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use a round or elliptical pitch lap for an elliptical flat?

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#1 greenglass

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Posted 22 April 2020 - 09:34 PM

Only 10 years and no luck figuring a 2.25'' secondary.  The silver coating was yellow and disappearing so I polished it off to try and make it flatter.  There was a slight TDE zone on one side.  Now it's a lot worse.   Mostly TDE, astigmatism and concave.  About 5 waves.



#2 Pinbout

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Posted 22 April 2020 - 09:38 PM

Round with lots of frequent press against a reference flat. Secondary on top.  Figuring with all pitch is like figuring with melting ice. That’s why you keep pressing the lap against the ref often. 
 

grin.gif


Edited by Pinbout, 22 April 2020 - 09:40 PM.


#3 DAVIDG

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Posted 22 April 2020 - 09:55 PM

 I use a round lap  and it is larger than the diagonal. The "secret" it that the pitch lap is pressed against another optical flat. This makes the pitch flat and the diagonal  you are  figuring follows the flat lap. Every 10 minutes or so I repress the lap against the optical flat.  The "pressing" flat can be scratched or chipped . I fixed this  4" MA axis diagonal using an 8" round lap and using a figure 8 stroke and kept rotating the diagonal make sure the stroke covered all the lap. You need to make sure the glass covers all of the surface of the pitch or the pitch  will not stay flat. 

 

                - Dave  

coulterdiagonalstart.jpg

 

coulterdiagonalfinished.jpg


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#4 greenglass

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Posted 23 April 2020 - 02:00 AM

000_0038.jpg fringes of the secondary


Edited by greenglass, 23 April 2020 - 02:04 AM.


#5 Lognic04

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Posted 23 April 2020 - 02:41 AM

How do you press a lap against a ref. without ruining it? Also for large secondaries do subdia laps work?

Cheers


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#6 Pinbout

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Posted 23 April 2020 - 08:07 AM

attachicon.gif000_0038.jpgfringes of the secondary

Im tryin to eat breakfast.... lol.gif


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#7 DAVIDG

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Posted 23 April 2020 - 08:48 AM

How do you press a lap against a ref. without ruining it? Also for large secondaries do subdia laps work?

Cheers

 When you press a lap against a mirror do you  ruin it ? No. It  is  a very common practice in optical shops to condition a lap against a "pressing flat"  You can find flats on places like Ebay for not much money that have scratches and/small edge chips. yet the surface  can be very flat. These make excellent "pressing" flats to condition a lap. The 8" flat that I'm using to test the 4" MA diagonal in the pictures I posted I found on Ebay for $35. It is flat to 1/10 wave across the whole surface with a perfect edge. The coatings were bad so it looked ugly.

 

                   - Dave 


Edited by DAVIDG, 23 April 2020 - 05:37 PM.

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#8 macdonjh

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Posted 23 April 2020 - 08:56 AM

I'm curious: can you also use three pieces of glass?  Figure A-with-B for a while, then figure B-with-C for a while, then figure C-with-A for a while.  Continually test as you lap the three pieces together until you meet your flatness spec.



#9 DAVIDG

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Posted 23 April 2020 - 09:27 AM

I'm curious: can you also use three pieces of glass?  Figure A-with-B for a while, then figure B-with-C for a while, then figure C-with-A for a while.  Continually test as you lap the three pieces together until you meet your flatness spec.

 Yes that is the classic way to make a flat but a fair amount of work.  If your trying to refigure a flat or diagonal you already have, then it is easier to use a pressing flat to condition the lap.  The pressing flat can also be used to test the surface of the flat your refiguring.

 

                          - Dave 



#10 greenglass

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Posted 23 April 2020 - 12:00 PM

made a round 4" lap medium hard resin and pressing a 6" unfigured flat (float glass that's a bit wavy fringed) every polishing wet.  wish me luck, will post new pics hopefully flatter. MOT. figure 8 strokes, rotating, using all the lap...



#11 DAVIDG

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Posted 23 April 2020 - 12:18 PM

 I don't think your float glass is going to work. You need to make the pitch as flat as the results your trying to achieve. Each time you press the pitch against the float glass your making it to  that level of flatness and the glass will polish to that shape.

    A flat is a spherical mirror with a super super long radius. Unlike figuring a concave mirror whose radius is many times shorter,  what you are trying to do is make that radius as long as possible and smooth. So any change in the flatness of the pitch is going to change the radius on the flat ie it's flatness.  So just like figuring a concave mirror the stroke used  is critical put even more so. As Danny (Pinbout) said, figuring a flat with pitch is like figuring with melting ice cream since the pitch is going to change shape just from the act of polishing on it. If the stroke you use doesn't cover all of the surface of the pitch, the pitch is going to change shape more in one area then the other. For example if  you only polish using the center of the lap, the lap is going to go concave and the glass convex. Too much over hang of the glass and the lap goes convex and the glass goes concave. Also be sure that the channels you cut result is equal size facets and don't make the channels wide or deep. If they are wide or deep then the pitch can flow and change shape quickly. 

