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Goniometers...
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Rylee Isitt



Joined: 13 Apr 2012
Posts: 474
Location: Canada

PostPosted: Tue Mar 19, 2013 11:29 am    Post subject: Goniometers... Reply with quote

One of the things I learned recently is that I really, really need a pair of goniometers for positioning a specimen. Struggling with tiny movements with a very coarse specimen clip over the weekend occupied more time than shooting, processing, and retouching the result.

I am looking for Newport, Thorlabs, or Opto-Sigma goniometers on eBay. But I also noticed a lot of other brands (Kohzu, Suruga Seiki, Melles Griot).

Is there any real distinction between these brands, or should I just worry entirely about the specs?

Regarding specs, is a +/- 10 degree shift likely to be enough?

I assume it might be nice if the lower goniometer has a higher point of rotation than the upper, so that when stacked they still rotate around a centered point in space... or is that a realistic goal?
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ChrisR
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Joined: 14 Mar 2009
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PostPosted: Tue Mar 19, 2013 1:42 pm    Post subject: Reply with quote

I haven't done a market survey but I think they all come in a pair of radii, so you can do what you suggest.
Then there's the base side length which varies enormously, and whether you want a micrometer on the tilt, or not. Much less common s/h and much more $$. Standard seems to be +/- 20º on the smallish ones. Mine's small M-G, rather too small perhaps, but quite a bit cheaper than some of the others.
I think it's about 30mm square, - it's packed away somewhere. I don't see any on the currrent M-G website. 65mm would be good - depending, obviously!
You'll find crystallographer's versions from time to time - this one allows 30º on top and 25 under:
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Chris S.
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PostPosted: Tue Mar 19, 2013 4:57 pm    Post subject: Reply with quote

Rylee, once you start working with a good pair of goniometers, you’ll wonder how anyone survives without them!

I only have experience with my Thorlabs goniometers, but the makers you’ve listed all strike me as serious players in the world of opto-mechanics. If you see appropriately specified and priced goniometers with any of those nameplates, it’s probably worth considering them.

Quote:
I assume it might be nice if the lower goniometer has a higher point of rotation than the upper, so that when stacked they still rotate around a centered point in space... or is that a realistic goal?

Very realistic. In fact, it is the way that matched goniometer pairs are designed to be used. Thorlabs’ goniometer stages page has a good description of this on the “overview” tab.

BTW, a problem with many of the goniometers I’ve seen on eBay is that often, they are not listed in matched pairs. Sometimes, you’ll see two identical goniometers paired together, instead of two complementary ones. Or you’ll see an offering for only one goniometer, and buying it puts you at risk of not being able to find a complementary model.

As you have probably noticed, among different offerings for matched goniometer pairs, the big differences are physical size, degrees of adjustability, and the height of the point of rotation above the stage. My Thorlabs GNL goniometer set has its point of rotation one inch above the platform. I’ve found this to be very workable. I would avoid having a point of rotation that is lower than this. Some of the offerings out there have a point of rotation that is two or three times higher; I suspect this would also be quite workable (in some cases, maybe a little better, in other cases, maybe a bit worse).

I haven’t found the need for a great number of degrees of rotational freedom. My set offers +-10 degrees in one axis, and +-5 degrees in the other. I’d gladly accept more rotation, if offered, but have not had many times when these limits were much of a problem.

A couple of years ago, Elf gave me the excellent advice to mount a thin sheet of steel on top of my (aluminum) goniometers so that I could use magnets to position the subject. This has proven to be an incredibly useful tip. Now I keep a supply of small, disk-shaped magnets on hand, along with epoxy and some bits of brass wire and thin rod. When I need a particular subject holder, it’s usually easy to whip up something on a magnetic base and plop it on the subject stage. (And of course, by now, I have quite a few of these made up in different configurations). By sliding the magnet and bending the brass, I can quickly rough-in the subject position. Then, with the goniometers and other micrometer adjustments, do the fine positioning.

