I switch to adjusting the tilt of the primary centering the primary center spot in the Cheshire's ring.

The collimation won't be perfect but it will be much closer than what I see depicted on your laser, I think. Orion provides a collimation cap which is similar to a Cheshire. When you put it in the eyepiece holder and look at your primary, is the center spot of the primary centered in the collimation cap's donut reflection? That would be a quick way to tell whether your collimation is badly off maybe.

Regards,

Jim

Ivan Deane

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Re: Collimation/star test question
« Reply #2 on: January 10, 2018, 04:12:20 PM »
If all you used was the Glatter laser and TuBlug, the secondary mirror placement is undefined as yet.
The simple thin beam Glatter laser lets you assess and correct the focuser axial alignment (laser dot is aligned/centered in the primary mirror center spot/donut). You usually make any corrections by tilting the secondary mirror.
The TuBlug Barlows the laser to let you assess and correct the primary mirror alignment (the silhouette of the primary mirror center spot is aligned/centered on the TuBlug laser aperture). You usually make any corrections by tilting the primary mirror.

Neither of these alignments address the secondary mirror placement (and it can be way out of alignment and the two axial alignments can still be corrected). You need to use either a combination Cheshire/sight tube collimating tool or a collimation cap to assess the secondary mirror placement.

Your attached image appears to be showing the laser diffraction pattern. When you use the TuBlug, you should remove the 1mm aperture stop...

Brian Ross

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Re: Collimation/star test question
« Reply #3 on: January 11, 2018, 11:13:20 AM »
Quote
Quote

I recently got my first Newtonian, an Orion XT12g and I had it out for the second time a few nights ago. I did a brief star test last night and I'm not sure what to make of what I saw. Im a total noob when it comes to collimation so I was hoping someone could give me some input.

Temps where dropping fast all night and seeing was only ok, so this was more of a seeing/collimation test than a test of the optics. I collimted the scope using a Glatter laser and tublug (and a parallizer) and to my untrained eyes it looked about perfect.
When I pointed at Arcturus 160 and 230x I was able to focus it to a relitivly fine point but not as fine as what I saw the first time I had the scope out out. I noticed some hazy spikes protruding out from the point. Inside focus the "donut" looked quite unsymmetrical/lopsided with the secondary shadow off to one side. I beleive this is what you see when a scope is improperly collimated? The thing that threw me off was that outside focus the "donut" looked fairly symmetrical (I could see the primary mirror clips, is this normal?).
Does this sound like something was wrong? A couple things that came to mind where focuser alignment or maybe just tube currents? Any thoughts?

This is what I'm seeing in my tublug. All the off center rings kind of through me off but if I'm understanding correctly you all I need to look at when using the tublug is the reflection of the center marking donut on the primary. Does this look right?

Thanks!

Looks miscollimated to me. I think it may be an issue with the tilt of your secondary.

I don't use lasers because I can never be sure that they are orthogonal due to potential focuser fitment issues and the like. Instead I use a sight tube and a Cheshire.

Using the sight tube, I adjust the tilt of the secondary until the reflection of the edge of the primary is centered in the sight tube. That is all three retainers are visible at the edge of the mirror reflection.

Using the Cheshire I switch to adjusting the tilt of the primary centering the primary center spot in the Cheshire's ring.

The collimation won't be perfect but it will be much closer than what I see depicted on your laser, I think. Orion provides a collimation cap which is similar to a Cheshire. When you put it in the eyepiece holder and look at your primary, is the center spot of the primary centered in the collimation cap's donut reflection? That would be a quick way to tell whether your collimation is badly off maybe.

Regards,

Jim
I snapped that pic before I had the primary perfectly aligned (although it was close), is that what your referring to or something beyond that?

The first time I used the laser collimator I did double check with the stock collimation cap and everything looked right to me at the time. My primary moved a little bit since and I didn't double check the laser collimators results with the collimation cap this time so its possible something is off with the secondary.

Cameron Artist

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Re: Collimation/star test question
« Reply #4 on: January 11, 2018, 03:10:00 PM »
Quote
...Using the sight tube, I adjust the tilt of the secondary until the reflection of the edge of the primary is centered in the sight tube. That is all three retainers are visible at the edge of the mirror reflection.

I hope that when you adjust the tilt of the secondary, you also confirm that the primary mirror center spot is aligned with the intersection of the sight tube crosshairs. Once this alignment is correct, then you can assess (and correct) the secondary mirror placement using the primary mirror clips as a reference. I usually suggest secondary mirror tilt for the cross hair alignment, and secondary rotation and/or position closer to or farther from the primary mirror to correct the secondary mirror placement (what is often referred to as "centering" the secondary mirror under the focuser).

