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Pinbout
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Re: Thinking of making a flat... new [Re: DAVIDG]
      #6370294 - 02/11/14 11:46 AM

Quote:

Advanced Telescope Make Technique- Mechanical on how to test a non polished surface for flatness using a large prism or flat strip of glass and some organic solvent like Acetone. So now your only grind two surface and the "tool" can be dental stone one.




Dave,

I have a 10" blank that zero's out on my 5" spherometer. I wanted to check if it was a throw-away from a 3 disc grind. I got it with a bunch of old grinds from some club that want to get rid of old projects that never happened.

could I bring it to delmarva so you can demonstrate it?


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Biff
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Re: Thinking of making a flat... new [Re: Pinbout]
      #6478220 - 04/20/14 11:01 PM

Picking this thread up again, Turn out I am relocating (again) so I won't be able to start on this for another few months at least. However I decided to go the three flat method, not saying I think it's a better method than the water test, it's just an avenue I wanted to explore. So I just finished ordering up three 8" x 1" quartz blanks for this project.

I'll need to grind/polish all 6 surfaces - so I can see through them for the tests. I've read the order in which the surfaces should be ground against each other when working 3 surfaces but I'm wondering if there is a procedure or order that should be followed when using 6 surfaces?


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Pinbout
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Re: Thinking of making a flat... new [Re: Biff]
      #6478656 - 04/21/14 09:19 AM

here's the mechanical test dave was talking about. Dick Parker set up my flat in question so you can see its not mechanically flat. so now I won't feel guilty making a f4.5 out of it.

https://www.youtube.com/watch?v=Hi8lQ8CFAkU


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dan_h
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Re: Thinking of making a flat... new [Re: Biff]
      #6478741 - 04/21/14 09:56 AM

Can't say I have any experience at all with this but I would think that it would be easiest to treat the project as two sets of three surfaces. It would certainly keep the arithmetic to manageable proportions and you can't test the surface on one piece against the other side of the same piece. Easier yet would be to beg, borrow or steal a reference flat.

Where's you new home going to be?

dan, oshawa


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DAVIDG
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Re: Thinking of making a flat... new [Re: Biff]
      #6478791 - 04/21/14 10:34 AM

Quote:

I'll need to grind/polish all 6 surfaces - so I can see through them for the tests. I've read the order in which the surfaces should be ground against each other when working 3 surfaces but I'm wondering if there is a procedure or order that should be followed when using 6 surfaces?




Your making things more difficult for yourself then need be. If you use the "acetone" test as shown in Danny's video, your only grinding two surfaces and one them doesn't need to be glass. You can get the glass surface that will become the flat, to a flatness of about 1 wave using this test. Then you can use a smaller master flat and/or the water test to finish the flat.

- Dave


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MKV
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Re: Thinking of making a flat... new [Re: Biff]
      #6478792 - 04/21/14 10:35 AM Attachment (20 downloads)

Hi Ryan,

I made my first flats in the early 1980's using the three disk (so-called ABC) method, and have used it ever since. As far as the order is concerned, just follow the same procedure on the back as on the front (A on B, B on C, Con A and for the the other side D on E, E on F, F on D). This will assure that all six sides will be about 2 weaves flat at the end of grinding.

Polish the back first. The back doesn't have to be super polished; just polished enough to see through it. Make sure the disk have good beveled edges. You may wish to remove any wedge on the blanks before starting.

The picture below shows my three 10-inch flats. One is for autocllimation, one is a "master", and one is for general use (i.e. testing the backs of pcx lenses, zeroing your spherometer, testing objective lenses, etc).

A freeware program called Three Planes will make judging the fringes easy. Otherwise just follow the simple arithmetic described in the books.

Making three flats is never a waste of time or effort. You could damage one (it happens!), and you'll have spares. Or you can sell the other two and more than recuperate the cost of the blanks and material several times over.

Good luck!

regards,
Mladen

Edited by MKV (04/21/14 10:44 AM)


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Biff
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Re: Thinking of making a flat... new [Re: dan_h]
      #6480304 - 04/22/14 12:16 AM

Quote:

Where's you new home going to be?

dan, oshawa




Hey Dan! How are things? Surprisingly we are moving BACK! so you'll probably start seeing my face at the meetings again. Pffft tired of all these palm trees and warm sunny weather anyway. I hear I missed a hell of a winter there.

Mladen, thanks for the info. I plan to keep two, use one and the other a spare. If they turn out well I'll hopefully be able to break even on the project or get close enough by selling the third (assuming they turn out well enough of course). Worse case I got a nice starting point to make another mirror.


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Mark Harry
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Re: Thinking of making a flat... new [Re: Biff]
      #6480756 - 04/22/14 08:45 AM

Not sure I understand the acetone test in the video. it only shows a certain fit, which -CAN- ocurr with either flat, or curved surfaces....UNLESS the comparative piece is a TRUE flat...correct?
M.
****
I cheat at all this. I use a spherometer to get within a couple fringes of flat, then I use a reference flat to tweak it in. Fewest surfaces to make, least amount of work.