--Chris
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johan



Joined: 06 Sep 2011
Posts: 898

PostPosted: Wed Mar 20, 2013 2:00 am    Post subject: Reply with quote

The commercial offerings look very sensitive and solid but they're also quite steep. I know where you're coming from though, I was finding that the positioning was taking as long as the stack, comical. I ended up making my own, some other people have done so as well. Mine isn't nearly as accurate as these goniometers... but it's good enough and has much more of a movement range than these things so kind of better for my purpose as far as I'm concerned. With your skills you should be able to pull off what I originally envisaged, one of these but with the rotating arms and needle remote controlled by motors, that'd be neat.
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Chris S.
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PostPosted: Wed Mar 20, 2013 3:19 am    Post subject: Reply with quote

Johan, I followed your project with interest. I would call what you created a gimbal mount, as opposed to a goniometric stage. Both gimbal and goniometric approaches have substantial overlap, yet differ in some important regards.

If my memory is correct, Elf also built and documented an effective, simple, gimbal subject mount.

Where gimbals and goniometers are similar is that both approaches permit a subject to be rotated in all three axes, while staying in place at its center of rotation--a characteristic of great practical value when observing a subject under high magnification.

Where the gimbal and goniometer approaches differ is in their amount of adjustability and their blockage of light. My sense is that well-constructed gimbals offer greater angular adjustment than goniometers, at the cost of blocking subject illumination with bits of the subject stage that block light at various locations. Conversely, a well-constructed goniometric stage offers less angular adjustment than a gimbal stage, but obstructs light placement much, much less.

My own sensibilities value lighting freedom very highly, so I prefer a goniometric approach. But I appreciate other clever approaches, and see your gimbal approach as equally legitimate, but matched to a different set of priorities--more positioning freedom, with less lighting freedom. Both approaches belong in the world! May we continue to vie so, to the improvement of our shared art. Very Happy

Peace,

--Chris
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johan



Joined: 06 Sep 2011
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PostPosted: Wed Mar 20, 2013 4:06 am    Post subject: Reply with quote

Yes, I'm sure you're right, mine is more of a gimbal construct than a goniometer, I remember googling gimbals extensively to be able to source parts! The arms can get in the way yes although now that I've discovered continuous lighting with Jansjos rather than huge flash units it's not a problem. But maybe that's my adapted diffusion methods - I tend to use either the end of a poly cone or some other sheet suspended with plamps or a table tennis ball, always quite small. In fact the main thing that gets in the way is actually the needle that the specimen is on because it's sometimes at an angle. The arms can get in the way in the background with less magnification images but I can generally get around this by different rotation configurations. The freedom of movement is absolutely great though, getting the thing to sit right on the insect examination stage I used to use was driving me bonkers.
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elf



Joined: 18 Nov 2007
Posts: 1283

PostPosted: Wed Mar 20, 2013 6:50 pm    Post subject: Reply with quote

Little motors exist and they're not terribly expensive: http://www.namiki.net/product/dcmotor/micro.html

The next step is to build a really small wireless controller
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Craig Gerard



Joined: 01 May 2010
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PostPosted: Wed Mar 20, 2013 8:02 pm    Post subject: Re: Goniometers... Reply with quote

Rylee Isitt wrote:
One of the things I learned recently is that I really, really need a pair of goniometers for positioning a specimen. Struggling with tiny movements with a very coarse specimen clip over the weekend occupied more time than shooting, processing, and retouching the result.

A matched pair (dual axis) goniometer combined with a rotational platform will be very useful for this purpose. Such devices are a pleasure to use. We've seen them used in horizontal setups, but I can envision them as being just as useful with a vertical arrangement.....something more for me to contemplate and explore.

Quote:
I am looking for Newport, Thorlabs, or Opto-Sigma goniometers on eBay. But I also noticed a lot of other brands (Kohzu, Suruga Seiki, Melles Griot).
Is there any real distinction between these brands, or should I just worry entirely about the specs?

The main specs I'd be concerned about is purchasing a matched pair. I have a matched pair consisting of a Thorlabs GNL10 (top) and a GNL18(base). I also have a Kohzu matched pair (although the base unit of the latter is somewhat bumpy and I'm intending to replace it). If you get a good matched pair, the Kohzu are superb. They are much larger than the aforementioned Thorlabs units and as a consequence offer more displacement and are possibly overkill at magnifications above 10X, but this is only a presumption on my part.

