Author Topic: Different Collimation Question  (Read 101 times)

rennlispuring

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Different Collimation Question
« on: December 29, 2017, 06:10:17 AM »
OK, need this explained to me, more a 'how come' kinda question...

Read my thought process and tell my if or why I'm seeing this wrong.

1) A laser is supposed to be good to point the 2ndary at the primary center, but not as accurate if 'used as advertised' to collimate the primary. What I've read is, that if the laser isn't perfect, and not perfect in the focuser (maybe angled a teeny bit), it throws it off. OK, makes sense.

2) So if we use a barlowed laser, it actually collimates it more accurate since focuser errors are eliminated.

Now here are my questions/issues with that.....

1) When we do a barlowed laser, we center the shadow over the center hole in the covering on the bottom of the barlow, so if the focuser is a bit off, then that center hole is off also from true perfect center. If the barlow itself shifts, then it's bottom hole shifts changing 'where' we center the target anyway.

2) If we do a star collimation, we use an eyepiece to 'see' the star, so aren't we in effect collimating to the 'view' of the eyepeice, which is also influenced by any slight errors in focuser placement, or how 'tight' you clamp in the eyepiece etc?

In my mind, it kinda seems like a laser used by itself, is collimating so the primary is pointing at 'laser center', which would be similar to 'eyepiece' center since both would be held the same by the focuser?Does that make sense?
Basically, trying to understand why it would be better to have the primary not lined up with the eyepiece center when we use eyepieces to 'view' objects and 'view' a star collimation? Isn't doing a star collimation (the best final true method) (since if the star isn't in the perfect center of view it'll be slightly out of collimation) dependant on 'where' the eyepiece is 'seeing' the center of view?



tingdermeli

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Re: Different Collimation Question
« Reply #1 on: December 30, 2017, 11:51:47 PM »
You have several questions, so let's start with the numbered ones.

The first question (on the Barlowed laser) is dependent on where the collimation target is relative to the effective focal point of the Barlow lens. Properly configured, the target ("the center hole in the covering on the bottom of the Barlow") will be inside the OTA while the effective focal point of the Barlow will be somewhere outside the OTA. This has been referred to as a "balanced" Barlowed laser. Configured this way, if the target moves (due to tilt in the focuser) downward, the effective focal point will move upward, making the shadow (silhouette of the primary mirror center spot) actually track the movement. Barlowing a windowed laser moves the target outside of the OTA, which unbalances the alignment and defeats the tracking.

The second question (on star collimation) has several conditions. You mention eyepiece registration ("clamping") and alignment star centering (the star must be carefully centered in the field of view to eliminate false reads), but you seem to be asking whether or not "slight errors in focuser placement" would influence the star collimation. The simple answer is residual focuser tilt errors have little impact on the star collimation. Here's why: let's assume the focuser tilts a couple hundredths of an inch downward (we'll say due to gravitational flexure). If the focuser drawtube is racked in most of the way (as it usually is to keep the focal plane close to the top of the focuser) this small amount of tilt as seen from the focuser (for this example, using a thin beam laser) will get increasingly worse farther from the focuser, ultimately magnifying itself (10X, 20X or more depending on the focal length of the scope) by the time the beam reaches the primary mirror. The upshot is that this type of error tilts the focal plane more than it decenters the alignment axis.

I don't quite understand what you mean by "a laser used by itself" unless you're trying to say an unBarlowed laser. If so, the reason a Barlowed laser is better (even if it's not balanced) is that the unBarlowed laser's outgoing thin beam's alignment must be perfect for the reflected beam to be the equivalent of the primary mirror axis. If there's any residual error in the outgoing beam, the return (reflected from the primary mirror) beam can't be the equivalent of the primary mirror axis, and is in fact, only a reflection of the (errant) focuser axis. With the Barlowed laser, the silhouette of the primary mirror center spot is reflected back to the target as part of the parallel bundle of light (parallel because the Balowed laser focal point is very close to the telescope's focal plane). Since the primary mirror center spot is "centered" on the primary mirror and the Barlowed laser's small target aperture is "centered" (equivalent to the center of an eyepiece) in the focuser, small, residual focuser tilt errors have very little impact on the primary mirror alignment. Any "centering" error caused by "clamping" the laser/eyepiece in the focuser drawtube should also be inconsequential since both "see" the same error.

Finally, I'm not sure where you're heading in the last italicized paragraph when you ask, "...why it would be better to have the primary not lined up with the eyepiece center..." For optimal image performance, the goal is to bring the primary mirror axis and the focuser axis into coincidence. This means the primary should absolutely be "lined up with the eyepiece center."

