Author Topic: Lightbridge Collimation help needed!  (Read 263 times)

ransgesislu

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Lightbridge Collimation help needed!
« on: December 28, 2017, 09:55:05 PM »
brand new Lightbridge, 12", 1 week old and yet to see first light.
i have collimation cap, cheshire, laser, tublug
when looking at the secondary, its too far into the tube and about 15% of the secondary is cut off. if i fix that, then the laser is about 2" off of the center circle and can't come to alignment.
if i use the laser and adjust the secondary, i get alignment with the center circle, but then i am back to 15% cut off.

also the collimation cap shows perfect secondary, but the cheshire shows it off alignment and the secondary is cut off.

could my trusses have anything to do with making the tube assemblies square?

i replaced the primary springs and bottomed out the mirror, that is my start position for the primary, but the secondary seems to be the problem.
any ideas?

david



Eric Hayes

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Re: Lightbridge Collimation help needed!
« Reply #1 on: December 30, 2017, 06:16:17 AM »
"when looking at the secondary, its too far into the tube and about 15%"

Did you attempt to tighten the central secondary alignment screw to pull it up closer to the UTA opening?

tiodiacontti

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Re: Lightbridge Collimation help needed!
« Reply #2 on: January 03, 2018, 10:28:30 PM »
David -- ideally, a sight tube with crosshairs would be really helpful getting the secondary in the right position. But with the tools you have, the collimation cap will work. I'd recommend setting aside the cheshire and tubelug for now. You don't want to mess with the primary until the secondary is right.

It sounds like your secondary is not positioned well, but once you have it right, you shouldn't need to do a lot of adjustments. When I remove my secondary, I will follow this procedure to properly position it on my lightbridge. What I typically do is while looking into the focuser with sight tube in place, (in your case with the collimation cap in place), is hold a blank sheet of white paper between the primary and secondary. That will eliminate any reflections that may cause confusion. Loosen the three secondary adjustment screws so you can freely move the secondary tilt and rotation. Move the secondary (tilt and rotation) by hand until it is centered and round as seen through the collimation cap, adjusting the center secondary screw if necessary to move it along the optical tube's center axis. Remove the white piece of paper and since you don't have a crosshair, make sure the reflection of the primary center spot is fairly well-centered in your view of the rounded secondary. Slightly move the position of the secondary if necessary and snug the three secondary adjustment screws. Drop in the laser, and adjust the 3 secondary adjustment screws to center the beam on the center spot. Remove the laser, insert the collimation cap again and using the white sheet of paper as before, make sure the secondary is still round and fully visible. If not, keep making slight adjustments until it is round and fully visible when the laser beam hits the center spot.

Once that's right, then you can use the tubelug or cheshire to adjust your primary. I also recommend marking your trusses so you always insert them in the same position during assembly. This will usually result in very little or no adjustments to the secondary after assembly in the future.

Freddy Banks

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Re: Lightbridge Collimation help needed!
« Reply #3 on: January 04, 2018, 12:55:54 AM »
finally got it.
it took a call to Woodland Hills, thank you Daniel, some photos, and backing out the secondary about 3/4 of an inch!

Marvin Alexander

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Re: Lightbridge Collimation help needed!
« Reply #4 on: January 04, 2018, 10:16:48 AM »
Quote
i have collimation cap, cheshire, laser, tublug
when looking at the secondary, its too far into the tube and about 15% of the secondary is cut off. if i fix that, then the laser is about 2" off of the center circle and can't come to alignment.
if i use the laser and adjust the secondary, i get alignment with the center circle, but then i am back to 15% cut off.

also the collimation cap shows perfect secondary, but the cheshire shows it off alignment and the secondary is cut off.

could my trusses have anything to do with making the tube assemblies square?


Let's see--I've got some questions and some answers.
First, by "Cheshire", I assume you mean Cheshire/sight tube combo tool (longish tube with cross hair reticle on the bottom aperture and either window type or other bright Cheshire target under the pupil)--is this correct?

What it sounds to me like you're doing is "fixing" the secondary cutoff by adjusting the secondary mirror tilt, and then when you use the laser, you fix the laser misalignment by undoing what you did to "fix the secondary".

You also mention that the collimation cap shows "perfect secondary"--but the Cheshire doesn't. The collimation cap is not a very good secondary mirror alignment tool--it doesn't show secondary centering very well and it doesn't show where the focuser is pointing either. This means it can trick you into thinking your secondary is "perfect" when it really isn't.

Jason (and now I see Daniel at Woodland Hills) have the right idea. You need to separate the two alignments--secondary mirror placement (sight tube) and where the focuser is pointing (laser). This is usually accomplished by correcting the secondary placement with adjustments in rotation and/or fore and aft in the tube assembly, and correcting where the focuser is pointing by adjusting the secondary mirror tilt.

And of course, your trusses can indeed impact the squareness/alignment of the upper tube assembly relative to the lower tube assembly. Jim gave you some good advice there.

