Dimensionally Stable Metal

I haven't gottena all of the particulars worked out yet, but I am working on designing a camera stand and trying to decide on the best
material to make most of it out of.
I'm not trying to reinvent the wheel, but a special project makes it necessary to be able to take a series of as many as 180 equally spaced photos across a 45° swing and be be able to do the same again without any perceptable deviation.
In other words if I take a picture at 12-1/4° and then continue turning the camera all the way to the end, when I return the camera to the same spot I took the photo I will need to be able to stop and lock the camera so that the photo next photo I take there is *exactly the same as the previous one I took.
This is not plausible with conventional camera stands. So I'm trying to decide on the best metal to make the moving parts out of. A metal that will best retain it's dimensions under changing temperature conditions.
To start. Would cast Iron be a plausible candidate?
Thanks.
Darren Harris Staten Island, New York.
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On 2/5/2013 4:20 PM, Searcher7 wrote:

there's something like this on the market already, for not a very high price.
http://gigapan.com/
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It's a fool's errand to try and find a metal you can work with that won't expand and contract with changes in temperature.
Instead, you should be 'focusing' on a design where the various dimensional changes don't affect the aim angle. For instance, where the camera pivots at the exact centroid of all the expansion or contraction.
That, and a solid base, like granite, to hold all the other stuff.
Even granite changes dimensions with temperature... <G>
LLoyd
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On Feb 5, 6:39 pm, "Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote:

e
LLoyd is correct. Theodolites are not made of metal that do not expand or contract.
Dan
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On Tue, 5 Feb 2013 15:52:04 -0800 (PST), jon_banquer

He's not trying to make a CMM, more like an index or dividing head.
There's no engineering reason to use an exotic material with a low CTE. There's plenty of reason to look at runout in bearings and that sort thing.
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On Wed, 6 Feb 2013 11:30:58 -0800 (PST), jon_banquer

I gave him feedback from a licensed engineer.
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I haven't gottena all of the particulars worked out yet, but I am working on designing a camera stand and trying to decide on the best material to make most of it out of.
I'm not trying to reinvent the wheel, but a special project makes it necessary to be able to take a series of as many as 180 equally spaced photos across a 45° swing and be be able to do the same again without any perceptable deviation.
In other words if I take a picture at 12-1/4° and then continue turning the camera all the way to the end, when I return the camera to the same spot I took the photo I will need to be able to stop and lock the camera so that the photo next photo I take there is *exactly the same as the previous one I took.
This is not plausible with conventional camera stands. So I'm trying to decide on the best metal to make the moving parts out of. A metal that will best retain it's dimensions under changing temperature conditions.
To start. Would cast Iron be a plausible candidate?
Thanks.
Darren Harris Staten Island, New York.
================================= Having experience building telescope mounts, I can say your main issue will not be thermal expansion. Instead you will need to worry about stiffness of the rigid structures, repeatability of the detents, stability of the rotation axis, and quality of the bearings. You will probably need a shaft of some length with bearings at either end.
If a metal structure is heated or cooled uniformly, that does not affect the angular dimensions on the part.
Aluminum is fine for telescope mounts and it should be fine for what you are doing.
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On Tue, 05 Feb 2013 15:20:56 -0800, Searcher7 wrote:

Well, if you really mean exact, then it can't be done.
Off by one pixel? Off by ten pixels? What's _really_ acceptable? And, how much angle does a pixel subtend, on your system?
If I were going to try this, if I were going to make just one, and if I felt my time was worth anything at all, I'd base things on a rotary table. You can get them with scary accuracies, if you're willing to spend money on it.
Then, depending on the accuracy I _really_ needed, I'd decide what I needed to mount it on.
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Is the camera itself stable? how big is it, how long are the exposures, how much does the camera weigh, what time frame does it take to take 180 exposures?
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Use a 5 axis cnc stage.
http://en.wikipedia.org/wiki/Motion_control_photography
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On Tue, 05 Feb 2013 15:20:56 -0800, Searcher7 wrote:

You probably *are* trying to reinvent the wheel. A rotary table or an index set probably would work with little modification or adaptation needed. See for example items in ebay Rotary Tables -- <http://www.ebay.com/sch/Rotary-Tables-/41943/i.html -- in particular, <http://www.ebay.com/itm/Shop-Fox-M1077-4-Rotary-Table-with-Divider- Plates-New-in-Box-/150985935956? pt=BI_Tool_Work_Holding&hash=item2327768c54>, <http://www.ebay.com/itm/Dividing-plate-for-6-8-10-and-12-rotary- tables-/180856888044?pt=BI_Tool_Work_Holding&hash=item2a1be91aec>, <http://www.ebay.com/itm/NEW-PRECISION-MINI-ROTARY-TABLE-3-75MM- HORIZONTAL-VERTICAL-/170983650642? pt=BI_Tool_Work_Holding&hash=item27cf6b7552>.
The first of those is most likely to work conveniently and reliably as-is, but costs $259.00 shipped. The table itself has a 1:72 ratio, and the indexing plates appear to have 18 different counts, which should cover all but about a dozen of the counts-per- rotation up to several hundred CPR. The second item, at $80.90 shipped, is just two index plates, with 22 different counts (5 and 6 tracks on top and bottom of each of two plates) and to use the plates you would need to make a chuck (ie a camera carrier), an index pin, and a sector arm. The third item, at $60.00 shipped, is just a small cheap rotary table with a 1:36 drive ratio and 15' (1/4 degree) graduations on the hand wheel. It would be accurate enough to repeat well (as long as you approach each position from the same direction, ie if you compensate for backlash) but much more tedious to use than an index set. Also see wikipedia's Indexing Head article: <http://en.wikipedia.org/wiki/Indexing_head
--
jiw