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DAVIDG
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Re: Thinking of making a flat... new [Re: Mark Harry]
      #6480853 - 04/22/14 09:47 AM

The curvature of the acetone tells you if the surface is concave or convex. In the video it is showing a concave surface since the acetone/air interface is concave. A piece of float glass has a surface that is flat enough. So if you were grinding the piece of glass in the video to make it into a flat surface you would place the tool on top and grind until the acetone/air interface was straight. Now the mechanical flatness is good to about 1 wave and you can start to polish from there. It is also a useful test in judging the flatness of machinist granite gauge blocks or metal gauge blocks.
The other method that I have used is to use a spherometer as well. I zero the spherometer on the piece that is going to be the flat and then test the tool. I keep zeroing the spherometer on the one piece as I grind and testing the other until the spherometer reads zero deviation. At that point I'm again mechanically flat to within a few waves and I polish from that point to make it optically flat.
Both of these methods have an advantage in both time and money. One only need one piece of glass, which is the one that is going to become the flat and your only polishing one surface.

- Dave


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MKV
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Re: Thinking of making a flat... new [Re: DAVIDG]
      #6481237 - 04/22/14 01:28 PM

Dave, just curious, how was it determined that the acetone test is good to about one wave? Has anyone tested two known surfaces with a 1 wave gap between them, and how much acetone curvature can one expect at 1 wave?

I also think Mark Harry made a valid observation that (in absence of a master flat) just testing two surfaces with acetone (or by contact fringes, for that matter) doesn't tell you which surface is what. That's why you need a third surface.

So, in absence of a master flat, the acetone test -- if it's truly that sensitive (i.e. to 1 wave) -- should also be performed using three, and not two, pieces of glass. This is because both tests using two glasses only indicate there is a space between the surfaces, but they don't tell you anything about which surface contributes what to that space.

Assume, for the sake of argument, that you make an acetone test and the two surfaces (A on B) test -2 waves (concave) . What conclusion can you draw from that about each surface? Which one is concave, and how much, or which one is possibly flat?! You can't determine this using only two glasses.

But if you then introduce a third piece of glass and you obtain the following results

A on B = -2 waves (concanve)
A on C = + 1 wave (convex)
B on C = 0 (flat)

After you do the math, this tells you that A is 0.5 waves concave, B is 1.5 waves concave and C is 1.5 waves convex.

Using only two glasses you can assume only that either A and B are 1 wave concave or that one surface is flat and the other 2 waves concave, which is totally misleading!

To use a spherometer, it needs to be almost as big as the flat diameter, and if it accurately reads to 0.0001" (which is another topic), it will read to no better than 5.5 waves.

The good news is that, for autocollimation flats, a "flat" can be many waves concave or convex*. Even if testing something as fast as an f/2 mirror by the AC method, the "flat" can be as many as 7 waves concave/convex -- or 3.4F˛/D˛ fringes off (where F is the focal length of the mirror, D its diameter, and a fringe = 1/2 wave).

Unless you plan on testing surfaces faster then f/2, any blank that's less than 7 waves concave/convex will do -- as long as it turns out to be a smooth figure of revolution. Which means: the acetone test, even with just two glasses is good enough.

regards,
Mladen

*See C. R. Burch, Tolerances permissible in flats for autocllimation (1938)


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DAVIDG
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Re: Thinking of making a flat... new [Re: MKV]
      #6481444 - 04/22/14 03:08 PM

Mladen,
Both myself and Dick Parker have checked the results of the test and it works just like the article states. Dick has made at least 3, 12" flats and one 16" using this method to grind only one surface. I also belive a couple of students in his mirror making class that he teaches he year up in New England has used this method as well.
The purpose of the test is to allow one to check a ground surface that won't allow interference testing and to get it flat enough that once you start to polishing you can be close enough to make it optically flat. It is not to determine the exact flatness. So as I stated it allows you to grind only one surface and check it to determine if it is concave or convex as your grinding it and adjust the grinding technique accordingly. To me this save both time and money since I'm only working one surface. Once you get it mechanically flat ins which the surface is within a few waves you then polish it and start the typical optical testing, via contact interference against a Master flat and/or the water test or using spherical mirror.

- Dave


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MKV
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Re: Thinking of making a flat... new [Re: DAVIDG]
      #6481555 - 04/22/14 04:08 PM

Hi Dave, I agree, the acetone test serves well its purpose for non-polished surfaces. So does a spherometer, but it's much easier to get acetone than a spherometer. And, I think just grinding the tool and the flat, and alternating them top-to-bottom, will produce a sufficiently flat surface. Progress can be checked with a good machinist's straightedge and a bright light. One can be pretty sure that when no light is showing underneath the straightedge, the surface is more than flat enough for autocllimation purposes.

