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Biff
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Thinking of making a flat...
      #6288791 - 01/03/14 12:04 AM

I'm itching to push some glass but don't want to commit to any construction projects right now so I'm thinking of getting into making a reference flat. I've read Texereau's three flat method as well as Ed's water test method. Both are well documented, however I have a couple questions about each...

With the water method, a collimated source of light is needed but how can I make this source of collimated light and how accurate does it need to be. I haven't decided on a flat size yet but say I go for 8", if I need to buy a precision lens at least that size to get my collimated light then that could be a real issue. Especially if it needs to be a significant amount more accurate than the flat I want to make.

The Texereau method uses testing one piece next to another - basically comparing errors. I'm sure it works out in the end but it doesn't give me a warm fuzzy having to judge how good my end product is by comparing it to a potentially imperfect reference.

So which is the best way to go to make a flat that I can be confident about it's quality?


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Pinbout
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Re: Thinking of making a flat... new [Re: Biff]
      #6288817 - 01/03/14 12:43 AM

You can always search eBay for a comparator mirror which maybe flat to some degree and try to make it flatter. Save all that grinding. I have an 8" that has an edge issue otherwise it would be nice for a ref for double pass tests. Surplus Shed has 3" flat that you can subdia test.

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kfrederick
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Re: Thinking of making a flat... new [Re: Pinbout]
      #6288977 - 01/03/14 03:58 AM

You could make three good primarys in the time of a flat not easy to get it very flat What you plan on needing it for ? Be better to spend time polishing a flat than watching TV I see the fun .. all good I do think it harder than a primary

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ccaissie
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Reged: 09/13/10

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Re: Thinking of making a flat... new [Re: kfrederick]
      #6289133 - 01/03/14 07:44 AM

Needs to be the exact shape (flat) AND the right radius (infinite).

Check out Advanced Telescope Making Techniques Vol 2. Articles by Browne and Cox show the use of a smaller spherical mirror, (Ritchey-Common) which I have determined works well. Still, I used three 8" disks for the grinding, and a precision straightedge.

As Kfrederich asks, what's it for? For autocollimating, it needs to be smooth, not exactly flat.

A smaller ref flat will be useful in checking edge via fringes. Maybe you can borrow a flat?

C


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Ed Jones
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Re: Thinking of making a flat... new [Re: Biff]
      #6289181 - 01/03/14 08:34 AM

Ryan,
For the water test the collimator doesn't need to be anything precision, I use the center section of a fresnel lens from an old big screen projection TV and it works just fine. I would avoid too fast an F number lens as many fresnels are however. You could use a plano-convex lens or a telescope mirror as well, SA doesn't matter.


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Biff
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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6290391 - 01/03/14 08:06 PM

I don't have an immediate need for a flat... was just sort of a make work project and I think it would be handy to have a reference flat from time to time. Eventually though I would like to make my own secondaries which would require one as well.

Ed, thanks for the info. Is it necessary or usefull to use a slanted piece of 50/50 glass so the fringes are viewed 'on axis' similar to other light boxes I've seen people build for interference testing? Also what would you consider to be a cutoff point for f/# if I happen to come across a fresnel?

Thanks


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Ed Jones
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Re: Thinking of making a flat... new [Re: Biff]
      #6290666 - 01/03/14 10:41 PM

Ryan,
Light boxes with slanted glass don't usually have a collimator which is required for the water test. However if you are making a flat for an autocollimator you can use a light box. Without a collimator a true flat will show some rings of power depending on the water thickness, viewing distance and mirror size. You could use fringe analysis software to correct to absolute flatness but a collimator is easier.

For testing diagonals you wouldn't need so big a collimator. How fast is too fast? Not really sure, I'd like 4:1 but maybe faster would still work?


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Pinbout
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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6290927 - 01/04/14 01:11 AM

$50 4" 1/20~ ref flat? that would be a steal.

http://www.bmius.com/p-30630-optical-flat-4.aspx


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MKV
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Re: Thinking of making a flat... new [Re: Biff]
      #6291044 - 01/04/14 03:11 AM

Ryan, you don't need a collimating lens if you build a simple flat viewer as described in ATM books. All you need is some a box with a 45-degree window pane beam splitter, as described here:

http://www.turbofast.com.au/astrotel/flattester.html

As I said, you can test with a flat without the colliamitng lens if you observe at a distance equal to at least 10 diameters of the mirror being tested. Thus, for a 10-inch mirror, a distance of 100 inches or 2.5 meters (or 8 feet). To observe the fringes, use a camera with a telephone lens, or a small finder scope.

The collimator only allows you to view the fringes closer, by removing the parallax effect which is made negligible by increasing the viewing distance.

regards,
Mladen


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Ed Jones
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Re: Thinking of making a flat... new [Re: MKV]
      #6291250 - 01/04/14 08:21 AM

Mladen,
Unfortunately you are correct only for contact testing of flats. Not using a collimator for the water test would result in a non-flat optic unless you do fringe analysis.


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DAVIDG
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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6291646 - 01/04/14 11:43 AM

Another thing that makes, making a flat so much easier is a "pressing flat". This is a flat that you use to press your pitch lap against to make it flat. Your trying to make an optically flat surface so if you already have a flat to make your lap, flat the glass will wear much faster to that shape. So you press for 10 minutes and polish for 10 minutes. "Pressing flats" are usually an old flat that the surfaces is scratch up.

- Dave


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greenglass
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Re: Thinking of making a flat... new [Re: DAVIDG]
      #6298269 - 01/07/14 02:50 PM

I recommend using a round blank then cutting it elliptical if needed. I'm starting to hate working with (Newtonian)ellipses. The sides aren't even vertical. How much is gained using an ellipse than a round one? 3% light loss and contrast?

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Pinbout
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Re: Thinking of making a flat... new [Re: Biff]
      #6298330 - 01/07/14 03:27 PM

a 6.5" x1" thk flat for $89

you can test and return it in 14days if its not flat...

it could always be a good pressing flat.


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GlennLeDrew
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Re: Thinking of making a flat... new [Re: greenglass]
      #6298450 - 01/07/14 04:26 PM

Quote:

I recommend using a round blank then cutting it elliptical if needed. I'm starting to hate working with (Newtonian)ellipses. The sides aren't even vertical. How much is gained using an ellipse than a round one? 3% light loss and contrast?




Far better to start elliptical, and block up pieces of 'junk' glass surrounding it, making a rough circle of larger diameter. The buffer thus provided eliminates turned edge. It's rather easier to get good flatness on a *portion* of a larger construct. It might be initially attractive to cut out your piece from a larger single one, but the risk of damage is present, not to mention the possibility of some strain relief and hence warping.


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MKV
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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6298459 - 01/07/14 04:33 PM

Quote:

Mladen,
Unfortunately you are correct only for contact testing of flats. Not using a collimator for the water test would result in a non-flat optic unless you do fringe analysis.



Ed, you're right I should have made that distinction, but since I don't use liquid water flats (I find the test too "fluid" if you know what I mean) I didn't think of it.

regards,
Mladen


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Ed Jones
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Re: Thinking of making a flat... new [Re: MKV]
      #6298656 - 01/07/14 06:37 PM

The fringes do wiggle around but it's not a big deal. Take a look at this video of a 4 1/4 diagonal tested on the water test. It doesn't detract from the usefulness of the test.