To clarify what I mean by focuser and primary mirror axes, consider the alignment process using a simple thin beam laser. The outgoing beam (from the focuser to the primary mirror center spot) is the focuser axis. The return (reflected) beam (assuming the outgoing beam is perfectly aligned--from the center of the primary mirror back to the laser emitter in the center of the focuser) is the primary mirror axis. The two axes are independent of each other (you can have "perfect" primary mirror axial alignment with less than perfect focuser axial alignment, and vice versa). And here's the final twist--each axial alignment has a prescribed tolerance for optimal (high magnification) image performance, and the primary mirror axial alignment error tolerance is usually much more critical than the focuser axial alignment error tolerance. So, while the outgoing thin beam laser can miss the exact center of the primary mirror by 3- or 4-percent of the primary mirror diameter and still deliver optimal performance, the reflected beam (assuming a "perfect" outgoing alignment) must hit the center of the laser emitter almost perfectly (0.005mm times the focal ratio cubed). For your 8-inch f/6, that's a little more than 0.25-inch on the outgoing beam, and about 0.04-inch for the return beam (again, assuming a perfect 0.0-inch outgoing alignment). IF you use the return beam and the outgoing alignment isn't perfect, you must add 1/2 of any residual error to the final primary mirror alignment. This means if you use a thin beam laser, the outgoing beam alignment error shouldn't be much more than about 0.04-inch (which leaves you a 0.02-inch return beam alignment tolerance--and that's going to be tough).

But if you use a Barlowed laser after your outgoing thin beam alignment, you only need to keep the outgoing beam around 0.25-inch, and the return silhouette "shadow" of the primary mirror center spot a little less than 0.1-inch (another benefit of the Barlowed laser is that the read is 2X the actual residual error).

If you're going to do a star collimation for "high magnification performance", not only will you need to keep the star accurately centered in the field of view, you'll also need to use high magnification (30X per inch of aperture or more depending on your visual acuity) and you'll need good seeing and thermally equilibrated optics. Generally speaking, above f/4, a good Barlowed laser or equivalent primary mirror collimating tool is faster and more precise than star collimation. This doesn't mean you won't be evaluating performance while you're observing--we all do that! It just means, if you "see" what looks like a comatic star in the center of the field of view or you notice planetary detail is not meeting your (realistic/proven) expectations, you can quickly assess the axial alignment and determine whether the collimation is the culprit, or if it's something else.

Danny Cruz

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Re: Different Collimation Question
« Reply #2 on: January 03, 2018, 03:18:13 PM »
First, thanks for that really great detailed reply
after reading it, I think I can now more clearly state my question ...
ok, we assume a non barlowed laser might not be perfect in its axis and not perfect with the actual focuser axis.
But, that means the eyepiece axis also might not be perfect with the focuser's axis
If we star collimate, we are collimating according to how the eyepiece is 'pointing'/angled.
(If the star moves from center you can see its 'collimation' change, so we center the star in the eyepiece center, not the focuser's actual perfect axis)
What makes me wonder about this, is perfect as I could get, Barlow collimation was still off with a star collimation. Then was thinking, star collimation is based on the eyepiece axis (where it is truly pointing at the 2ndary, and where that axis is truly intersecting the primary.) So it 'seems' that a non barlowed laser would emulate that same axis that the eyepiece is 'seeing' and emulate a star collimation.
So its 'primary axis aligned to eyepiece axis' >vs 'primary axis aligned to focuser axis' (even if eyepiece center differs.

ridafimist

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Re: Different Collimation Question
« Reply #3 on: January 03, 2018, 04:19:48 PM »
Oh, this'll be easier to see what I mean.
You collimate with barlowed laser, then fine tune on a star, when your back is turned, someone bends your focuser, now the star is moved in the FOV, so you re-center the star but now have to collimate the primary for the star in its new center view. But Barlow would still show good since it doesn't care about the focuser angle.

Johnny Jeep

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Re: Different Collimation Question
« Reply #4 on: January 04, 2018, 07:45:55 AM »
"...or, does star collimation NOT change if the eyepiece angle changes in the focuser?"

It doesn't if the pupil position remains stationary. And even if it does move, as long as it doesn't move more than the prescribed error tolerance, the high magnification star image will not be significantly impacted.

The alignment error tolerances for an 8-inch f/6 are generous and forgiving (even more so if the seeing or thermal equilibrium is less than optimal). If, on the other hand, your properly executed star collimation result does not match the axial alignment result from a properly executed tool alignment, and you can confirm the error consistently (with different eyepieces and different tools), then at least one of the references (focuser or primary mirror center spot) is suspect. The most common error is a poorly centered primary mirror center spot.