The way I look at it, making tools is a one-time deal in a long while. So why not go the extra mile and go with three disks, and get one spare, one master and one coated out of it, or sell two for a good profit. A set of flats will last you a liftetime. I don't see a need to cut corners when setting up a well tooled workshop. And the experience is another thing. If we look only at how quickly we can start using optics, or how much work is involved in making optics, why not just buy a flat, or a telescope? But, as always, let everyone decide what suits them best. This is just informational to show different aspects of different approaches.

regards,
Mladen


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Pinbout
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Re: Thinking of making a flat... [Re: MKV]
      #6481708 - 04/22/14 05:31 PM

Quote:

 This is just informational to show different aspects of different approaches. 




And that's why I posted the vid. It doesn't get talked about that much. Most go right to the three disc method. And with the price of supremax33, why spend money...I know I don't have.


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DAVIDG
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Re: Thinking of making a flat... new [Re: Pinbout]
      #6481781 - 04/22/14 06:23 PM

Mladen,
As the name states, the three disks method requires three disks and it requires grinding and polishing all of them if you want to make three good flats. If you only want to make one flat surface it still requires grinding three surface and two of which won't be used.
I would rather save both the time and money by only grinding and figuring one surface. The time and money I save can be used to make a much larger flat, which to me is more useful then 3 smaller ones.
If one is going to go through the time to make three flats via the three disk method then one should use three disks of glass of the correct material ie low expansion glass of the correct thickness. With the cost of 10" disks of low expansion material costing about $250 each, and 12" disks cost around $500 each that it fair amount of money when you need to purchased three of them. I personally find having one large autocollimation flat much more useful then three smaller flats so I would spend the money to get one large piece of glass and the acetone test will allow me to grind that piece to a mechanical flatness that then can be optically polished flat.
To each his own.

- Dave


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Ed Jones
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Re: Thinking of making a flat... new [Re: DAVIDG]
      #6482094 - 04/22/14 09:13 PM

Quote:

The other method that I have used is to use a spherometer as well. I zero the spherometer on the piece that is going to be the flat and then test the tool.



I've used this method and it's quite sensitive with a good spherometer gauge. I zero on the tool an read the flat which doubles the difference as well. It's better and cheaper than the 3 part method IMHO.


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Pinbout
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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6482125 - 04/22/14 09:25 PM

for my 10", I used a 4.75" dia spherometer and I guess I didn't move it around enough to see the side rising up. it would've helped to have one the same dia.

with the acetone you can see the shape and where it starts to rise. no measuring and then guessing where it deviates from flat.


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MKV
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Re: Thinking of making a flat... new [Re: Pinbout]
      #6482427 - 04/23/14 12:34 AM

Quote:

for my 10", I used a 4.75" dia spherometer and I guess I didn't move it around enough to see the side rising up. it would've helped to have one the same dia.




Danny, if you're going to use a spherometer to assess the flatness of a disc, not only should it be nearly as large as the disk itself, but it should read to 0.0001" or preferably better (0.0001" is about 5 waves or 10 fringes)!

If the acetone test is truly 5 times more accurate, as claimed, then it beats the spherometer hands down. I would like to read more about this test.

regards,
Mladen


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Mark Harry
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Re: Thinking of making a flat... new [Re: MKV]
      #6482734 - 04/23/14 08:13 AM

I get to within a couple fringes, and my sphero is 5" and graduated to only .001" between divisions. -Flat- lies exactly a half-mark (.0005") above zero on this particular tool.
I see that the acetone test requires use of a reasonable flat. That was my original question.
But I have various plate glass in various thicknesses. Blue tinted, and water white; along with a couple pieces of B270.
Absolutely -NONE- of it is what I would call REASONABLY flat. Not by any stretch. I can get a far better indication of flatness with the sphero, period. My 2 cents. (Of course, I'm totally familiar with it!)
M.


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DAVIDG
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Re: Thinking of making a flat... new [Re: Mark Harry]
      #6482895 - 04/23/14 09:42 AM

I agree that one can get a more accurate reading of the exact flatness using spherometer but that is not the point of the acetone test. It will allow one to grind only one surface and get it mechanically flat enough to be polished optically flat. It doesn't require fabricating a precision spherometer and it doesn't require an optical flat of known flatness to zero the spherometer on. A cheap piece of relativity flat glass and $5 worth of Acetone from the hardward store is all that is needed. It is a "close enough" test.

- Dave


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MKV
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Re: Thinking of making a flat... new [Re: Mark Harry]
      #6483005 - 04/23/14 10:31 AM

Quote:

I get to within a couple fringes, and my sphero is 5" and graduated to only .001" between divisions. -Flat- lies exactly a half-mark (.0005") above zero on this particular tool.



Mark, a surface that departs flatness by 0.0005" is 25 waves or 50 fringes deep. In order to read flatness to a couple of fringes, your spherometer should be able to read to one wave, or 0.00002".

regards,
Mladen


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