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Ed Jones
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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6298663 - 01/07/14 06:42 PM

I should have added that if you intend to use the flat for auto-collimation testing then a light box will work just fine. If you view from some distance away the few fringes that may result wouldn't be an issue.

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JohnH
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Re: Thinking of making a flat... new [Re: Pinbout]
      #6298671 - 01/07/14 06:53 PM

Quote:

a 6.5" x1" thk flat for $89

you can test and return it in 14days if its not flat...

it could always be a good pressing flat.




This is most likely an optical window, not quite the same as a flat


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Pinbout
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Re: Thinking of making a flat... new [Re: JohnH]
      #6298707 - 01/07/14 07:19 PM

maybe, you have 14days to find out.

I got a 4" piece of glass for a laser and its at least 1/8~ for $40.


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MKV
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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6299283 - 01/08/14 02:36 AM

Quote:

The fringes do wiggle around but it's not a big deal. Take a look at this video of a 4 1/4 diagonal tested on the water test. It doesn't detract from the usefulness of the test.



Ed, I can't tell which is the true shape of the fringes, as they move incessantly. It's not about usefulness, but preference. If I didn't have a reference flat, I would use the Ricthey-Common test with a spherical mirror simply because I prefer a steady image. .

regards,
Mladen


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Pinbout
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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6301002 - 01/08/14 10:14 PM

Quote:

The fringes do wiggle around but it's not a big deal. Take a look at this video of a 4 1/4 diagonal tested on the water test. It doesn't detract from the usefulness of the test.




Ed, So is that flat convex?


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ccaissie
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Re: Thinking of making a flat... new [Re: Pinbout]
      #6301051 - 01/08/14 10:47 PM

Back to square one....

You were looking for a glass project, how about a reference sphere?

Just like in toolmaking, you make your own tools, and build from first principles. First a sphere..can test it reliably to impeccable quality and then use it to make a larger flat of impeccable quality...to make a paraboloid of impeccable quality, etc. How about a 6 or 8" sphere, f/12-15 of 1/40 wave? If nothing else it will make a fine planetary scope for someone.


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Pinbout
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Re: Thinking of making a flat... new [Re: ccaissie]
      #6301100 - 01/08/14 11:23 PM

Thats a nice workflow.

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greenglass
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Re: Thinking of making a flat... new [Re: MKV]
      #6304211 - 01/10/14 01:19 PM

my 2 inch flat was easy to make compared to the elliptical one I haven't got right yet.

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Ed Jones
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Re: Thinking of making a flat... new [Re: Pinbout]
      #6304386 - 01/10/14 02:37 PM

Quote:

So is that flat convex?



Actually it's been a while, I can't remember, I tested this flat for an Ohio Club. The important point was that it wasn't flat.


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Pinbout
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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6307765 - 01/12/14 10:03 AM

was wonderring if you can tell because of the bullseye

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Ed Jones
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Re: Thinking of making a flat... new [Re: Pinbout]
      #6307852 - 01/12/14 10:42 AM

A hill or a hole both look like a bulls-eye in an Igram, you have to change the tilt between the water and glass to tell them apart. A good flat will have straight evenly spaced fringes.

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DAVIDG
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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6308407 - 01/12/14 03:25 PM

You can also very slowly chance the level of the water and the fringes will move and tell you if the surface being tested is convex or concave. Dick Parker who uses the water test on regular bases discover that if he used a clay pot to hold the water, that the wicking action of the clay slowly changed the level of water and he could observe the fringes moving. If they moved inward the surface was concave and if outward convex.

- Dave


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ccaissie
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Re: Thinking of making a flat... new [Re: DAVIDG]
      #6316152 - 01/16/14 10:20 AM

Quote:

Dick Parker who uses the water test on regular bases discover that if he used a clay pot to hold the water, that the wicking action of the clay slowly changed the level of water and he could observe the fringes moving. If they moved inward the surface was concave and if outward convex.
- Dave




Elegant.


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greenglass
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Re: Thinking of making a flat... new [Re: ccaissie]
      #6364993 - 02/08/14 09:56 PM

Why isn't my 4 inch flat getting less concave with the tool on top? Same method as grinding?

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DAVIDG
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Re: Thinking of making a flat... new [Re: greenglass]
      #6366063 - 02/09/14 01:07 PM Attachment (30 downloads)

Take a good look at the shape of the pitch squares on your lap. It's all about equal wear to make a flat surface. If the pitch squares aren't the same size then your wearing more glass were they are larger and even thou your polishing with the lap on top, you can still be wearing more glass away in the middle and making the figure more concave.
Here is a picture of the flat surface of singlet lens and pitch lap I'm making out of red glass for a H-alpha solar telesope. The flat surface is almost done, just need to remove the little bit of turned edge.
- Dave


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Pinbout
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Re: Thinking of making a flat... new [Re: greenglass]
      #6367139 - 02/09/14 08:41 PM

Waite just uploaded a youtube about figuring a secondary

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greenglass
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Re: Thinking of making a flat... new [Re: Pinbout]
      #6368196 - 02/10/14 12:10 PM

http://www.youtube.com/watch?v=Vd6RiCjmzfE&feature=player_detailpage

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DAVIDG
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Re: Thinking of making a flat... new [Re: greenglass]
      #6368417 - 02/10/14 02:04 PM

Interesting video but he doesn't show the results or how he tests. Since he states he uses a Master to test. He would have been better off to press a pitch lap into his Master flat and use that to polish the diagonal. The pitch would take on the flat surface of the Master and the surface of the diagonal would follow the pitch. It is a common practice to use "pressing flats" to precondition the pitch lap when making flats and it greatly reduces the time required to "scrub" the surface optically flat.

- Dave


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Pinbout
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Re: Thinking of making a flat... new [Re: DAVIDG]
      #6368422 - 02/10/14 02:09 PM

Dave,

do you use gugolz 73 for flats also?


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DAVIDG
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Re: Thinking of making a flat... new [Re: Pinbout]
      #6368493 - 02/10/14 02:36 PM

Quote:

Dave,

do you use gugolz 73 for flats also?




Danny,
The pitch I use started out as #73 but I keep remelting and softening with olive oil to what I like and for the job I use it for. For flats I use pitch on the hard side so the stuff is most likely harder then #73.
Like the old saying goes thou. "Ask three mirror makers how to make a mirror and you'll get 6 different answers, all of them right"
All I can do is post what has worked for me in the past and try to include a picture or two to show the results so it is not just me saying it.

- Dave


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Mark Harry
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Re: Thinking of making a flat... new [Re: DAVIDG]
      #6370013 - 02/11/14 08:38 AM

I have 3 masters, and the rule is never use them to press out pitch. It's best to work a separate piece of pyrex or plate to do the pressing out. That way, the masters won't get screwed up.
***
My pressing plate is 1/2" thick by about 6" dia. plate glass, that can be weighed strategically to make a slightly convex or concave polisher to do a bit of tweaking. Works well for doing small pieces on a spotblock.
M.


