Author Topic: To tweak collimation? Which first, secondary or primary?  (Read 171 times)

Stephen Artman

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To tweak collimation? Which first, secondary or primary?
« on: December 30, 2017, 05:31:03 AM »
Hopefully this makes sense!

After my dob has been sitting a few weeks the collimation needs minor adjustments. I'll put in the Cheshire and see the primary needs adjustments but before I move anything I'll put in the autocollimator and see the secondary is also off.

I remember reading that once collimated one mirror usually stays set but I can't remember if that was the secondary or primary.

So when collimating after a few weeks which mirror do I start with? Because I would think one mirror would need more tweaking than the other and if I start with the opposite mirror then I would be doing too much unnecessary adjusting!

Like I said, hopefully that made sense!



Stephen Gupta

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Jon Venning

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Re: To tweak collimation? Which first, secondary or primary?
« Reply #2 on: December 31, 2017, 05:32:49 PM »
Once properly set, the secondary should pretty well stay set. The secondary also needs to be properly aligned under the focuser before you can collimate the primary.

Daniel Ross

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Re: To tweak collimation? Which first, secondary or primary?
« Reply #3 on: January 08, 2018, 02:40:06 AM »
The above post makes a good point. Make sure you do the secondary FIRST.

carmiphypart

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Re: To tweak collimation? Which first, secondary or primary?
« Reply #4 on: January 08, 2018, 03:26:31 AM »
I start with the secondary then the primary. I usually go back and forth a few times to get it perfect.

calbeyrefrows

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Re: To tweak collimation? Which first, secondary or primary?
« Reply #5 on: January 08, 2018, 11:59:10 AM »
Quote
Once properly set, the secondary should pretty well stay set. The secondary also needs to be properly aligned under the focuser before you can collimate the primary.

Side question, can the secondary and primary seem properly collimated even though the seconday is not properly aligned under the focuser? I awlays wondered about that. I have the laser accurately pointed at the center spot on the primary and the return beam centered properly in my Hotech laser. The secondary seems to be centered under the focuser from what I can see. If was not, how could the laser be right on?

Thanks!
Bob

curnarenche

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Re: To tweak collimation? Which first, secondary or primary?
« Reply #6 on: January 10, 2018, 11:49:16 PM »
Quote
Quote

Once properly set, the secondary should pretty well stay set. The secondary also needs to be properly aligned under the focuser before you can collimate the primary.

Side question, can the secondary and primary seem properly collimated even though the seconday is not properly aligned under the focuser? I awlays wondered about that. I have the laser accurately pointed at the center spot on the primary and the return beam centered properly in my Hotech laser. The secondary seems to be centered under the focuser from what I can see. If was not, how could the laser be right on?

Thanks!
Bob

Bob:

When understand the answer to your question you will understand the value of the Barlowed Laser technique..

Jon

byhodete

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Re: To tweak collimation? Which first, secondary or primary?
« Reply #7 on: January 11, 2018, 01:19:14 AM »
Quote
...can the secondary and primary seem properly collimated even though the seconday is not properly aligned under the focuser?...I have the laser accurately pointed at the center spot on the primary and the return beam centered properly in my Hotech laser.

The laser is only showing the axial alignments. The outgoing beam (aligned to the primary mirror center spot) shows where the focuser (axis) is pointing. You typically correct this alignment by adjusting the secondary mirror tilt. Once the outgoing beam is accurately aligned, the return beam shows where the primary mirror is pointing. Optimal image performance (in the center of the field of view) is achieved when both of these alignments are correct.

Getting the secondary mirror correctly placed is usually sorted out by correcting the secondary mirror rotation and the secondary mirror's position closer to or farther from the primary mirror. The alignment is correct when these three circles are concentric:
The bottom edge of the focuser drawtube
The actual edge of the secondary mirror, and
The reflected edge of the primary mirror.
Optimal field illumination is achieved when all three circles are concentric.

To summarize, secondary mirror placement delivers optimal field illumination (brightness), and the axial alignments deliver image performance (detail resolution).

Bobby Javier

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Re: To tweak collimation? Which first, secondary or primary?
« Reply #8 on: January 11, 2018, 04:26:33 AM »
Quote
Quote

Once properly set, the secondary should pretty well stay set. The secondary also needs to be properly aligned under the focuser before you can collimate the primary.

Side question, can the secondary and primary seem properly collimated even though the seconday is not properly aligned under the focuser? I awlays wondered about that. I have the laser accurately pointed at the center spot on the primary and the return beam centered properly in my Hotech laser. The secondary seems to be centered under the focuser from what I can see. If was not, how could the laser be right on?

Thanks!
Bob
Some say close enough is good enough, , getting it centered under the focuser ensures a"evenly illuminated FOV,

Michael Zamora

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Re: To tweak collimation? Which first, secondary or primary?
« Reply #9 on: January 11, 2018, 02:24:26 PM »
Quote
Quote

Once properly set, the secondary should pretty well stay set. The secondary also needs to be properly aligned under the focuser before you can collimate the primary.