I would, however, like to modify Jim's advice about putting a piece of paper between the primary and secondary mirrors to obscure the confusing reflections in the primary mirror. Instead, since you have a truss OTA, I recommend cutting a piece of paper the same size as the primary mirror and laying it over the primary (you can even tape it to the mirror clips if you like). This leaves the edge of the primary visible when aligning the secondary position, which is a very useful reference. You can also mark the center of the paper so you don't have to take it off to use your laser when you adjust the secondary mirror tilt.

And, glad you had someone to help you sort it out "hands on" this time.

itupmenra

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Re: Lightbridge Collimation help needed!
« Reply #5 on: January 04, 2018, 06:00:27 PM »
Remember, 3 steps:
1) center the secondary under the focuser (using the sight tube+cheshire tool).
2) adjust the tilt of the secondary using either the sight tube or laser*
3) adjust the tilt of the primary using either the cheshire part of the combination tool, the collimation cap, or the Tublug*

Repeat all 3 once or twice because big adjustments in one will result in adjustments needed in the others.
When all 3 are good at the same time,

helppomgido

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Re: Lightbridge Collimation help needed!
« Reply #6 on: January 10, 2018, 11:11:54 PM »
Newtonian secondaries are not centered under the focuser. Lightbridge secondaries are positioned at the factory. Moving it up results in displacement of the fully illuminated spot from the center of the field and in light loss.

erenlinra

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Re: Lightbridge Collimation help needed!
« Reply #7 on: January 11, 2018, 09:56:11 AM »
Ivan,
Standard collimation techniques require centering the secondary under the focuser IN ORDER TO make illumination uniform around the edges of the field.
LightBridge secondaries ARE positioned at the factory, and often incorrectly.

Steven Autio

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Re: Lightbridge Collimation help needed!
« Reply #8 on: January 13, 2018, 04:41:35 AM »
Don, these techniques are a fine approximation for long-focus Newtonians. The proper offsets can be learned from a ray diagram. Once seen enough times, they can be eyeballed. But here is a story about, concidentally, a Lightbridge. A new owner disliked the view down the drawtube so much, and distrusted the Meade Instruments Corporation so thoroughly, he cut the secondary from the stalk (it's on silicone) and re-glued it so that the sides were parallel to the drawtube. Needless to say, that was like masking the primary. It has since been repaired, so we can all have a good laugh.

Owen Khan

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Re: Lightbridge Collimation help needed!
« Reply #9 on: January 13, 2018, 12:04:56 PM »
it looks right now, and this is not my first newt, if we can get the clouds to move along to the east, i will give the first light test!

exmartata

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Re: Lightbridge Collimation help needed!
« Reply #10 on: January 22, 2018, 08:08:21 PM »
Quote
...A new owner disliked the view down the drawtube so much, and distrusted the Meade Instruments Corporation so thoroughly, he cut the secondary from the stalk (it's on silicone) and re-glued it so that the sides were parallel to the drawtube. Needless to say, that was like masking the primary. It has since been repaired...

I don't exactly understand what you mean by "...sides were parallel to the drawtube...", but I'm very interested to hear how it was "repaired". I spend a lot of time (both in these forums and out in the field) correcting focuser/secondary mirror geometry errors. Don's simple 3-step technique works for most situations--even with incorrect geometry. "Centering" under the focuser with a good sight tube (or thin beam laser/collimation cap using Jason's technique (post #412 at http://www.cloudynig...ignment/page-17 ) is pretty much fundamental.

FWIW, with reasonably accurate focuser/secondary mirror geometry and a correct centering procedure, there's no prerequisite to learn the proper offsets via ray tracing. My current scope is a 22-inch f/4 StarStructure (with a mechanically centered secondary mirror) and my collimation procedure delivers an offset secondary mirror placement (relative to the optical axis). Optical performance is excellent, and Go-To accuracy is also very good (even with the residual ~0.2-degree orthogonality offset error). I've used this same procedure on many sub f/4 large aperture Go-To Dobs with similar success.

Mortimer Concepcion

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Re: Lightbridge Collimation help needed!
« Reply #11 on: January 31, 2018, 09:25:13 AM »
Vic, in the Lightbridge the secondary has a cylindrical side surface whose axis is at 45 degrees to the flat face. In the imbroglio, this axis was made parallel to the geometrical axis of the focuser tube. To put it back right, the silicone had to be cut again.

ovhercayvic

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Re: Lightbridge Collimation help needed!
« Reply #12 on: January 31, 2018, 10:09:29 AM »
Do you mean the user mounted the secondary mirror upside down (front surface rotated 180-degrees)?

Danny Cruz

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Re: Lightbridge Collimation help needed!
« Reply #13 on: February 09, 2018, 09:53:46 AM »
Now is my time to be uncertain, Vic  Apparently it's unimaginable. I'll look you up at the Winter Star Party and show you (but no silicone cutting).