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DAVIDG
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Re: Thinking of making a flat... new [Re: Mark Harry]
      #6370096 - 02/11/14 09:45 AM

I have 6 or more different Master's that I have picked up over the years. I have no problem pressing a lap into one of them and polishing away on the piece I'm working on. It saves me a ton of time and so far there hasn't been any change in the figure on my Masters or damage to surface. I'm not doing production optics so I only need to make a flat about once a year. It is more of a concern to me that I'll damage the surface of a Master doing the contact interference testing.
You can pick up a scratched and chipped Master on Ebay for good prices to be used as pressing flat so you don't have to worry about damaging the surface of a scratched up one. Even a 1/2 wave pressing flat will save a good amount of time in conditioning the lap and get the surface your working on close to be flat.
Like the old saying goes thou. "Ask three mirror makers how to make a mirror and you'll get 6 different answers, all of them right"

- Dave


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MKV
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Re: Thinking of making a flat... new [Re: DAVIDG]
      #6370235 - 02/11/14 11:10 AM

It pays to make three flats simultaneously using the ABC method and three blanks. It's more work but it has its rewards.

1) Ground against each other (all six surfaces) will automatically be flat to within a wave or so before you start polishing.

2) By pressing the lap against all six will produce enough randomness to maintain the flatness.

3) You don't need a master flat to determine accurately how flat each flat is.

4) Perforate one and aluminize it together one non-perfrated flat for autocollimation, and leave one un-aluminized for contact testing.

5) While you're at it, finish all surfaces flat, so that way you have six functional flats!

6) Of course, one can also sell two flats and recoup the money you spent on three blanks and materials, and then some. A decent double-sided 12.5 inch flat can sell for a good buck!

regards,
Mladen


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DAVIDG
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Re: Thinking of making a flat... new [Re: MKV]
      #6370283 - 02/11/14 11:40 AM

The three disk method works well but if you don't want to spend the money on three banks there is great article in 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. By looking at the curvature of the liquid between the two glass surface you can determine if the surfaces is concave or convex and adjust your grinding technique. I know of couple of people that have made excellent flats using this technique to grind the surface within a few waves of flat and go from there and start to polish. One of those flats is 16" in diameter and couple of them are 12" in diameter so using the three blank method would have been costly. The technique also works great for check machinist granite flats.
- Dave


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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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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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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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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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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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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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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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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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Re: Thinking of making a flat... new [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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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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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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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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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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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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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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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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Re: Thinking of making a flat... new [Re: MKV]
      #6483076 - 04/23/14 11:07 AM

Quote:

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)



Quite right. I use a Mahr gauge that resolves 1/2 micron with a large base. That's about 2 fringes.


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Re: Thinking of making a flat... new [Re: DAVIDG]
      #6483085 - 04/23/14 11:11 AM

Quote:

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.



Dave, the main advantage of a 3-disk method is that it favors flatness. You don't have to measure or test during grinding and lapping. Of the three disks, at least one should be a stable substrate that will be used as a flat; the others can be plate glass disks affixed to cement tools. They can be used later for making pitch laps for polishing and figuring of the flat itself.

So, the cost doesn't have to be excessive. And neither does one have to polish and figure all three disks! You can finish only one of the three, but if all three are stable substance, you can make parabolic mirrors out of them later on.

The stable substrate flat (for autcollimaiton flats, a Pyrex disk is stable enough) can be figured using using alternate methods other than the ABC reduction. You can use the Ritchey-Common test, with a long spherical mirror, as you hinted, or an interferometer, etc. but in either case, when you're figuring a single flat you need additional optics and/or equipment, such as a really good (1/20 wave RMS) long focal length sphere, which most people don't just have lying around.

So, short of borrowing one (chances are if someone has such a quality long focus sphere, he may also have a master flat...), you'll have to make a sphere, and that involves grinding and figuring another set of optics to a high precision -- and by that time you could have finished your three flats! The ABC method is costlier if all three disks are stable substrate, but if you make 3 good large flats you will more than recuperate the cost by selling just one of them.

regards,
Mladen

Edited by MKV (04/23/14 11:21 AM)


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Re: Thinking of making a flat... new [Re: MKV]
      #6483143 - 04/23/14 11:39 AM

Also, for those who're interested in testing unpolished surfaces for flatness, the pros use grazing interferometry, but there is also a simpler version called grazing Ronchi test.

Again, this is just informational, not a critique of other tests. There is no doubt that the acetone test is cheap, and easily performed. I have not found any source, outside of those already mentioned, that actually prove the accuracy of the acetone test being about 1 wave -- in which case it beats the spherometer hands down, except for the Mahr gauge; and a Mahr gauge, such as the one used by Ed Jones, is not cheap.

I was hoping that someone would offer an empirical proof of that claim, namely a test of a known 1 wave concave/convex surface tested by acetone and showing measurable curvature. But since an autocollimaiton flat can be many fringes concave or convex as long a sit has a smooth figure of revolution, it really doesn't matter if the acetone test has such accuracy. It may as well be 10 fringes off, it's still good enough for starters.

Mladen


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Re: Thinking of making a flat... new [Re: MKV]
      #6483169 - 04/23/14 11:46 AM Attachment (8 downloads)

Mladen,
One can use the water test which is cheap and very accurate vs making a sphere to test a flat. You don't have to a use collimated light source that is as large as the surface your testing when using the water test and scan over the surface without any loss of accuracy. I have a copy of an article from the Journal of the British Astronomical Society from the 70's that goes into the details and since that article was written I know of others that have used the water test using this method with excellent results. So between the acetone test and the water test I can make a larger flat in less time and less money then the three disk method.
I know that it has been done with excellent results because Dick Parker and Alan Hall made a 16" autocollimation flats to test their optics in their award winning twin 16" Cassegrains using these methods.
There are many ways to accomplish the same task. I'm just trying to shows a different method that others have used successfully.

- Dave


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Re: Thinking of making a flat... new [Re: DAVIDG]
      #6483277 - 04/23/14 12:38 PM

Dave, Ed Jones has a Youtube video which shows how the test looks. Personally, I would find that test very difficult to use, especially in the final figuring stages of a flat, when you do short figuring spells, long cooling spells, and short testing spells.

The tests seems to be a nice cheap alternative to testing finished surfaces, and the sensitivity seems to be about 3 fringes (judging by the video) or 1.5 waves. That's plenty accurate for an autocllimation flat, but it seems to take much longer to obtain the results then by other tests.

By the way, you can use the Ricthey-Common test with a very small mirror compared to the size of the flat; it's just that the test is most sensitive when the two surfaces are angled at 54°45' -- which dictates the optimum sphere size.

regards,
Mladen


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Re: Thinking of making a flat... new [Re: MKV]
      #6483768 - 04/23/14 04:30 PM

Quote:

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



***********
No it doesn't. With visual acuity (paying attention to what your eye resolves) and a magnifier, I can get well down in the single # fringes rather easily. Of course I've made thousands of them, so I'm sorta familiar in how to do such expeditiously. (practice makes perfect!)
M.


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Re: Thinking of making a flat... new [Re: MKV]
      #6483783 - 04/23/14 04:33 PM

If you re-read what I said, -FLAT- is EXACTLY @ +.00050; not +.00049 or +.00051" This difference can be easily seen with a moderately strong glass. (if your eyes are up to it)
M.


