Author Topic: Collimation Perfect Except for One Thing  (Read 126 times)

Isaac Griffin

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Collimation Perfect Except for One Thing
« on: December 29, 2017, 02:29:38 AM »
Hello all,
Scope: Obsession Classic 12.5
After appropriate adjustments in collimating my scope:
I have the focuser (JMIEV1) leveled.
The Glatter holographic laser shows the focuser is centered with the secondary mirror.
The primary is equidistant from each mirror clip.
With center axis bolt and nut adjustments, the laser dot hits the center mark of the secondary
The laser strikes the center of the primary.
The Tublug shows the primary marker in the center of the circle.
Back at the secondary - the outgoing and incoming laser dots appear nearly superimposed. So, what is not to like?
Thru the sight tube I see only two mirror stays. Should I disregard this? How would you proceed?
Carl,
Ledyard, CT



rubnirootcount

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Re: Collimation Perfect Except for One Thing
« Reply #1 on: December 29, 2017, 09:30:39 AM »
Quote
....I have the focuser (JMIEV1) leveled.
The Glatter holographic laser shows the focuser is centered with the secondary mirror.
...
With center axis bolt and nut adjustments, the laser dot hits the center mark of the secondary
...
Back at the secondary - the outgoing and incoming laser dots appear nearly superimposed. So, what is not to like?
Thru the sight tube I see only two mirror stays. Should I disregard this? How would you proceed?...

A few questions...
What is the "leveled" focuser's reference?
I'm not sure what you mean by "the Glatter holographic laser shows the focuser is centered with the secondary mirror" and "the laser dot hits the center mark of the secondary". Did you center spot the secondary mirror? Is the center spot offset? You might gain some additional insight here:https://www.cloudyni...ence/?p=4119578 If the center spot on the secondary is centered, the secondary mirror placement will also be centered, which does not utilize the full primary mirror and would explain why you only see "two mirror stays" through the sight tube.

Finally, you note that, "...the outgoing and incoming laser dots appear nearly superimposed." I would expect them to be superimposed if both of the axial alignments are fully corrected.

To answer your closing questions, I personally wouldn't disregard the secondary mirror placement error. I would proceed with an offset secondary mirror placement using the sight tube to set the offset. Then use the Glatter to set the secondary mirror tilt, and the TuBlug to set the primary mirror tilt.

wallnewsspheryz

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Re: Collimation Perfect Except for One Thing
« Reply #2 on: January 04, 2018, 04:24:36 PM »
Hello Vic,
Thanks for taking the trouble to wade in on my description of the current state of my optical axis.
"Level": On the focuser, I had to loosen the fastening bolts and move the focuser left about 2 mm and tweak two of the canting screws to get the shadow of the secondary to fall in the center of the holograph pattern along the axis of the OTA. Then locked it down.
By center mark on the secondary, I refer to the mark placed by the scope's maker at Obsession.
The in and out beam on the secondary is within a mm. The tech at JMI commented that trying to get it spot on, so to speak, wold make no difference in observing. So, I let it ride.
Seems I need to learn about offset mirror placement. Thanks for suggesting a path for inquiry and understanding.
Carl

grounincalpay

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Re: Collimation Perfect Except for One Thing
« Reply #3 on: January 08, 2018, 06:33:49 AM »
Quote
"Level": On the focuser, I had to loosen the fastening bolts and move the focuser left about 2 mm and tweak two of the canting screws to get the shadow of the secondary to fall in the center of the holograph pattern along the axis of the OTA. Then locked it down.
By center mark on the secondary, I refer to the mark placed by the scope's maker at Obsession.
The in and out beam on the secondary is within a mm. The tech at JMI commented that trying to get it spot on, so to speak, wold make no difference in observing. So, I let it ride.
Seems I need to learn about offset mirror placement. Thanks for suggesting a path for inquiry and understanding.

Your method of "leveling" the focuser, specifically, the alignment signature, could be incorrect (see the link I provided in my earlier post--you'll see what I mean). Properly centered (offset), the shadow of the secondary may not fall in the center of the holographic pattern!