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This could be as simple as an index plate with raised pins and an arm on the rotating camera mount that stops against them. As long as the plate and arm are made from the same metal they will both expand identically with temperature without changing their relative shapes, the same as a photographic enlargement of a watch face that retains the 30 degree angle between the hour marks no matter how big it is..
You could lay out and drill the plate by hand and the camera positions will still be as repeatable as you are careful, even if the angles between positions varies a tiny bit.
jsw
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For an exact answer to your question, look up Invar. It's an alloy developed for watch and clock making. However, if you need exact repeatability, it's going to cost. Such things CAN be done, but need lasers and interferometers as well as a pretty massive pier going down to bedrock. Active computer controlled positioning is the key against a stable reference surface. I've seen such setups in university optical labs, temperature control becomes an issue, even with Invar. Air density changes as well as the optics involved move with temperature. Just depends on whether a thousandth of an inch is sufficient or a fraction of a wavelength of light. How exact are your requirements? How deep is your wallet?
Stan
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On Wed, 6 Feb 2013 23:22:58 -0800 (PST), Stanley Schaefer

That's a 3 second shot with a fast movie camera, Darren. Match up individual frames from the sequences using most any video editing software. An electric motor should pan that for you pretty easily. (What on Earth requires a quarter degree turn photo?) Perhaps a motorized equatorial telescope movement would work for you. Some are GPS guided, which would ensure your exact placement.

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Sorry for the delay in getting back to this thread. I lost my internet serv ice and can only get access on my trips to the library.
I know all metals will expand and contract with changes in temperature. (Di mensionally stable being relative, of course). I don?t think that I?m t rying to reinvent the wheel here, because nothing mentioned will work for m e. No stand and no tripod head that is already in production.
On Tuesday, February 5, 2013 7:30:34 PM UTC-5, anorton wrote:

without

Yes. That?s exactly what this is. :) But I made an error. The camera will have to cover 90° of the horizon. (Not 45°).

That wouldn?t work, because this would have to be manual (and simple). I think it is easier than having a motor stop the camera at each interval and then wait for any vibration to dissipate before taking the next picture. ( Each frame must be the equivalent of a high definition image).
To elaborate further, I want to make what would amount to a 3 second video, but I need to have *complete control* over each individual frame. I can th en manually pan *at various speeds*. Only 1/3 of the 90° of horizon will be viewable at any one time. This of course is going to take software not y et written. (And I haven?t even mentioned the other effects I?ll need t o incorporate).
This would be a lot easier if there was a panoramic video camera that had a lens that would cover 90° and not have the customary distortion. Then I wouldn?t have to worry about panning.
Thanks.
Darren Harris Staten Island, New York.
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snipped-for-privacy@mail.con.com fired this volley in

Not all. Cerro-Tru expands only 0.0002 in./in. from molten to solid. <G> LLoyd
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"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> fired this volley in

My fingers autonomously hit return before I finished...
(then, after Cerro-Tru...),and some bismuth/cadmium-containing alloys have essentially zero temperature coefficient between molten and solid.
LLoyd
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And this boom doesn't sag, vibrate or expand with temperature?
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On Wednesday, March 13, 2013 5:59:26 PM UTC-4, Jim Wilkins wrote:
need to be >at the end of a boom. The pivot radius will probably >be in th e neighborhood of 12 feet. (I don?t know the >mathematically formula off hand, but isn?t that almost >14 feet between the far right and far left i mages taken?). >Darren Harris >Staten Island, New York. And this boom doesn 't sag, vibrate or expand with temperature?
Yes, of course. That is why it will have to be supported at both ends and t he reason for the initial question concerning what material to use.
The reason for the boom is to keep the realistic 3d aspect of the scene swe ep.
The detents whould probably be where the camera is as opposed to the axis l ocation.
Darren Harris Staten Island, New York.
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On Fri, 15 Mar 2013 12:34:04 -0700, Searcher7 wrote:

It would save time and you will get more and better help if you write a clear and complete description of what you are trying to do and the general idea of how you plan to do it. Do you have some reason to not describe what you are trying to do and the general idea of how you plan to do it?
At the moment it appears you plan to have a horizontal 12-foot boom pivoted at one end, and a quadrant of a 12-foot curved ring with detents to set boom positions to quarter-degree resolution, and there will be a camera mounted on the boom above the detents ring.
Will the camera point in, or out? If it points out and you plan to make ordinary panoramic pictures, the center of perspective in the camera should be located directly above the center of rotation. See the section called "Capturing the images" in [1] or "Using a panoramic tripod head" in the second half of [2] or the section "BACKGROUND: PARALLAX ERROR & USING A PANORAMIC HEAD" in [3].
If it points in and you plan to make Matrix-style photo or slo-mo of a moving subject as in [4] then accurate time synchronization probably is more important than physical location, although few details are available publicly.
[1] <http://www.cs.washington.edu/education/courses/cse131/12sp/applets/projection.html [2] <http://commonsensephotography.com/how_to_take_digital_panoramas/index.php [3] <http://www.cambridgeincolour.com/tutorials/digital-panoramas.htm [4] <http://smokingstrobes.com/matrixring
--
jiw

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