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Re: Thinking of making a flat... new [Re: Mark Harry]
      #6483835 - 04/23/14 04:55 PM

Mercury flats and water flats are very flat. Wonder how flat they are frozen ? https://www.youtube.com/watch?v=5I4rxfnCtxY&list=UUtESv1e7ntJaLJYKIO1FoYw

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Re: Thinking of making a flat... new [Re: MKV]
      #6483884 - 04/23/14 05:25 PM

Quote:

The tests seems to be a nice cheap alternative to testing finished surfaces, and the sensitivity seems to be about 3 fringes (judging by the video) or 1.5 waves. That's plenty accurate for an autocllimation flat, but it seems to take much longer to obtain the results then by other tests.



Mladen,
The water test is actually more sensitive than testing with a master flat since a fringe in water is only 3/8 wave rather than 1/2 wave in air. When using a master flat you still need the part and master to equilibrate to the same temperature and using the water test the part needs to set in water to equilibrate so there isn't much time difference. If you are making an autocollimation flat you can get away from needing a collimating lens and use a light box provided you observe as far away as possible and always observe from the same distance. It will be a few fringes from absolute flat however. You still need a collimator for making a master flat.


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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6484005 - 04/23/14 06:38 PM

With no collimating lens, at what viewing angle do ya need to be when your far away

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Re: Thinking of making a flat... new [Re: Pinbout]
      #6484091 - 04/23/14 07:16 PM

Quote:

at what viewing angle do ya need to be?


That's a ray trace problem. It depends on the size of the part and water thickness and how many fringes you can be off.

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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6484399 - 04/23/14 09:22 PM Attachment (13 downloads)

I forgot I had this graph, it shows how far away you need to be for a given number of fringes given a water thickness of 1 mm.

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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6484728 - 04/24/14 12:51 AM Attachment (5 downloads)

Quote:

If you are making an autocollimation flat you can get away from needing a collimating lens and use a light box provided you observe as far away as possible and always observe from the same distance. It will be a few fringes from absolute flat however



I have always tested flat sets (ABC) using the standard viewer (pictured below), from 25 feet, and with a telephoto lens. I am surprised this would still be a few fringes off due to parallax.

Edmund Scientific has 12x12 inch Fresnel lenses which may make satisfactory collimating devices.

regards,
Mladen.


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Re: Thinking of making a flat... new [Re: kfrederick]
      #6484839 - 04/24/14 02:29 AM

Quote:

With visual acuity (paying attention to what your eye resolves) and a magnifier, I can get well down in the single # fringes rather easily....If you re-read what I said, -FLAT- is EXACTLY @ +.00050; not +.00049 or +.00051" This difference can be easily seen with a moderately strong glass. (if your eyes are up to it)



That's not possible with digital readouts Mark. And one cannot go beyond manufacturer's accuracy rating and still claim accuracy. I don't know what equipment you have, so I'll take your word for it, but it's a little more than just visual acuity.

For those who don't have 0.001" dial indicator (0.001" = about 50 waves, or 100 fringes), each division is 1/16" (0.625") wide and the needle tip is 1/80" (or 0.0125") wide, so the needle tip fits inside the division exactly five times. That means each 1/5 of that division represents 10 wave or 20 fringes (letting wavelength = 0.00002" for practical purposes).

regards,
Mladen


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Re: Thinking of making a flat... new [Re: MKV]
      #6485020 - 04/24/14 07:22 AM

I do -NOT- use a digital readout.(It's a vernier; a very expensive one !!!) You're ASSUMING too much, and modify my statements to suit your own point of view, while trying to "preach to the choir". This "trolling" is an annoyance, and it clouds the facts.
***
ED JONES is precisely right; if you want to make a master, you HAVE to use a collimated light source, or make allowances for viewing angles not perfectly normal to the surface under test. His chart posted here should give you a clue.
Visualise looking straight down at the test. The thickness of the water/substrate will be minimal, and significantly less than if viewed at an angle.(being at an angle, the water/substrate will have an actual thicker layer for the light to travel through, which makes it act like a negative lens.) The thicker water/substrate distorts the fringe pattern and makes direct visual assessment difficult; particularly if you LACK EXPERIENCE with interpretation. To me this distortion makes the optic look as though it has a down edge, or oblate shape. With small pieces/references, I use a positive PCX lens placed right on top to get rid of this distortion.
If you're just trying to make an AC flat for typical Newt mirrors, or to check a telescope as a system, a fringe or two off won't hurt anything. But this isn't the case for -ALL- optical elements.
And if one is going to go through the trouble to MAKE a flat, and not just buy one to check so as to avoid the necessary work, it doesn't require much effort to make the piece truely flat instead of a couple or 10 or 100 fringes off. Visual assessment of regularity with a +- 10 fringe flat is coarse and crude, and +- 100 fringe flat is essentially impossible.
I'm fairly sure that most who have made any amount of flatwork will agree with this. I may have forgotten a point or two.
M.

Edited by Mark Harry (04/24/14 07:25 AM)


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Re: Thinking of making a flat... new [Re: Mark Harry]
      #6485206 - 04/24/14 09:46 AM

Mark,
Contact testing in air would be a much flatter graph. It depends on the air gap thickness. A collimator would be a good idea but I can't say it would be necessary either.


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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6485318 - 04/24/14 10:51 AM

Back when I was a poor student I made a 5 inch flat using a homemade spherometer. I cut a large triangle out of hardwood and put 3 aluminum nails in the corners, removed the heads and rounded them off with a file. I tapped a fine thread screw in the center and put in a brass screw, rounded the point off with a file and fitted it with a large wooden knob. These threads must be tight without any slop. To use it I would turn the knob so that when I pushed on a leg it would spin about the center. As I turned the knob it would eventually stop rotating on the center screw and move differently. I found you could find this cross over point with much sensitivity. I found this cross over point on the tool then checked my part and could always determine which part was more convex and successfully made my flat and cut it into a secondary mirror.

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Re: Thinking of making a flat... new [Re: Mark Harry]
      #6485377 - 04/24/14 11:31 AM

Quote:

I do -NOT- use a digital readout.(It's a vernier; a very expensive one !!!) You're ASSUMING too much, and modify my statements to suit your own point of view, while trying to "preach to the choir". This "trolling" is an annoyance, and it clouds the facts




Nothing was assumed. I specifically said that I don't know what equipment you have, but that I will take your word that you can read departure from flatness to a "single fringe #". If that is "trolling", "annoying", or "clouding the facts", then maybe -- unfortunately -- you may be reading too much into it.

Now, all 0.001" indicators I have seen have limited accuracy that is not even close to single digit fringe readings, but I believe that your very expensive 0.001" dial indicator is an exception, and that's why this is an issue that goes beyond just experience. Maybe it has something to do with a very expensive dial indicator. I would like to buy one just like yours. Can you tell me what make and model it is? Thank you, Mark.

regards,
Mladen

Edited by MKV (04/24/14 05:44 PM)


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Re: Thinking of making a flat... new [Re: MKV]
      #6487288 - 04/25/14 08:27 AM

Fowler; 0-6" range with replaceable spindles in 1" increments and I just double-checked the graduation range on a suspicion-

It's .0001", not .001". The coarse scale is .001"; my misquote.