  That is why is use the figure 8 stroke making  sure the glass covers all the surface of the lap but with no or very little overhang.  Keep turning the lap and keep turning the elliptical flat.  As I have said many times" Ask 3 mirror makers how to make a mirrors and you'll get 6 different answers. All of them right !"   There are many ways to figure optical flats. This works for me and  besides just posting a message about it  I provide images to show it works, at least for me.  You mileage may vary !

 

                      - Dave 


Edited by DAVIDG, 23 April 2020 - 05:37 PM.

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#12 Pinbout

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Posted 23 April 2020 - 12:27 PM

these radioactive russian flats are pretty cheap

 

https://www.ebay.com...oUAAOSwP4hdYk-0



#13 Mark Harry

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Posted 24 April 2020 - 03:20 AM

To clarify just a bit-
"pressing flat" is -NOT- a "reference flat"
(references are used for testing ONLY.)
Long time ago, I made a pressing plate for polisher pressing, roughly 6" dia. x 1/2" thick. With thoughtful placing of weight distribution, polisher can be made slightly 'vex, or CC. It simplifies things and speeds up the job significantly.
******
 How was breakfast, Danny??!!



#14 Pinbout

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Posted 24 April 2020 - 06:28 AM

To clarify just a bit-
"pressing flat" is -NOT- a "reference flat"
(references are used for testing ONLY.)
Long time ago, I made a pressing plate for polisher pressing, roughly 6" dia. x 1/2" thick. With thoughtful placing of weight distribution, polisher can be made slightly 'vex, or CC. It simplifies things and speeds up the job significantly.
******
 How was breakfast, Danny??!!

today was better than yesterday lol.gif



#15 macdonjh

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Posted 24 April 2020 - 07:07 AM

 I don't think your float glass is going to work. You need to make the pitch as flat as the results your trying to achieve. Each time you press the pitch against the float glass your making it to  that level of flatness and the glass will polish to that shape.

    A flat is a spherical mirror with a super super long radius. Unlike figuring a concave mirror whose radius is many times shorter,  what you are trying to do is make that radius as long as possible and smooth. So any change in the flatness of the pitch is going to change the radius on the flat ie it's flatness.  So just like figuring a concave mirror the stroke used  is critical put even more so. As Danny (Pinbout) said, figuring a flat with pitch is like figuring with melting ice cream since the pitch is going to change shape just from the act of polishing on it. If the stroke you use doesn't cover all of the surface of the pitch, the pitch is going to change shape more in one area then the other. For example if  you only polish using the center of the lap, the lap is going to go concave and the glass convex. Too much over hang of the glass and the lap goes convex and the glass goes concave. Also be sure that the channels you cut result is equal size facets and don't make the channels wide or deep. If they are wide or deep then the pitch can flow and change shape quickly. 

  That is why is use the figure 8 stroke making  sure the glass covers all the surface of the lap but with no or very little overhang.  Keep turning the lap and keep turning the elliptical flat.  As I have said many times" Ask 3 mirror makers how to make a mirrors and you'll get 6 different answers. All of them right !"   There are many ways to figure optical flats. This works for me and  besides just posting a message about it  I provide images to show it works, at least for me.  You mileage may vary !

 

                      - Dave 

That's why I asked about the "three glass" method.  Perhaps it's more work, but it seems like it would be more "direct"- less chance for errors inherent in the process.  Besides, once you've finished your first flat mirror, you could save the other two pieces and have a head-start on your next flat mirror, right?

 

To anyone trying to learn something, don't mind me, listen to DAVIDG and Pinbout who have actually made a mirror.  I'm just asking questions. smile.gif



#16 DAVIDG

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Posted 24 April 2020 - 08:25 AM

 Inexpensive but excellent flats are out there. I just bought this Zerodur 5" flat off of Ebay for $50. Edmund sells the same one for $700. 

  With    just a bit of care,  pressing a lap against will do no harm.  I have never scratched or sleeked any of my flats doing this over many years. 

    Here is a  picture to show the flat being tested against the 8" one I got for $35 both flat test to 1/10 wave completely across the surfaces and with no turned edge.

    Again there are many way to do things, this works for me and I post  pictures to show my results. 

 

               - Dave 

 

 

zerodur flat.jpg


Edited by DAVIDG, 24 April 2020 - 08:26 AM.

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#17 Mark Harry

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Posted 25 April 2020 - 07:03 AM

I saw something in the 4" elliptical flat; enhanced it a bit, and superimposed 2 straight lines on the dark interference bands just underneath.
There should be more parallelism; and in fact, (actually being closer) the right side of the superimposed lines should be a smidge further apart.
Make your own judgements. It would help if the pic was made straight-on to the test arrangement. At such an angle as taken, sensitivity is reduced.
If the flat on top is of equal thickness, it should appear to show a slight convex fit if the distance to the test is shorter than recommended.