With respect to the center mark on the secondary mirror, your subsequent assessment with a sight tube indicates it's either centered or incorrect. My honest advice is not to use it. Use the sight tube to align these three circles:
the bottom edge of the sight tube,
the actual edge of the secondary mirror, and
the reflected edge of the primary mirror.
When all three circles are concentric, the secondary mirror tilt should be nearly perfect. But the simple thin beam laser should allow you to align the secondary mirror tilt a little more precisely. If the subsequent adjustment significantly impacts the concentricity of the three circles, tweak the secondary mirror placement without adjusting the tilt component--just use rotation and/or secondary mirror placement closer to or farther from the primary mirror. When the concentricity is good and the secondary mirror tilt is correct, you should have no problem getting the unBarlowed laser to return back on the same path it left on the way to the primary mirror center spot. The TuBlug may show a slight residual error in the primary mirror tilt that you can further tweak, but if the correction is substantial, you may want to verify both alignment via rotation in the focuser (be sure to resecure the tool in the focuser drawtube before making the alignment read).

From your questions, the alignment issue you're dealing with is all secondary mirror placement. The problem could be offset, or tilt/rotation error, or some other geometry error in the focuser/secondary mirror alignment. The good news is that a small residual error in the secondary mirror placement has little or no impact on image performance. The axial alignments deliver image performance. The alignment of the outgoing thin beam laser relative to the primary mirror center spot (focuser axial alignment) and the alignment of the return beam (preferably a Barlowed laser, like the TuBlug, which shows the primary mirror axial alignment magnified 2X).

Robert Porter

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Re: Collimation Perfect Except for One Thing
« Reply #4 on: January 08, 2018, 09:25:44 PM »
I suspect that you are conflating two secondary mirror issues that must be managed independently of each other.
I also read that you are dealing with the dreaded center spotted secondary mirror. That center spot routinely causes collimation problems. You should completely ignore the secondary mirror center spot.

1. Centering the secondary mirror under the focuser. Best done utilizing a sight tube. Not a critical adjustment. Star images can be good even if not perfectly centered. The worst thing that will happen if your secondary is not centered under the focuser tube is that you will waste a little bit of the light concentrated into the light cone by the primary mirror.

2. Secondary mirror axial alignment. Use the plain laser to adjust the rotation and tilt of the secondary mirror so that the beam is centered on the middle of primary mirror center spot.

Once you have centered your secondary under the focuser with the help of a sight tube and you have perfected your secondary mirror axis alignment, the laser beam will not impact the secondary mirror center spot. You can bank on it.

If you try to shift the secondary so that the laser beam shines on the secondary center spot and, then, perfect your secondary axial alignment, your secondary mirror will not appear centered under the focuser tube. You must resist the temptation to pay any attention to the secondary mirror center spot. The mutually exclusive scenarios are based on an inviolable principle of conic geometry.
--------
C

vicareeti

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Re: Collimation Perfect Except for One Thing
« Reply #5 on: January 10, 2018, 02:38:33 AM »
I copied your response to my Obsession User thread (at that time unresponded to) and got the following:
"The Cloudy Nights poster is the authority. I like his terminology. You can bank on it. That center dot is a waste of ink.
Regards and clear skies,"

The secondary center dot figures large in the Obsession Telescope instructions on line as to how to collimate their scopes. The dot is factory placed. The quote below is their guidance:

"So to achieve the best collimation you it is imperative that you "center dot" both mirrors. If you have not done so you simply cannot collimate the optics. Instructions for center dotting the mirrors as well as a template for dotting the secondary mirror are found in the Assembly Instructions that came with your Obsession.Collimating the secondary mirror:
Install the secondary mirror in its holder and the holder in the spider. Set the scope up. It helps if you do this in the shade or at night so the laser beam is easier to see. Look through the focuser barrel at the secondary mirror. You will see the secondary holder and mirror in some odd position. Rotate the holder so that mirror is exactly face-on to the focuser. Put the laser in the focuser. Next use the adjustment nuts on the stem of the holder to move it up and down until the laser beam hits the dot on the surface of the secondary mirror. The secondary mirror is now centered under the focuser."