So it's better than you & I thought.
But it doesn't go beyond experience at all. You should know better. -THAT- is always beneficial. That's why I don't get into prescribing aspirin or statins.
I will repeat, it's a VERNIER. Not a dial indicator, or any other -digital- readout device !!!
Have you ever read a true vernier scale? Are those flats the only ones you personally have made?
M.


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Re: Thinking of making a flat... new [Re: Mark Harry]
      #6487711 - 04/25/14 12:23 PM Attachment (5 downloads)

Mark, I checked Fowler's vernier depth gages (both with mircometers and dial indicators). I am confused how a dial indicator or a micrometer can have 0.001" and 0.0001" scales? I could only find vernier depth gages with 0.001" or 0.0001" resolution, and they weren't really very expensive, so these couldn't possibly be like yours. A model number, or a picture, would really be helpful, Mark. Thanks.

Other than that, in case someone is interested, there is a nice 0.0001" Fowler dial indicator on eBay, selling for $50!I have an old indicator (shown below), with 0.05" travel and a 0.0001" scale, for which I paid $30 a few years back. Please note that dial indicators with 0.0001" scales will have a short spindle, around 0.05" in all; not one inch or longer.

Spherometer heads typically have longer spindles and may be more suitable for spherometers with that kind of accuracy. An example of such a device is Mitutoyo micormeter head with 0.00001" (1/2 wave or 1 fringe) subdivisions, and a nice price of $1,400 and change!, but notice that its accuracy is guaranteed only to 0.00004" (2 waves or 4 fringes)!


That's why I said in a post previous to this one that even if you can estimate 0.00001" using a 0.0001" dial indicator, your results will never be certain beyond 0.0004" (a whopping 20 waves or 40 fringes), or simply four times the reading limit!

So by estimating smaller quantities with magnifiers you're just fooling yourself into believing that you're reading something down to single fringe numbers!

These devices are mechanical and their gears have internal error which no amount of "guesstimate", magnification, or experience will overcome.

regards,
Mladen

Edited by MKV (04/25/14 07:29 PM)


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Re: Thinking of making a flat... new [Re: MKV]
      #6489381 - 04/26/14 08:56 AM

As what I thought. You accept what you -READ- as sacred absolute truth, but you don't bother to VERIFY any particular example to a compared reference flat--- perhaps because you don't have access to a truely verified reference???
Why do you think mine indicares exactly +.00050" when a reference indicates single-# fringes variance- how do I know that??? (3 guesses, and first 2 don't count!)
There's a lot to learn out there, fella. Have fun.
M.


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Re: Thinking of making a flat... new [Re: Mark Harry]
      #6489385 - 04/26/14 08:59 AM

Another clue for your thoughts-
A VERNIER DOESN'T HAVE GEARS. (doh?)


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Re: Thinking of making a flat... new [Re: Mark Harry]
      #6489522 - 04/26/14 10:28 AM

Quote:

Another clue for your thoughts-
A VERNIER DOESN'T HAVE GEARS. (doh?)



Every measuring device, be it mechanical or digital, uses gears or rack and pinion for movement, Mark.

A vernier is not a device. It's a small movable graduated scale for obtaining fractional parts of subdivisions on a fixed main scale of micrometers, dial indicators, calipers, setting circles, etc, for greater precision.

This is a vernier

Digital devices, obviously, don't have verniers, so smaller units cannot be extrapolated.

Based on your description your device is a "Fowler; 0-6" range with replaceable spindles in 1" increments". Those devices are either micrometers or dial indicators, and the Fowler models I found had scales reading only to 0.001", and are not very expensive.

Example of a depth micormeter 0-6"

Yet, you say that your device is a vernier and not a dial indicator (?), and that it has dual scales (0.001"/0.0001") and is very expensive (your post #6487288). I am interested in that model. Can you post a link where it can be found? Thanks.

reagrds,
Mladen


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Re: Thinking of making a flat... new [Re: Mark Harry]
      #6489570 - 04/26/14 10:46 AM

Quote:

As what I thought. You accept what you -READ- as sacred absolute truth, but you don't bother to VERIFY any particular example to a compared reference flat--- perhaps because you don't have access to a truely verified reference???
Why do you think mine indicares exactly +.00050" when a reference indicates single-# fringes variance- how do I know that??? (3 guesses, and first 2 don't count!)
There's a lot to learn out there, fella. Have fun.



The advantage of making flats using a three disk method is that you don't need a reference flat. Also, if you're making a reference flat against a master flat no mechanical device will help you. No mechanical measuring device can distinguish between a 1/4 wave flat and a master flat. Even a $1,400 Mitituyo -- which reads down to 1 fringe (1/2 wave) -- will not certify its accuracy to anything smaller than 4 fringes (2 waves)! There is, indeed, a lot to learn, Mark.

ragards,
Mladen


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Re: Thinking of making a flat... new [Re: MKV]
      #6491498 - 04/27/14 10:00 AM

Tell you what.
SEND YOUR FLATS HERE. I will test them thru contact with a VERIFIED and CALIBRATED testplate. Or have a go with another career optician nearby with a 14" certified flat. PM me if interested. A refusal will also be most telling.(!)
*****
"Every measuring device, be it mechanical or digital, uses gears or rack and pinion for movement, Mark. "
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Nope, wrong again! You haven't had any machinist experience?
As I said before, there's a lot for you to learn....sorry.
M.


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Re: Thinking of making a flat... new [Re: Mark Harry]
      #6492209 - 04/27/14 04:57 PM

While an interesting discussion, perhaps we could change the tone.

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Re: Thinking of making a flat... new [Re: davidpitre]
      #6492583 - 04/27/14 08:04 PM

Yes, we should. I appologize. I have the fault that I HATE mis-information and inuendo, and will never change as long as I take a breath. Someday, I hope it proves to be beneficial at the correct time.
M.


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Re: Thinking of making a flat... new [Re: Mark Harry]
      #6492586 - 04/27/14 08:05 PM

And I'm thru with posting for the sake of arguement.
M.


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Re: Thinking of making a flat... new [Re: MKV]
      #6493618 - 04/28/14 11:15 AM

Quote:

I have always tested flat sets (ABC) using the standard viewer (pictured below), from 25 feet, and with a telephoto lens. I am surprised this would still be a few fringes off due to parallax.

Edmund Scientific has 12x12 inch Fresnel lenses which may make satisfactory collimating devices.

regards,
Mladen.




I used a set-up similar to yours, and it seemed to work quite well with a Fresnel lens as the collimator and a sodium lamp at the lens focus (about 12") as the source.

Unfortunately I never got around to finishing the flats. And probably never will. Hmmm...wonder what three 8" pyrex not-yet-flats, both sides partially polished, would sell for on the Classifieds?

Best -- Terry


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Re: Thinking of making a flat... new [Re: Mark Harry]
      #6493672 - 04/28/14 11:39 AM

Quote:

Tell you what.
SEND YOUR FLATS HERE. I will test them thru contact with a VERIFIED and CALIBRATED testplate. Or have a go with another career optician nearby with a 14" certified flat. PM me if interested. A refusal will also be most telling.(!)




My flats are fine for autocllimation. I have no need to to have them tested. I tested them against each other when I made them by the ABC method. So, thank you for the offer, Mark, but my flats are not the topic.