"...when your back is turned, someone bends your focuser, now the star is moved in the FOV, so you re-center the star but now have to collimate the primary for the star in its new center view."

It depends on how the focuser is bent.
If the focuser drawtube is racked out all the way, it's quite possible that the pupil position will move off of the primary mirror axis more than the allowable tolerance. If the focuser drawtube is racked all the way in, the tilt will move the bottom of the focuser drawtube and the pupil will stay close to the primary mirror axis (and the star will remain near the center of the field of view).

"But Barlow would still show good since it doesn't care about the focuser angle."

This isn't entirely true. The typical tilt error encountered with "sloppy" focusers (especially inexpensive Crayford focusers) is a poor fit between the outer cylinder of the eyepiece/collimating tool and the inner cylinder of the drawtube--often called a "registration error." In this case, the focuser drawtube moves in a linear fashion as designed, but the eyepiece/collimating tool can be tilted inside the drawtube because the loose fit allows the locking screw to cause the cylinders to wedge instead of align along the cylinder sides (something the Glatter Parallizer does a good job eliminating for 1.25-inch eyepieces/collimating tools when used in a 2-inch focuser).

Remember, the reference axis to which all else is aligned is the focuser axis (all collimating tools are mechanically mounted on the focuser axis before any assessment or correction is attempted). If the focuser is incapable of maintaining a reasonably accurate alignment because of poor build or overwhelming load, it's probably time to consider a better focuser...

stalafovkith

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Re: Different Collimation Question
« Reply #5 on: January 08, 2018, 09:53:52 PM »
I remember an account of a question that Ralph Dakin answered under the tent at Stellafane in the mid- or late-60s.

(It was well known that the Dakin Barlow cost a good deal more than sundry Barlows on the market at the time).

Q.: "Ralph, what's the difference between your Barlow and the Edmund Barlow?"

A.: (Mr. Dakin): "Mine is centered".

So, centering of the Barlow lens in its cell in its tube can be an interesting first detail to check. Say, by chucking it in a lathe and sending laser light through its presumed center (or etc. ... ) while rotating it slowly and watching the behavior of the laser spot on a target beyond the Barlow.

--Joe

redoroto

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Re: Different Collimation Question
« Reply #6 on: January 10, 2018, 10:11:39 PM »
Quote
So, centering of the Barlow lens in its cell in its tube can be an interesting first detail to check.

This may be a concern with the tracking feature of the "balanced" Barlowed laser, but is unlikely to have a significant impact if the Barlowed laser is properly registered to the focuser drawtube. Consider the Krupa collimator ( http://www.iceinspac...55-0-0-1-0.html which uses a simple LED instead of a Barlow to illuminate the primary mirror center spot. The important consideration is to fully illuminate the mirror surrounding the primary mirror center spot. Since the LED is very close to the axis, the return bundle is essentially parallel to the primary mirror axis when the alignment is corrected.

Brenton Crosby

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Re: Different Collimation Question
« Reply #7 on: January 11, 2018, 06:08:34 AM »
Ok, interesting. I think I see now, if the center of the focuser did move, then the barlow would also move that same amount so using a barlow would still align it properly. I hadn't thought of that, that the barlow laser target center stays with the center of the focuser, duh

I found why I was a bit off with my combo, I didn't have that paper target 'perfect' on my barlow. Yesterday I remade a new center holed paper target, when I put it on the end of my barlow, this time, instead of trying to measure center or use a compass for center, I put my laser in on it's lowest power, in the barlow, and it's tiny dot showing on the back lens of the barlow, helped me center the paper target accurately. Last night, I collimated with that barlowed laser combo and got it spot on. Last night I checked collimation on a star and it was as perfect as it could be that I could see at high power. So now I understand, AND found out my issue, and can totally trust my barlowed laser combo.

Thanks a ton for all your help !

Mark Rivera

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Re: Different Collimation Question
« Reply #8 on: January 11, 2018, 11:31:57 AM »
Correct me if I'm wrong (a distinct possibility...).

Re: Barlowed laser.  I have the Glatter Tublug. When I use it I consider the laser light, having come through the barlow lens, to be nothing more than a light source to illuminate the ring at the centre of my mirror. The blob of light on the mirror is about 3 inches wide so it lacks any precision at all in and of itself.

Yes?

