Dimensionally Stable Metal

there's something like this on the market already, for not a very high price.

Searcher7 fired this volley in news:370407bd- snipped-for-privacy@k14g2000vbv.googlegroups.com:

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...

LLoyd

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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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.

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.

LLoyd is correct. Theodolites are not made of metal that do not expand or contract.

Dan

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?

Use a 5 axis cnc stage.

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 -- -- in particular, , , .

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:

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

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.

I gave him feedback from a licensed engineer.

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

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.

A Haas indexer would do the job nicely.

And you can program it to whatever positions you need.

The methodology of the left has always been:

1. Lie
2. Repeat the lie as many times as possible
3. Have as many people repeat the lie as often as possible
4. Eventually, the uninformed believe the lie
5. The lie will then be made into some form oflaw
6. Then everyone must conform to the lie

I didn't see the OP mention if this panoramic camera has to survive outdoor weather that will ruin an expensive indexer. jsw

I worked for a company that had a ruling engine for diffraction gratings. Gratings could be ruined from vibrations caused by heavy trucks going by (this was in the company's former location). Besides building sway, weather, and all the other complications I think that the wheel is being re-invented here. To start:

Any good indexed panorama tripod head will already do this. The Manfrotto 303 comes to mind

My favorite is to "fix it in software" If there's a fixed reference item in frame, can't you just align it in software?

That sounds like something I'd look at, but it seems that the OP is looking for unconventional solutions to a problem that is not well described. Without knowing the variables (and their relative importance), constraints (including budgetary ones), and what has been already tried, it's hard to understand the problem or give advice. Whatever he is trying to do, I wish him luck.