We're talking here about mechanical estimate of flatness, as a flat-making method. You said your device, whatever it may be, can estimate single digit fringe depths even though it is not rated for single digit accuracy. Like I said, a $1,400 Mitituyo micrometer head will read to 1 fringe but Mitituyo will not certify anything more accurate than 4 fringes with it. But you say your device reads to 100/10 fringes respectively, yet you claim accuracy to single fringe numbers. What is the manufacturer's stated accuracy of your device?

regards,
Mladen


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Re: Thinking of making a flat... new [Re: prfesser]
      #6493709 - 04/28/14 11:56 AM

Quote:

I used a set-up similar to yours, and it seemed to work quite well with a Fresnel lens as the collimator and a sodium lamp at the lens focus (about 12") as the source.

Unfortunately I never got around to finishing the flats. And probably never will. Hmmm...wonder what three 8" pyrex not-yet-flats, both sides partially polished, would sell for on the Classifieds?




Hi Terry, sorry you didn't finish them. As far as selling them, I have no clue.

It would be nice, however, to relate your experience of making them.

regards,
Mladen


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Re: Thinking of making a flat... new [Re: MKV]
      #6493754 - 04/28/14 12:13 PM

Quote:

Spherometer heads typically have longer spindles and may be more suitable for spherometers with that kind of accuracy. An example of such a device is Mitutoyo micormeter head with 0.00001" (1/2 wave or 1 fringe) subdivisions, and a nice price of $1,400 and change!, but notice that its accuracy is guaranteed only to 0.00004" (2 waves or 4 fringes)!

That's why I said in a post previous to this one that even if you can estimate 0.00001" using a 0.0001" dial indicator, your results will never be certain beyond 0.0004" (a whopping 20 waves or 40 fringes), or simply four times the reading limit!

So by estimating smaller quantities with magnifiers you're just fooling yourself into believing that you're reading something down to single fringe numbers!

These devices are mechanical and their gears have internal error which no amount of "guesstimate", magnification, or experience will overcome.

regards,
Mladen




Mladen is correct. Readability (or estimate-ability) and accuracy are two different things.

Let's say we measure the thickness of a piece of metal with eBay micrometer mentioned above, and the reading is 0.12245 inch. The inherent uncertainty** of the device is 0.00004 inch. That means that the actual thickness is*** somewhere between 0.12241" and 0.12249".

Best -- Terry
**This is called indeterminate error--the uncertainty when a system of measurement is extended to its maximum. It's present in every measuring device from a wooden ruler to a scanning-tunneling-microscope. Rarely is the indeterminate error of a device smaller than the device's readability; usually it's the other way around.

***That 0.00004" probably represents one standard deviation for the micrometer, as that's how uncertainties are often quoted in ads. If so, that actually means that there is only a 68% chance that the reading will be accurate to +-0.00004"! (96% chance that it will be within 0.00008". Statistics are sadistic. )


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Re: Thinking of making a flat... new [Re: MKV]
      #6493777 - 04/28/14 12:23 PM

Quote:

Unfortunately I never got around to finishing the flats. And probably never will. Hmmm...wonder what three 8" pyrex not-yet-flats, both sides partially polished, would sell for on the Classifieds?




Hi Terry, sorry you didn't finish them. As far as selling them, I have no clue.

It would be nice, however, to relate your experience of making them.

regards,
Mladen




Unfortunately, I can't. I last worked on them over 20 years ago, and that notebook was lost somewhere in our last two moves.


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Re: Thinking of making a flat... new [Re: prfesser]
      #6494531 - 04/28/14 07:20 PM

Quote:

This is called indeterminate error--the uncertainty when a system of measurement is extended to its maximum. It's present in every measuring device from a wooden ruler to a scanning-tunneling-microscope. Rarely is the indeterminate error of a device smaller than the device's readability; usually it's the other way around.




Thanks for expanding on this very relevant point, Terry. In Advanced Telescope Making Techniques, Volume 2, William M. Browne, explains in "Accuracy of Spherometers" how to calculate the uncertainty error of spherometers, since they are made up of two components, each having its own indeterminate error.

Readability (precision) and accuracy are truly two different things, and anyone using measuring devices would be well served to read that article. The main thing to remember is that just because one obtains the same readings over and over doesn't affect the inherent (in)accuracy of a device.

regards,
Mladen


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Re: Thinking of making a flat... new [Re: prfesser]
      #6494590 - 04/28/14 07:44 PM

Quote:

Unfortunately, I can't. I last worked on them over 20 years ago, and that notebook was lost somewhere in our last two moves.



That's too bad. I think the OP would have appreciated it, and so would many others I am sure.

All the best,
Mladen


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Re: Thinking of making a flat... new [Re: MKV]
      #6494678 - 04/28/14 08:39 PM

Keep it civil please people otherwise someone else's valuable thread will be locked.

Cheers,
John B


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Re: Thinking of making a flat... new [Re: ausastronomer]
      #6495104 - 04/29/14 01:54 AM

I guess no-one here (except me)has calibrated a sphero on an absolute flat. Or perhaps doesn't know -HOW-. It solves the range issue precisely, but only to a specific individual tool.
If I paid $1400 for a Mitsy, I would most definitely check it on a reference flat. And it would likely come with instructions how to calibrate it. There are -NO- calculations necessary. Not doing so, would relegate even the Mitsy to $15 status.
And knowing how to calibate will save a ton of money. fwiw,
M.


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Re: Thinking of making a flat... new [Re: Mark Harry]
      #6495889 - 04/29/14 01:33 PM

A provocative discussion.

Mark, what do you mean by "calibrated a sphero on an absolute flat"?

And when you say "absolute flat" what do you mean?

Thanks,

Gary


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Re: Thinking of making a flat... new [Re: Gary Fuchs]
      #6495972 - 04/29/14 02:15 PM

You set it on a well acclimated REFERENCE (certified error) flat, and adjust/MARK the scale where precise flat is indicated.
SIMPLE!
There are a few here that are reading too much into this. After thousands of flats in my career, I'm here to tell you this simple method works VERY well.

M.

Edited by ausastronomer (04/29/14 05:38 PM)


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Re: Thinking of making a flat... new [Re: Mark Harry]
      #6514589 - 05/08/14 07:27 PM

Here's a video.

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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6515076 - 05/09/14 12:32 AM

Great video, Ed! Just one question: what is the factory stated accuracy of your Mahr gage?

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Re: Thinking of making a flat... new [Re: MKV]
      #6515377 - 05/09/14 08:41 AM

All I can find from their on line catalog is 1/2 micron graduations.

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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6515409 - 05/09/14 09:03 AM

how do you adj. the adjustable tolerance markers, aren't they behind the window?

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Re: Thinking of making a flat... new [Re: Pinbout]
      #6515480 - 05/09/14 09:48 AM

Back to the "acetone" test. I just purchased a copy of the second edition of "Prism and Lens Making" by Tywman who is the same guy that invented the Tywman-Green interferometer. For advanced ATM's this is a very good book on the many techiques that were developed over the years to make optics, many of which were kept as trade secrets.
On page 29 of my copy it discribes the "acetone" test and he stated that one can determine the flatness of a ground surface to 2 waves or better using it. It was very late last night when I was reading that section so I need to go back and understand the math behind his statement. More to come.
- Dave


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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6515481 - 05/09/14 09:48 AM

Thanks, Ed. The markings (graduations) are your resolution; accuracy is what the manufacturer sets as a limit of confidence of readings, which depends on the mechanical components put together. For example, this Mitituyo micrometer head has a resolution or graduations down to half a micron (0.0005 mm), which is one wave, or two fringes, but the accuracy is listed as one micron (0.001mm), or two waves (four fringes).