I only mention this because it's not uncommon to lose visibility (partially or completely) of one or two mirror clips when the tilt adjustment is made using the cross hair/center spot alignment. If one or more of the clips is only partially visible when the cross hair/center spot alignment is correct, at least the axial alignment is correct.

Marcus Kucrud

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Re: Collimation/star test question
« Reply #5 on: January 13, 2018, 01:27:07 AM »
Do you have a catseye XLS?  Before using the laser and tublug did you adjust the secondary ( and perhaps primary axial location) to get the concentric images : google collimation newtonian Don Pensak cloudynights.

I have a hot spot and use the glatter laser and tublug.  My dark hot spot image is centered over the tublug hole and I do not see any multiple dark offset images. However, I have noticed and still see a very, very faint secondary image of the hot spot which is about 1 cm away from the primary dark image. I assume its from some secondary or tertiary reflections due to some minor axial offset of something.

My star images are sharp and my infocus and outoffocus image of bright stars shows many round centered concentric rings.

I see you're corresponding with Vic.  He is one of the most helpful guru's on this site.

Philip Price

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Re: Collimation/star test question
« Reply #6 on: January 14, 2018, 04:52:16 AM »
Quote
If all you used was the Glatter laser and TuBlug, the secondary mirror placement is undefined as yet.
The simple thin beam Glatter laser lets you assess and correct the focuser axial alignment (laser dot is aligned/centered in the primary mirror center spot/donut). You usually make any corrections by tilting the secondary mirror.
The TuBlug Barlows the laser to let you assess and correct the primary mirror alignment (the silhouette of the primary mirror center spot is aligned/centered on the TuBlug laser aperture). You usually make any corrections by tilting the primary mirror.

Neither of these alignments address the secondary mirror placement (and it can be way out of alignment and the two axial alignments can still be corrected). You need to use either a combination Cheshire/sight tube collimating tool or a collimation cap to assess the secondary mirror placement.

Your attached image appears to be showing the laser diffraction pattern. When you use the TuBlug, you should remove the 1mm aperture stop...

Just to make sure I'm following you, when aligning the secondary, I should use the 1mm aperture stop with the laser right in the focuser, but when aligning the primary I shouldn't use the aperture stop and I should have the Tublug between the laser and the focuser? This would explain a lot because I have been using the aperture stop in both cases.

So there can be cases where something can be out of alignment but the laser and tublug combo won't show it? To check this I use the collimation cap to make sure the secondary is center under the focuser draw tube? Is that all?

Thanks

anstirabnas

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Re: Collimation/star test question
« Reply #7 on: January 15, 2018, 03:04:36 PM »
Quote
Just to make sure I'm following you, when aligning the secondary, I should use the 1mm aperture stop with the laser right in the focuser, but when aligning the primary I shouldn't use the aperture stop and I should have the Tublug between the laser and the focuser? This would explain a lot because I have been using the aperture stop in both cases.

Are there cases where something can be out of alignment but the laser and tublug combo won't show it? If so what?

Correct--do NOT use the 1mm aperture stop when you are using the TuBlug.

The laser and TuBlug ONLY show the axial alignments (focuser axis pointed correctly toward primary mirror center spot, primary mirror axis pointed correctly toward the center of the focuser). Although the secondary mirror is typically used to correct the focuser's aim (using the simple thin beam laser), the secondary mirror placement is assessed and corrected relative to the edges of three circles: the bottom edge of the focuser drawtube (or if you have one, the bottom edge of the sight tube), the actual edge of the secondary mirror, and the reflected edge of the primary mirror. When these three circles are concentric, your secondary mirror placement is correct.

You can use your simple thin beam laser AND your collimation cap to sort out the secondary mirror alignments. The process is explained here:https://www.cloudyni...ment/?p=5260727

Jeff Swan

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Re: Collimation/star test question
« Reply #8 on: January 15, 2018, 10:17:02 PM »
NIckwin,

I have to confess that I leave the aperture stop on when I use the Tublug. The only real downside is that I think it can contact the barlow lense. Just don't push it down hard and crank on it. The 2 set screws and the tight tolerances of the Glatter equipment should properly register the laser in the Tublug even if it isn't pushed down all the way. And the beauty of the barlow technique is that it doesn't matter!

I really like the variable brightness option on the laser because it makes it much easier to see the laser dot in the center spot when you are aligning the secondary (using the laser). I usually don't even use any other tools.

Mike R.