Recently another forum member and I were discussing the principals involved when comparing the Kohzu units to the Linof Levelling Head.
http://www.linhof.com/download_e/Linhof_3D_Micro_2013_engl_i.pdf (the information on page 4 and "parallelism" is what generated my interest)

Quote:
Regarding specs, is a +/- 10 degree shift likely to be enough?

This is a question I'd like to expand and publicly ask of the more experienced goniometer users. Does the degree of apparent, usable shift increase at higher magnifications? I need to think about this some more but wanted to introduce it into this discussion...

Quote:
I assume it might be nice if the lower goniometer has a higher point of rotation than the upper, so that when stacked they still rotate around a centered point in space... or is that a realistic goal?

This is how goniometers were designed to be used, as far as I know, however, even when used as simple swivel stages they are useful devices, but that 'rotation around a centered point in space' is most desirable.


Craig
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AndrewC



Joined: 14 Feb 2008
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PostPosted: Thu Mar 21, 2013 1:19 pm    Post subject: Re: Goniometers... Reply with quote

Craig Gerard wrote:

This is a question I'd like to expand and publicly ask of the more experienced goniometer users. Does the degree of apparent, usable shift increase at higher magnifications? I need to think about this some more but wanted to introduce it into this discussion...

Craig


Not sure I fully understand the question but if you are completely centred the rotation doesn't change with magnification - little circles and big circles are still 360deg. If you are off centre you add a linear movement in xyz so magnification does change the amount of apparent movement. Odds are you are never completely centred so you have to mix both types of motion. As an aside, people often forget that (depending on your set up) you may have an extra degree of motion by rotating your camera body around the optical axis. Easy if you've got a collared lens or bellows with rotatable standards.

Goniometers are just as useful vertical as horizontal - a vertical setup is just a horizontal setup standing up Smile I've been building a stage for a while (got distracted by building a 3D printer) which is designed to rotate the camera around the subject so it can be at any angle between horizontal and vertical. There are certain times when you want gravity to work in a certain direction but your POV needs to be neither horizontal nor vertical. Haven't quite mastered black holes yet so decided to move the camera angle.

So far as prices / brands are concerned - it doesn't really matter what brand for amateur work. Prices have never recovered since Chris first posted his pair of Thor Labs Sad
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dmillard



Joined: 24 Oct 2006
Posts: 501
Location: Austin, Texas

PostPosted: Thu Mar 21, 2013 7:16 pm    Post subject: Re: Goniometers... Reply with quote

Craig Gerard wrote:

A matched pair (dual axis) goniometer combined with a rotational platform will be very useful for this purpose. Such devices are a pleasure to use. We've seen them used in horizontal setups, but I can envision them as being just as useful with a vertical arrangement.....something more for me to contemplate and explore.
Craig


Something like this? It has been a very useful combination, although it has suffered from neglect for the past few months, as you can probably tell by the accumulation of dust Sad
Regards,
David

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Rylee Isitt



Joined: 13 Apr 2012
Posts: 474
Location: Canada

PostPosted: Thu Mar 21, 2013 9:19 pm    Post subject: Reply with quote

Great information, everyone!

Yes, I've seen almost entirely single goniometer offerings on eBay. I'll keep searching, eventually I'll find something that will work.

Until then, I suppose I will just have to suffer!
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Rylee Isitt



Joined: 13 Apr 2012
Posts: 474
Location: Canada

PostPosted: Thu Mar 21, 2013 9:42 pm    Post subject: Reply with quote

Here's one:

http://www.ebay.ca/itm/OptoSigma-123-2895-GONIOMETER-65-x-65mm-2-Axis-Metric-Travel-30-x-20-Degrees-/160949984693?pt=LH_DefaultDomain_0&hash=item25795dddb5

It's new in box, close to full retail price. But it sure looks nice! 65mm base, with the center just over an inch above the surface.

If I see something like that used, I'd probably jump on it. $900 is a bit rich for me, the shipping ($60) is oddly high, and I can expect some hefty taxes when it arrives, so I'll probably pass.

But it's certainly tempting...
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rjlittlefield
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PostPosted: Thu Mar 21, 2013 10:13 pm    Post subject: Reply with quote

That one looks good, except for the relatively high price.