Good news is that any optical surface used as an optical flat with a sag across its full aperture of less than 2 um will have essentially zero optical power (optical power = 1/f), like an optical window, which is why AC flats do not have to be physically absolutely flat, and why achieving an impeccable surface quality, free of zones, ripple, TDE, etc. is of much greater importance in these tools.

R. C. Burch (1938) worked out the limit of concavity/convexity in terms of fringes allowed for an AC flat as fringes = 3.4*(focal ratio squared). For an f/4 mirror, curvature depth of 54 fringes or 27 waves is the limit. For an f/2 it's 14 fringes or 7 waves. For an f/8 it's 218 fringes or 109 waves -- that's 0.002" folks! So, either Ed's method seen in the video or the acetone test Dave mentioned is more than sufficiently accurate. They couldn't be cheaper or simpler!

regards,
Mladen

Edited by MKV (05/09/14 10:01 AM)


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Re: Thinking of making a flat... new [Re: Pinbout]
      #6515515 - 05/09/14 10:15 AM

Danny,
No they are in front of the glass and can be moved manually.

Mladen,
The point of the video is that you can make for essentially nothing a gauge that will get you flat to a few fringes to where you can bring it into flat in polishing. If your indicator isn't sensitive enough you may end up with too many fringes difficult to correct in polishing.


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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6515698 - 05/09/14 12:01 PM

Quote:

Mladen, The point of the video is that you can make for essentially nothing a gauge that will get you flat to a few fringes to where you can bring it into flat in polishing. If your indicator isn't sensitive enough you may end up with too many fringes difficult to correct in polishing.



Yes, Ed, you're absolutely right. It's just that at one point in your video you're 'certifying' your larger flat as being flat to half a micron over a given diameter based on resolution, and not accuracy.

My comment was not disputing the fact that your simple wooden triangle and a brass screw will get you to within a few fringes, but simply pointing to others that 'certifying' any measurement without knowing the accuracy can be misleading as it seems to be a common mistake. Graduations (resolution) and accuracy are not necessarily one and the same.

I was personally amazed how well that plywood triangle worked for this purpose. In terms of simplicity, effectiveness and easy of use, it's ingenious, just as the acetone test. The only problem is that with both you still need a reference flat, which begs the question -- why make another flat if you already have one?

In that case, why not make three? The beauty of three flats made simultaneously is that you end up with a very flat surfaces without any measurements, versatility, and can actually make one a master flat by testing all three against each other without a master flat.

regards,
Mladen

Edited by MKV (05/09/14 12:06 PM)


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Re: Thinking of making a flat... new [Re: MKV]
      #6515819 - 05/09/14 01:01 PM

Quote:

The only problem is that with both you still need a reference flat



Actually you don't with my gauge. Assuming that the tool and part are mated surfaces then either 1. one will be convex and the other concave or 2. both are flat and you can't measure any difference which is your goal in lapping. Once you're polishing then water becomes your master flat.

If you need three flats then the traditional 3 flat method is great but if you need only one it's also no problem to make just one. Large pieces of glass can run into some money.


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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6515868 - 05/09/14 01:22 PM

Ed,
Something isn't working with your video. I clicked
on your link and it brought me to the you tube site
and where the video should be is blank (white). I did a refresh
and the picture went black like it was going to play and
then went white again.
Regards Norm


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Re: Thinking of making a flat... new [Re: Norm Meyer]
      #6515969 - 05/09/14 02:11 PM

The link in this thread works for me. Try this: https://www.youtube.com/user/opticsed

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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6516173 - 05/09/14 04:05 PM

I see your point, Ed. I guess a water flat is the way to go, even though I have yet to see one with steady fringes. If I am wrong I stand corrected, but a video that shows fringes that are motionless for more than a fleeting second would be much appreciated.

regards,
Mladen

Edited by MKV (05/09/14 09:16 PM)


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Re: Thinking of making a flat... new [Re: MKV]
      #6516179 - 05/09/14 04:08 PM

Quote:

I guess a water flat is the way to go, even though I have yet to see one with steady fringes.




I haven't been able to get any fringes.

but omg I saw this on ebay last night, someone got a 12.5" flat for the price of a blank...

http://www.ebay.com/itm/231222214495?ssPageName=STRK:MEWAX:IT&_trksid=p39...


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Re: Thinking of making a flat... new [Re: Pinbout]
      #6516435 - 05/09/14 06:38 PM

Quote:

but omg I saw this on ebay last night, someone got a 12.5" flat for the price of a blank...



I would say much cheaper than a 12.5 inch full thickness Pyrex blank, and many time cheaper than a finished 12.5 inch AC flat.

Mladen


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Re: Thinking of making a flat... new [Re: Pinbout]
      #6518691 - 05/11/14 02:52 AM

Quote:

Quote:

I guess a water flat is the way to go, even though I have yet to see one with steady fringes.




I haven't been able to get any fringes



Did you use a collimating lens? I believe Ed obtained a Fresnel lens from an old TV projection set. Pretty nifty, imo.

regards,
Mladen


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Re: Thinking of making a flat... new [Re: MKV]
      #6519524 - 05/11/14 03:18 PM

no but I should be able to get concentric circles without the fresnel.

lately been busy making other things...



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Re: Thinking of making a flat... new [Re: Pinbout]
      #6519619 - 05/11/14 04:25 PM

[quoteI haven't been able to get any fringes]


You might get or make an autocollimating telescope with an illuminated cross hair. Aim it down at the flat in water and bring the 2 reflections together. Check to make sure the water level in on the order of 1 mm too.

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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6519627 - 05/11/14 04:32 PM

Shouldnt a newt provide collimated light with the led at the focuser hole.

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Re: Thinking of making a flat... new [Re: Pinbout]
      #6519769 - 05/11/14 06:17 PM

I haven't needed to use a collimated light source to see fringes. You do need it to get an accurate read of the flatness thou. The secret is to use a laser pointed down at about 60 degrees to water and be sure you setup were there is no vibration and no drafts that can disrupt the surface of the water. I use my basement. You'll see a number of reflections of the laser on the ceiling from the water and the glass flat in the water. You need to adjust the tip and tilt of the pan of water that contains the flat so all the reflections fall on top of each other. Now shiny the monchrome light on the water and watch carefully for a tiny orb of round fringes. The water has to be nearly perfectly still. Once you see it, just wait to see if it spreads out into bands. If not I use a eye dropper and remove some water and try again. When the water level is correct the fringes will start out as a tight circle and when the water is perfectly still spread out into fringes.

- Dave


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Re: Thinking of making a flat... new [Re: Pinbout]
      #6519947 - 05/11/14 08:32 PM

This is an autocollimator used a lot in optics labs. You would use it to align the submerged flat to the surface of water, not to see fringes. It's sort of overkill but would be the best method. You can make something similar with a beam splitter and a small telescope.