Jermaine Conner

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Re: Collimation/star test question
« Reply #9 on: January 17, 2018, 01:36:46 AM »
If you look at the image Nickwin included in his opening post, you will see the issue that can arise if the 1mm aperture stop is left attached when using the TuBlug. The diffraction pattern in this case is meaningless, and removing the 1mm aperture stop will remove the unwanted pattern. The Barlow in the TuBlug is used to spread the laser sufficiently to cover the primary mirror center spot (and to move the "virtual" emitter to a point outside the OTA near the focal plane). Small misalignments between the laser and the Barlow are not an issue because of the inherent insensitivity of the Barlowed laser to these kinds of errors. Unfortunately, the diffraction pattern created by the 1mm aperture stop can end up embedded in the Barlowed laser beam, revealing this misalignment. This is the reason Howie Glatter suggests removing the 1mm aperture stop when using the TuBlug (or the Blug). (FWIW, I've seen a few TuBlugs that project a perfectly aligned diffraction pattern--but Nickwin's pattern is more common.)

FWIW, I normally use my Glatter with the 1mm aperture stop (no TuBlug or Blug) to facilitate aligning the secondary mirror tilt. My primary mirror has a perforated CatsEye triangular center spot. The silhouette of the triangle is easily seen superimposed on the diffraction pattern under the focuser. By adjusting the secondary mirror tilt, I can move the pattern into alignment with the silhouette, and I can keep my reading glasses on for the entire process without leaving the front of the scope. When the pattern and the silhouette are correctly aligned, I can move to the back of the scope and adjust the primary mirror tilt to move the pattern and the silhouette into alignment with the 1mm aperture stop--just like using a Barlowed laser, except without the Barlow!

ardrivunla

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Re: Collimation/star test question
« Reply #10 on: January 18, 2018, 11:11:49 AM »
I was going to say that picture doesn't show us enough to know about secondary positioning. And yes, ignore those diffraction rings. Perhaps the OP could post a picture looking down into the focuser?

Richard Reed

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Re: Collimation/star test question
« Reply #11 on: January 21, 2018, 01:01:26 AM »
Quote
Quote

...Using the sight tube, I adjust the tilt of the secondary until the reflection of the edge of the primary is centered in the sight tube. That is all three retainers are visible at the edge of the mirror reflection.

I usually suggest secondary mirror tilt for the cross hair alignment, and secondary rotation and/or position closer to or farther from the primary mirror to correct the secondary mirror placement (what is often referred to as "centering" the secondary mirror under the focuser).

I have question if you don't mind. If all three circles appear concentric and crosshairs of telecat intersect with center of hotspot, the only discrepency being as I raise the telecat, the primary clip that coincides with the focuser disappears before the other two. What might you deduct from that? Not enough offset, or am I wrong? I sometimes get confused on which direction to move the secondary if one of the clips is less prominent than the others and wind up chasing the positioning, if that makes sense.

canreosenbi

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Re: Collimation/star test question
« Reply #12 on: January 22, 2018, 10:27:10 PM »
Actually, it could be that the TeleCat pupil isn't correctly located for the three circle alignment. This is more apparent with imaging Newtonians (that typically have larger secondary mirrors). You might find this information helpful: http://www.vicmenard...rspectives.html scroll down to, Notes on matching a sight tube to your 'scope's focal length.

(I typically use a slightly shorter focal ratio sight tube and a slightly closer pupil to create easily visible steps between the three circles. If the steps are too wide, the offset will be incorrect, and the mirror clips won't disappear simultaneously. That said, if the secondary mirror is properly sized (translate, not too small), the slightly offset illumination profile will be undetectable in the eyepiece.)

Darkz Tousa

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Re: Collimation/star test question
« Reply #13 on: January 31, 2018, 05:31:50 AM »
Thanks Vic. I'll check that out.

reapriavoland

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Re: Collimation/star test question
« Reply #14 on: January 31, 2018, 12:38:04 PM »
Thanks for all the input guys! Extremely helpful. So, lots to try next time I'm in front of the scope.

A) Make sure secondary is centered under the focuser and looks perfectly round. It sounds like this is possible with just a collimation cap but more accurate with a sight tube? These things are pretty expensive, how much difference does it make at the EP?

B) Dim the laser for better visibility when adjusting secondary tilt.

C) Remove aperture mask from laser when adjusting primary.

D) Ensure that alignment holds whether scope is pointed horizontally or at zenith.

I'm curious about "autocollimators", how do these fit with the above? Again, they are quite expensive, how much difference does it make at the EP in a 12" f/4.9?

Getting back to my OP, does any of this explain what I was seeing in the star test? Basically with a defocused star image on one side of focus there appeared to be a gross misalignment, but on the other it looked fairly good? I saw an image in a Backyard Astronomy article on star testing that showed something similar for tube currents, is it possible that was all that was going on?

I'll post an update when I have a chance to play around with the scope.Thanks again!