Dave

Michael Greene

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Re: Different Collimation Question
« Reply #9 on: January 11, 2018, 02:37:54 PM »
Quote
Re: Barlowed laser.  I have the Glatter Tublug. When I use it I consider the laser light, having come through the barlow lens, to be nothing more than a light source to illuminate the ring at the centre of my mirror. The blob of light on the mirror is about 3 inches wide so it lacks any precision at all in and of itself. Yes?


Sort of... the precision of the diverging source is in the location of the emitter--in the case of a Barlowed laser, a virtual point on the optical axis near the focal plane. When the diverging source meets the surface of the primary mirror, it's reflected back to the focuser along the optical axis in a parallel bundle (which carries the focused silhouette/shadow of the primary mirror center spot). The smaller the source, the sharper the focus. If the target for the silhouette/shadow of the primary mirror center spot is kept close to the focal plane, the center spot shadow magnification will be 1X.

If the source is aimed a bit off center with respect to the primary mirror center spot, the silhouette of the primary mirror center spot will still return to the focuser as part of the parallel bundle. It's similar to how the primary mirror images an infinite source (like a star), only in reverse.

Anton Balderrama

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Re: Different Collimation Question
« Reply #10 on: January 11, 2018, 05:51:47 PM »
Another different collimation question.

The usual collimation steps using a laser are:
first make sure the diagonal is centered with the eyepiece holder drawtube
put the laser in the eyepiece holder and adjust the diagonal to ensure the laser spot is hitting the center of the primary
adjust the primary collimation screw until the return beam goes back to the center of the laser source
When we do the last step, moving the primary mirror on its collimation screws, the primary mirror's surface is changing angle so as to bring it's optical axis inline with the eyepiece's optical axis. BUT, moving the primary mirror this way will result in a linear displacement of the center of the primary mirror. The amount of movement depends, of course, on theamount of collimation adjustment but also on how far the adjustment screws are in relation to the center of the mirror, both behind and to the side. For a very large, thin mirrormounted on a very low profile mirror support, theeffect is much less. But for a 6 inchfull thickness mirror, mounted on a plywood cell made of one ¾" board under the mirror and another ¾" plate attaching to the tube, it's a lot more.

There exists a similar problem when diagonal mirror adjustment screws are located very far from the surface of the glass: the diagonal doesn't just do an angle change but also moves along an arc, since it's at the end of a long radius, creating a quasi-linear movement of the surface in addition to an angle change.

My question is: do some of you re-check the centering of the laser spot on the center of the primary mirror, especially after a large initial adjustment and then repeat the primary mirror collimation again (iterative process)?

llammenkudi

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Re: Different Collimation Question
« Reply #11 on: January 13, 2018, 04:14:28 AM »
I guess I accidentally did/do, as I check the 2ndary occasionally, even after I've collimated the primary, so at worst, the primary is almost spot on collimated when I have double checked the 2ndary.

I've been bugged about how the 2ndary moves too, they should make a 2ndary holder with a 'ball and socket' in it's true center, so moving it literally just changes it's angle.

Rasheed Grayson

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Re: Different Collimation Question
« Reply #12 on: January 13, 2018, 01:27:43 PM »
Like this... it would 'move' a LOT less.
One could do that with the primary too
With either, you could have 3 points of adjustment to tilt around the ball axis. 2 points would be screws, the 3rd point would be both a screw to lock down, and a spring to maintain tension while adjusting the other 2 screws.
You'd loosen point 3's screw so the spring is pushing the mirror against the other 2 screws and holding it, then adjust the other 2 as needed, when collimated, you'd then tighten screw 3 (that has the spring), to lock it as that position. It would really really lower movement, and make collimating either/both of them, MUCH easier.

Or, just thinking out loud here, you could even have ONLY two screws, but those screws would attach to the cell/holder with a little really tight precision ball and socket, so all you'd have to do is loosen or tighten one and it would push/pull the cell/holder back and forth, now that would be REALLY convenient. No binding, no too tight or too loose, just pure clean angle adjustement.Attached Thumbnails




Jayarajan Mcloven

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Re: Different Collimation Question
« Reply #13 on: January 18, 2018, 03:46:30 AM »
Ron 77 said:

> they should make a 2ndary holder with a 'ball and socket' in it's true center, so moving it literally just changes it's angle . .

 The true center would be at the center of the surface of the mirror, so it would not be possible to put a ball joint there, but it would be possible to design a holder that has a "virtual pivot" there.

Cameron Artist

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Re: Different Collimation Question
« Reply #14 on: January 20, 2018, 02:27:44 PM »
True, under would still be better than standard, but your idea better stiil