As others have described, the usual condition with stacked goniometers is that they have a common rotation point. So that's what I assumed when I picked up THIS STACK for an attractive price from a nearby location. Unfortunately the spec about "50mm Virtual Point" was not correct. In fact there are two virtual points, averaging about 60 mm above the top unit, and pretty far apart. On close examination of the parts, it looks like somebody stacked two units that are identical instead of properly matched for stacking.

Ask the seller, if there seems to be any question. Maybe even if there isn't...

--Rik
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Chris S.
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PostPosted: Fri Mar 22, 2013 12:07 am    Post subject: Reply with quote

Rik, there was a dead giveaway in the eBay photos of the pair you purchased, but it probably wouldn't have been apparent to anybody except a few weirdos like me who have spent way too much time looking at goniometers. I'll point it out because "identical" pairs (like those you purchased), rather than "complementary" pairs (which we should prefer), are pretty common on eBay, and forewarned is forearmed.

The giveaway is that the marked rotation scales and movement arcs are identical in the two goniometers you received. Below is a picture I posted a couple of years ago (here) of the complementary pair in the Bratcam. We see that the lower goniometer has fewer degrees of movement and a shallower arc than the top goniometer. For two goniometers to have a common point of rotation, but to lie at greater and lesser distances from this point of rotation, these differences in adjustment range and radius of arc are the most common engineering approach. I believe I recall a few models in which lower goniometer has had similar adjustment range to the top one, but in the examples in my mind's eye, the engineering necessary to permit this has been obvious, and it is not the norm. I cannot think of an example in which the lower goniometer has not had a flatter arc than the top one, in pairs with a common rotation point. I'll also include an image from the eBay auction Rik referenced, so that this thread can remain useful if the eBay auction becomes unavailable in the course of time.

Complementary (good) goniometer pair (Thorlabs GNL20). Note that the lower goniometer has less adjustability, +-5 degrees, than the upper goniometer, +-10 degrees. The rotational arc of the lower is also flatter than the arc of the upper:



Identical (bad) goniometer pair from the eBay auction Rik referenced (note that each has an identical +-8 degrees of rotation, and that each has the same apparent rotationial arc):





Andrew, we need to be careful what we wish for, don't we? Wink As I recall, it was you who encouraged me to come out of the dark about goniometers. But you were right, and I was working on it, anyway--it just took some time to get the post ready. It surprised me that good goniometer pairs became so much scarcer on eBay right after that post. And as Andrew says, the easy days of buying used goniometers have never returned since. There really was a decent selection before that post. The world may "largely note and long remember" what we say here at PMN.

For those suffering while waiting for a nice goniometer pair, I'd suggest just biting the bullet and purchasing the Thorlabs GNL20 pair new for $475. It's good equipment, fairly priced. And while you're waiting for something like this to come up on eBay, life is passing by. (I've purchased macro equipment on eBay whenever possible--but with scarce, badly-needed items, sometimes it's just better to pay retail and move on.)

The GNL20 recommendation seems like a natural choice for a horizontal rig. But for a vertical rig like David Millard's, having the point of rotation an inch above the stage might not be as important, if most of the potential subjects are reasonably flat. In that case, the cheaper Thorlabs GN2 goniometer pair ($260, new from Thorlabs), might be a good bet. The rotation point is only a half inch above the stage, but the rotational angle is greater.

BTW, Rik--the timing pulleys on your goniometers are interesting. Somebody may once have put a timing belt and motor drive on these?

Cheers,

--Chris


Last edited by Chris S. on Fri Mar 22, 2013 3:54 pm; edited 1 time in total
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Rylee Isitt



Joined: 13 Apr 2012
Posts: 474
Location: Canada

PostPosted: Fri Mar 22, 2013 12:17 am    Post subject: Reply with quote

Chris,

Thanks for pointing out the Thorlabs GNL20, that's a price I'm willing to pay.

I also have the same rotation stage as you... so I'd probably also want to draw up designs for an adapter.

My rig is looking more and more like the Bratcam. I guess you can call it a case of convergent evolution. When the same solution ends up coming together more than once by different paths, you know it's gotta be a good one. In other words, you've built a great rig!
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