Side question, can the secondary and primary seem properly collimated even though the seconday is not properly aligned under the focuser? I awlays wondered about that. I have the laser accurately pointed at the center spot on the primary and the return beam centered properly in my Hotech laser. The secondary seems to be centered under the focuser from what I can see. If was not, how could the laser be right on?

Thanks!
Bob
Deleted incorrect information.

James Etrheim

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Re: To tweak collimation? Which first, secondary or primary?
« Reply #10 on: January 12, 2018, 05:14:19 AM »
Quote
...Loosening or tightening the three screws on the secondary will move the dot around the face of the primary. This is because you are changing the distance (and the angle) between the focuser and the secondary. If you spin the secondary from side to side this angle doesn't change, so a simple laser collimator won't reveal this particular misalignment.

OK, when you loosen and tighten the three "tilt" screws, you are mostly changing the angle between the focuser and secondary, not so much the distance (which becomes apparent when you try to sort out the secondary mirror offset). The distance you have more control over is the distance between the secondary mirror and the primary mirror, and to change that distance you have to loosen or tighten the middle, mounting screw between the three "tilt" screws.

If you "spin" (rotate) the secondary mirror, you will most definitely change the angle between the secondary and the focuser. You can easily observe this change with the simple thin beam laser mounted in the focuser. This is also the cause of the common combined tilt/rotation secondary mirror placement error--without a doubt it's the most common problem I encounter when asked to "fix" someone's secondary mirror alignment problem.

The fact is, while the simple thin beam laser will readily show rotation, tilt and offset errors, it isn't able to easily identify what is causing the error. Even with a "center" mark on the secondary mirror (offset or not), there's no guarantee that getting that center mark aligned with a simple thin beam laser will in fact, optimally place the secondary mirror under the focuser. It's too complicated to easily explain why, but the complexity of the laser solution (even with a holographic grid) is why I recommend using the three circles I noted above. You can read more about using a holographic grid to place the secondary mirror here: https://www.cloudyni...ence/?p=4116531 and here: https://www.cloudyni...ence/?p=4119579

boysagiskest

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Re: To tweak collimation? Which first, secondary or primary?
« Reply #11 on: January 12, 2018, 08:26:08 AM »
Everything is properly collimated.

Im talking about after a few weeks or so.

I put in the autocollimator and notice that the colimation needs a slight adjustment. Which mirror is usually the one that needs the adjusting the most.

Or even after the mirror is collimated and as the night goes on I check the collimation and its not perfect like when I first put it out. Which mirror is it that moves out of adjustment the secondary or the primary?

Zachary Tenk

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Re: To tweak collimation? Which first, secondary or primary?
« Reply #12 on: January 14, 2018, 12:19:57 AM »
Try decollimating the primary mirror (to separate the reflections in the autocollimator). If the two reflections in the middle are still stacked, the problem was primary mirror alignment. If the two reflections in the middle are not stacked, the focuser axial alignment is off and the primary mirror axial alignment may or may not be involved.

fronenfiten

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Re: To tweak collimation? Which first, secondary or primary?
« Reply #13 on: January 14, 2018, 03:01:25 AM »
Looking back at your opening post,

Quote

After my dob has been sitting a few weeks the collimation needs minor adjustments. I'll put in the Cheshire and see the primary needs adjustments but before I move anything I'll put in the autocollimator and see the secondary is also off.

I remember reading that once collimated one mirror usually stays set but I can't remember if that was the secondary or primary.

So when collimating after a few weeks which mirror do I start with? Because I would think one mirror would need more tweaking than the other and if I start with the opposite mirror then I would be doing too much unnecessary adjusting!

The mirror that "usually" stays pretty much where you left it is the secondary mirror. This is because the secondary mirror has less mass/inertia than the primary mirror and it's support mechanicals are usually robust.

However, with a truss tube Dob, there are more mechanicals (including the trusses) that can impact secondary mirror alignment, and if accommodations (like plastic washers) have been implemented to facilitate easy rotational adjustment, rotation may be subject to change as well...

If you find the secondary mirror reasonably centered under the focuser, you can carefully decollimate the primary mirror to reveal the focuser axial alignment and make any necessary corrections. Then use your Cheshire to fine align the primary mirror alignment and you should be done with fine axial alignment and ready to observe.

facwindpsychco

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Re: To tweak collimation? Which first, secondary or primary?
« Reply #14 on: January 15, 2018, 12:01:39 PM »
Quote
Everything is properly collimated.

Im talking about after a few weeks or so.

I put in the autocollimator and notice that the colimation needs a slight adjustment. Which mirror is usually the one that needs the adjusting the most.

Or even after the mirror is collimated and as the night goes on I check the collimation and its not perfect like when I first put it out. Which mirror is it that moves out of adjustment the secondary or the primary?


Either mirror can shift. With a 12 inch Aperture, it could be the secondary because the spider tension is low, it could be the primary because the springs are not sufficiently compressed..

The primary alignment is the most critical so I adjust that last.

Jon