OK. Following these instructions from Obsession likely got me in to the fix I led with in my first post of this thread. I see that. So, if I start with manipulating the five bolts controlling the rotation and tilt of my secondary to achieve a "see all four primary mirror clips" solution, won't I then be moving off that positioning, when I use the same tilting knobs to center the laser in the primary mirror donut at a later stage of the process?
Carl

Justin Lewis

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Re: Collimation Perfect Except for One Thing
« Reply #6 on: January 12, 2018, 06:43:13 PM »
I use iteration in cases where components are grossly misaligned and correction will bodily move the parts in the course of correcting their alignment.

(Centering the secondary: using a sight tube is my preferred method and I already own a combination tool)

Centering the secondary under the focuser utilizing a sight tube:
Step 1 - twist the 4 (or sometimes 3) secondary mirror tilting screws until the mirror holder base looks pretty evenly supported and not tilted noticeably.

Step 2 - insert sight tube and center the secondary by adjusting the center bolt for antero-posterior positioning and reciprocating the spider vane nuts for lateral positioning.

Step 3 - remove the sight tube and insert the laser to roughly aim the laser spot close to the center spot of the primary mirror. That is when I use the 4 (or sometimes 3) secondary mirror tilting screws. I always begin adjusting the aim of the secondary by rotating the secondary holder until the laser spot falls on a line extending from the center spot to the reflection of focuser tube on the primary. Once on that imaginary line, I gradually drive the spot with the tilting screws so the secondary axis is pretty closely aligned. It doesn't have to be perfect at this stage.

Step 4 - once secondary axis is grossly aligned, I reinsert the sight tube and check the centering again. It may look a smidgen off center. I then recenter the secondary using center bolt and spider vanes again. Movement of parts this time will be far less.

Step 5 - remove sight tube, reinsert laser and again grossly re-aim the secondary with the 4 (or three) tilting screws.

Step 6 - back to the sight tube for inspection. It should be close enough by this iteration.

Step 7 - finish perfecting the secondary mirror tilt using the laser.

I guess that centering the secondary with a laser on the secondary center spot can get you pretty close and avoid the need for a sight tube. There are probably hundreds of telescopes out there aligned that way with nary a complaint. It's only if you are striving for exactness that the discrepancy becomes noticeable. As you have experienced, it is detectable and the question comes up from time to time. If you can live with a minor loss of gathered light for the extra convenience of easier mirror alignment, you have a lot of company out there. The difference in performance may not even be detectable visually in the eyepiece. The shorter the focal ratio of the telescope, the greater will be the amount of light loss.

It's a personal decision. You will be okay either way as long as you know what is happening. Clear, dark skies.
--------
C

vicareeti

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Re: Collimation Perfect Except for One Thing
« Reply #7 on: January 15, 2018, 04:24:11 PM »
Cames,
First, let me say what an enormously knowledgeable and generous set of folks respond to these posts from lost and often hapless observers.
I am wondering if there is some substitute other than reciprocating spider vane nuts for laterally manipulating the secondary axis. My vanes don't seem to be attached in a way that would allow for any fine tuning.
While you were composing your thoughtful response, the Sorcerer's Apprentice here was busy backing out the right side of the focuser base to where it was last summer and stuck washers in to keep the cant locked in. This got the holograph centered and the side to side spacing of the sec. mirror, as seen thru the sight tube, equal. The up/ down of the mirror in the longitudinal axis is pretty straightforward. I was able to fuss with the thing until I got three of the mirror pegs to show in the sight tube - none prominently, so I figured the one that was AWOL was just over the horizon. So, I went ahead and collimated the thing - very minor adjustments to achieve this. The outbound and inbound laser dots were precisely overlaid. Interestingly, the spot at which they met was now about a centimeter north of the center dot of the secondary mirror. I then rechecked the position of the mirror pegs and two showed prominently - the other two were not seen. I'll keep working at this. Trying different things...mostly chasing down the leads you all provide.
Best Regards,
Carl

dedlowsthima

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Re: Collimation Perfect Except for One Thing
« Reply #8 on: January 18, 2018, 12:17:45 AM »
Quote
...Following these instructions from Obsession likely got me in to the fix I led with in my first post of this thread. I see that. So, if I start with manipulating the five bolts controlling the rotation and tilt of my secondary to achieve a "see all four primary mirror clips" solution, won't I then be moving off that positioning, when I use the same tilting knobs to center the laser in the primary mirror donut at a later stage of the process?

