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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

Loc: Whitefield, Maine
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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Reged: 09/13/10

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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 (18 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 (13 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.
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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 (4 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 (4 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 (1 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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