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Re: Thinking of making a flat... new [Re: Pinbout]
      #6520208 - 05/11/14 10:47 PM

Quote:

lately been busy making other things...




Nice autocllimation bench!


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Re: Thinking of making a flat... new [Re: MKV]
      #6520652 - 05/12/14 09:14 AM

its getting there...

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Re: Thinking of making a flat... new [Re: Ed Jones]
      #6521673 - 05/12/14 06:43 PM

Quote:

This is an autocollimator used a lot in optics labs. You would use it to align the submerged flat to the surface of water, not to see fringes. It's sort of overkill but would be the best method. You can make something similar with a beam splitter and a small telescope.



Yes, autocllimators are used a lot in industrial settings. Quite doable, I think.

Instead, I use a simple cross-beam laser like this one. The principle is the same as with optical autocllimators you mention. Just make sure all reflections fall on top of each other. I use it to set up the Ross null, and the double-pass autocollimation tests. Quick and easy.

Mladen


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Re: Thinking of making a flat... new [Re: MKV]
      #6521698 - 05/12/14 06:58 PM

After this last delmarva trip, working with swayze, setting up the ross null was so much easier. Before I tried but didnt get it, or at least not anything useful.

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Re: Thinking of making a flat... new [Re: Pinbout]
      #6521706 - 05/12/14 07:01 PM

I see Biff left the building.

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Re: Thinking of making a flat... new [Re: mikey cee]
      #6521774 - 05/12/14 07:34 PM

Am I allowed to make any Canada joked?

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Re: Thinking of making a flat... new [Re: Biff]
      #6525205 - 05/14/14 03:29 PM

Quote:

So which is the best way to go to make a flat that I can be confident about it's quality?





you can always get something like this and if it has power work on it. save all that grunt work for someone else...

http://www.ebay.com/itm/6-x-925-Optical-Glass-Mirror-Blank-Laser-Telescope-/3...


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Re: Thinking of making a flat... new [Re: Pinbout]
      #6525222 - 05/14/14 03:37 PM

or look at this...already polished, maybe not flat...but for the price of a blank...i maybe flat, used in testing...they may not know what they have.

http://www.ebay.com/itm/Very-Nice-Optical-Glass-Window-14-5-Dia-1-8-Thick-Tel...


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Re: Thinking of making a flat... new [Re: Pinbout]
      #6525262 - 05/14/14 04:03 PM

Quote:

I see Biff left the building.



No, I'm still reading along.

Quote:

Am I allowed to make any Canada joked?



Of course you can!

I already have the three 8" blanks that I'm going to making flats out of. I'll be using it to figure my own elepticals and and various other flats should I proceed with some projects I have ideas for. But the 8" should be big enough for my uses.


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Re: Thinking of making a flat... new [Re: Biff]
      #6525274 - 05/14/14 04:11 PM

Quote:

Quote:

I see Biff left the building.



No, I'm still reading along.

Quote:

Am I allowed to make any Canada joked?



Of course you can!

I already have the three 8" blanks that I'm going to making flats out of. I'll be using it to figure my own elepticals and and various other flats should I proceed with some projects I have ideas for. But the 8" should be big enough for my uses.




but what about an autocollimator [with hotwheels music ]

https://www.youtube.com/watch?v=Ef8wOZY7jzU


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Re: Thinking of making a flat... new [Re: Pinbout]
      #6525276 - 05/14/14 04:13 PM

Quote:

or look at this...already polished, maybe not flat...but for the price of a blank...




The price is a current bid...the final auction price will be much higher, you can be sure of that.

Mladen


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Re: Thinking of making a flat... new [Re: Pinbout]
      #6525298 - 05/14/14 04:30 PM

Quote:

but what about an autocollimator [with hotwheels music ]

https://www.youtube.com/watch?v=Ef8wOZY7jzU



Not sure if I will go that route. I have parts to built a Bath I.F. so between that and foucault (which I may explore a robotic version ala James Lerch). I think between those two tests I will have my bases covered for projects I foresee in the near future. Also as I understand it, an A.C. 'flat' can have a few waves of power just no irregularities so fabrication can be a little more lax where I agree the three flat method would be a waste of time and materials.


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Re: Thinking of making a flat... new [Re: Pinbout]
      #6533827 - 05/19/14 09:31 AM Attachment (5 downloads)

Quote:

Quote:

I guess a water flat is the way to go, even though I have yet to see one with steady fringes.




I haven't been able to get any fringes.

but omg I saw this on ebay last night, someone got a 12.5" flat for the price of a blank...

http://www.ebay.com/itm/231222214495?ssPageName=STRK:MEWAX:IT&_trksid=p39...




So what do you think?


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Re: Thinking of making a flat... new [Re: Pinbout]
      #6533941 - 05/19/14 10:56 AM

Quote:

So what do you think?



Can't tell. You have to account for parallax either by looking from a large distance or by using a collimating lens.

Mladen


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Re: Thinking of making a flat... new [Re: MKV]
      #6533948 - 05/19/14 10:59 AM

its smooth.

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Re: Thinking of making a flat... new [Re: Pinbout]
      #6533962 - 05/19/14 11:08 AM

Quote:

its smooth.



That it is, but it could be many waves off. Also, how flat is your master flat? A slightly concave master and a slightly convex test flat can show perfect flatness.

Mladen


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Re: Thinking of making a flat... new [Re: MKV]
      #6534070 - 05/19/14 12:12 PM

Its at least 1/8

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Re: Thinking of making a flat... new [Re: Pinbout]
      #6546873 - 05/26/14 03:36 PM




Finally mount my 16" flat. And installed the vertical pivot screw.


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Re: Thinking of making a flat... new [Re: Pinbout]
      #6558321 - 06/02/14 12:51 AM

Hey just wondering... after I'm done making these flats, is there anywhere I can send them to get them independently tested or get some kind of 'certification'? If anything just for a sanity check.

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Re: Thinking of making a flat... new [Re: Biff]
      #6558497 - 06/02/14 06:15 AM

Send to ed jones.

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Re: Thinking of making a flat... new [Re: Biff]
      #6558899 - 06/02/14 11:36 AM

Quote:

Hey just wondering... after I'm done making these flats, is there anywhere I can send them to get them independently tested or get some kind of 'certification'? If anything just for a sanity check.



You can test three flats against each other if you make a simple Newton ring tester, such as

this one

It's just a box with a sheet of glass at 45 degrees nad a more or less monochrome light source.

You test all three flats against each other and then solve for each. It's a method descirbed in ATM books. There is also a small downloadable routine called "three planes" that will do it for you.

You can also test each flat individually using a simple spherical mirror using the Ritchey-Common test.

Sending them to Ed Jones is a great idea to verify your methods. You still need a tester in the final figuring phases. There is also a lot more to be said about making and testing the flats as you go along, as well as how to observe them correctly. Flats have to be allowed to cool down to room temperature and stabilize before testing them, and if you don;t have a collimating lens you need to observe them form a quite a distance to avoid parallax bias.

Your best bet is to ask questions as you go along, and as you read more on various tests and make suitable testing equipment.

Gridinng is the easiest part. Then things get progressively longer and more complicated. Read up on making flats and make what you need to test them.

regards,
Mladen


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