As I noted in my last post, when aligning the laser dot to the primary mirror center spot, you should ONLY adjust the secondary mirror tilt screws. When positioning the secondary mirror under the focuser (to make the three circles concentric), using the sight tube, you should ONLY adjust the secondary mirror rotation and/or the position of the secondary mirror closer to or farther from the primary mirror. By limiting the two alignments to entirely separate adjustments, you won't completely undo the previous adjustment (hope that makes sense). You may have to repeat the two alignments two or three times (each time should be noticeably closer) before both are fully corrected. This usually resolves the two most common secondary mirror placement errors--combined tilt/rotation error and/or combined tilt/offset error. Once you sort out the secondary mirror placement, the rest is easy.

(FTR, you should put a piece of white copy paper behind the secondary mirror so you can easily see the actual edge of the secondary mirror.)

haigeoredis

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Re: Collimation Perfect Except for One Thing
« Reply #9 on: January 23, 2018, 03:10:50 AM »
Quote
...I am wondering if there is some substitute other than reciprocating spider vane nuts for laterally manipulating the secondary axis.

Yes, there is.
Which brings us back to my earlier question, "What is the "leveled" focuser's reference?"
Normally, the method I described above to resolve combined tilt/rotation and tilt/offset errors is all that you'll need to get the secondary mirror placement sorted out. This assumes a lot of things: a reasonably squared focuser, a reasonably squared spider, a reasonably aligned UTA, a reasonably centered primary mirror, etc. If one or more of these mechanical adjustments is off more than a millimeter or two, you may find that there's no tilt/rotation and/or tilt/offset correction that makes all three circles concentric. FIRST--close is good enough. If you can see all of the mirror clips (pretty much equally) and the laser is aligned with the primary mirror center spot, it's pretty likely to be close enough. SECOND--if you're feeling "obsessed" about getting the secondary mirror placement as perfect as possible, now is the time to "tweak" the leveling screws on the focuser base. If it takes more than a tweak, something else is wrong, and you should probably sort that out first. I would start with the UTA, but I'll save the full discussion for after you've worked through the secondary placement adjustments.

There's some more good information here:https://www.cloudyni...rror-alignment/

aftilicomp

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Re: Collimation Perfect Except for One Thing
« Reply #10 on: January 31, 2018, 02:58:21 AM »
Carl,
the dilemma is (mainly) a matter of secondary "offset": the "upper" half of the secondary is nearer to the focuser than the "lower" half, and thus is seen larger from the focal plane. This is unavoidable, and means that even though the elliptic secondary will look circular from the center of the focal plane, the center of this circle is *not* the center of the ellipse (that you have marked, in the obsession tradition). So what are the tradeoffs?
The "fully illuminated field" is the part of the focal plane from where you can see the whole primary reflected in the secondary (including all of the mirror clamps <grin>), and thus the whole primary will contribute light from all stars in this part. This is usually smaller than the field stop of the lowest-powered wide field EP (you don't really need the outskirts of that field fully illuminated - some loss of light is quite acceptable and you like to keep the obstruction low), but high-powered EPs with smaller fields should be fully illuminated.
So here we have possible two distinct, and different, ways to place the secondary: A) use the mark on the center of the ellipse to aim the down-going laser beam, and B) center the circle you see in the peephole and accept the inevitable - the center mark on the secondary is not on the laser beam.
Actually, either will give acceptable results, and the difference may be too small to be noticeable, but if you don't understand and accept they are different, you will be permanently confused (as you are now...)
Again, what are the tradeoffs? With A), the traditional way of Obsession and some others (It has been - don't know if it is still), the fully illuminated field will shift some, away from the primary. This means that the secondary should be ever so slightly larger to ensure full illumination of the center of the focal plane - but still, with a peephole thingy there, you should see all of the primary clamps. On the other hand, the optical axis and aim of the telescope will be centered in the secondary cage [edit], so if both the primary and secondary are centered in the tube, so is the aim.
With B), I think on the whole more popular today, the fully illuminated field is centered, but the optical axis (and aim) is shifted a tiny bit towards the focuser - probably too little to matter, or do you folks with digital setting stuff disagree? Offsetting the secondary away from the focuser by the appropriate amount will remedy this, but if so, better implement at the design stage.
The difference in A) and B) is in practice small - but which way is simpler is a matter of taste.
Another possibly confusing thing if you use a Cheshire: with A), the reflection of the Cheshire will be centered in the dark reflection of the secondary - with B) it will be offset (inevitably, so if you don't understand and accept this, it will be *very* frustrating!) If you choose B), a sight tube might be convenient, but not necessary.
To check: use the laser and tilt the secondary to center the spot (never mind the return beam right now). Use a peephole cap of some sort in the focuser near the focal plane, and a sheet of paper in the UTA behind the secondary, to see its outline better (or at least half the outline). You will see the primary with all clamps in both A) and B), but if B) is the choice: the primary is on the optical axis, and the secondary should be centered around it. If not, the secondary should be moved up/down the tube (you see how much, but repeat until you are close enough - don't be obsessive here). If off sideways in the tube the most likely reason is rotation of the secondary (combined with a tilt error): look in the peephole, see if the secondary looks straight or rotate it slightly until it does, then adjust the tilt again and check. Only if this doesn't work, try shim the focuser.
Not until this is done, adjust the tilt of the primary.
Hope this can clarify some of the mysteries of collimation (Cross posted to the Obsession group))
Nils Olof

