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Equipment Discussions >> ATM, Optics and DIY Forum

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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
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
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
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
Postmaster
*****

Reged: 02/22/10

Loc: nj
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
scholastic sledgehammer
*****

Reged: 01/22/06

Loc: Hamilton. Ontario, Canada
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
Carpal Tunnel
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Reged: 04/06/04

Loc: Sin-sin-atti
Re: Thinking of making a flat... [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
Postmaster
*****

Reged: 02/22/10

Loc: nj
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
Carpal Tunnel
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Reged: 04/06/04

Loc: Sin-sin-atti
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
Post Laureate
*****

Reged: 12/02/04

Loc: Hockessin, De
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
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
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
scholastic sledgehammer
*****

Reged: 01/22/06

Loc: Hamilton. Ontario, Canada
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
Post Laureate
*****

Reged: 12/02/04

Loc: Hockessin, De
Re: Thinking of making a flat... new [Re: greenglass]
      #6366063 - 02/09/14 01:07 PM Attachment (20 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
Postmaster
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Reged: 02/22/10

Loc: nj
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
scholastic sledgehammer
*****

Reged: 01/22/06

Loc: Hamilton. Ontario, Canada
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
Post Laureate
*****

Reged: 12/02/04

Loc: Hockessin, De
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
Postmaster
*****

Reged: 02/22/10

Loc: nj
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
Post Laureate
*****

Reged: 12/02/04

Loc: Hockessin, De
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
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
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
Post Laureate
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Reged: 12/02/04

Loc: Hockessin, De
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
Carpal Tunnel
*****

Reged: 01/20/11

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
Post Laureate
*****

Reged: 12/02/04

Loc: Hockessin, De
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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