John Fimbres

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Re: Collimation Perfect Except for One Thing
« Reply #11 on: January 31, 2018, 10:06:54 AM »
Quote
...I guess that centering the secondary with a laser on the secondary center spot can get you pretty close and avoid the need for a sight tube. There are probably hundreds of telescopes out there aligned that way with nary a complaint. It's only if you are striving for exactness that the discrepancy becomes noticeable...

It depends on those pesky mechanical adjustments! Spotting the secondary mirror assumes everything is reasonably "squared" and "centered". It also assumes the secondary mirror rotation is correct and the secondary mirror placement is either centered or offset (or perhaps, somewhere in between). It helps to know if it's one or the other, especially if you're planning on using a sight tube. If you don't know, you may find using a secondary mirror center spot to be an exercise in frustration.

The other reason I don't use a secondary mirror center spot is it can get in the way of the outgoing laser beam, making it more difficult to use the laser to set the secondary mirror tilt. If the secondary mirror center spot is a small ring, that may help for the outgoing beam alignment...

Christopher Buchanan

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Re: Collimation Perfect Except for One Thing
« Reply #12 on: February 02, 2018, 03:07:38 PM »
Quote
...With B), I think on the whole more popular today, the fully illuminated field is centered, but the optical axis (and aim) is shifted a tiny bit towards the focuser - probably too little to matter, or do you folks with digital setting stuff disagree?

I used a mechanically "centered" secondary mirror with an offset alignment on an f/4.1 StarMaster with SkyCommander DSCs (2000 count encoders) and routinely achieved 0.1-degree go-to accuracy. My current f/4 StarStructure is similarly aligned and is ArgoNavis/ServoCAT go-to equipped (2500 count encoders)--I get similar go-to accuracy. I believe the Obsession has an f/5 primary...

(edit: I've also purposely used a "centered" alignment with my centered secondary mirror to determine if the accuracy could be significantly improved. IIRC, the results were better near the zenith, but overall accuracy away from the zenith was still about 0.1-degree. I probably could have achieved better with a TPAS model, but I get a 0.5-degree field of view with my 13 Ethos, so 0.1-degree pointing accuracy (most of the time) gets the job done.)

Theodore Inlaw

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Re: Collimation Perfect Except for One Thing
« Reply #13 on: February 09, 2018, 10:47:23 AM »
FWIW, I quickly learned to ignore the secondary center spot that I had dutifully placed on the mirror per the instructions. Eventually, after many years I used some solvent to remove the spot. The spot was just in the way/confusing as others have mentioned above.