Edit- added parentheses

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  • groski stig 3A.jpg

Edited by Mark Harry, 25 April 2020 - 07:06 AM.


#18 Pinbout

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Posted 25 April 2020 - 07:33 AM

When going off 90 the lines get thicker, could be causing the fanning, being further away off 90



#19 DAVIDG

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Posted 25 April 2020 - 09:08 AM

I saw something in the 4" elliptical flat; enhanced it a bit, and superimposed 2 straight lines on the dark interference bands just underneath.
There should be more parallelism; and in fact, (actually being closer) the right side of the superimposed lines should be a smidge further apart.
Make your own judgements. It would help if the pic was made straight-on to the test arrangement. At such an angle as taken, sensitivity is reduced.
If the flat on top is of equal thickness, it should appear to show a slight convex fit if the distance to the test is shorter than recommended.

Edit- added parentheses

     The purpose of the pictures  is to show that my technique of pressing the  pitch lap against a pressing flat and using that lap to  polish a  non flat surface works to flatten it out . The  pictures were not to show the exact wavefront of the finally result.  I didn't pay attention to the  angle they were taken or anything like that and I didn't use it to determine the exact wavefront but again to show the fringes now were greatly improved. The before and after pictures clearly show that the method took a surface that was nowhere near flat and in fairly short time I was able to get to an optically  flat surface. This thread started out from the author saying he has been trying to get his surface flat for years with no luck so I posted what continues to work for me. The bottom lines is that this methods  works for me and hopefully it will help others that  are trying to make flats. 

 We can have a new  thread on the exact method one needs to test flats to determine their exact flatness and then how to photograph the results  to post so there is no error shown in the images but again that wasn't the purpose of why I posted the images.

  I look forward to others posting about the  techniques they use and hopefully they can also post images to showing how it is done the results they obtain.

 

             - Dave 


Edited by DAVIDG, 25 April 2020 - 09:15 AM.

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#20 Mark Harry

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Posted 26 April 2020 - 06:26 AM

When going off 90 the lines get thicker, could be causing the fanning, being further away off 90

Correct. If the top flat is plano-parallel, there should be a slight indication of convexity, and lines should be actually -slightly- narrower; especially since the camera shot was too close. It appears polishing overshot somewhat, and in all likelihood the edge is slightly up.
The inscribed lines show astigmatism, and I didn't see anywhere near that degree in the original pic.
I did not bring wave ratings into the discussion, just calling out gross errors as I see them.  Corrective methods are up to the one that did the job.
Perhaps another thread is in order. Or at least, how to image a flat test correctly? It can't be discerned that the elliptical in question is optically flat, especially from just that one picture. (it's not)  Just saying.



#21 JulianShull

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Posted 26 April 2020 - 02:23 PM

I do not believe you can make an elliptical flat by just polishing. Only round ones. That is just the way the geometry works. Some friends of mine did make several elliptical flats, but they did it by making oversized round ones first then placing them in a container filled with plaster of Paris at a 45 degree angle. When the plaster cured they used a diamond hole saw and drilled a hole of the desired minimum diameter through the plaster holding the flat.



#22 JohnH

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Posted 26 April 2020 - 07:15 PM

Only 10 years and no luck figuring a 2.25'' secondary. The silver coating was yellow and disappearing so I polished it off to try and make it flatter. There was a slight TDE zone on one side. Now it's a lot worse. Mostly TDE, astigmatism and concave. About 5 waves.



The way flat were made is you "block" them in with other pieces of glass filling it the area.

#23 ed_turco

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Posted 27 April 2020 - 09:42 AM

Use hard pitch.  Soft pitch turns edges.



#24 Pierre Lemay

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Posted 27 April 2020 - 10:20 AM

I do not believe you can make an elliptical flat by just polishing. Only round ones. That is just the way the geometry works. Some friends of mine did make several elliptical flats, but they did it by making oversized round ones first then placing them in a container filled with plaster of Paris at a 45 degree angle. When the plaster cured they used a diamond hole saw and drilled a hole of the desired minimum diameter through the plaster holding the flat.

Julian, welcome to Cloudy Nights.

 

Contrary to your beliefs, as Dave Groski mentions in several posts, above, making an accurate elliptical flat can indeed be done by polishing against a larger, round polishing lap that is regularly pressed against a same size optical flat.

 

As for the method your friends used it can also work but the surface will be subject to the Twiman effect after the cut is made with the diamond saw.  To eliminate the surface distortion resulting from the Twiman effect, the ground periphery of the diagonal will need to be fine ground, which is not easy to do on the inclined edge surfaces of a diagonal. Read this to learn more about the impact of cutting away glass on a finished optical flat surface.

 

Of course, in addition to the Twiman effect, there must be no strain in the glass before cutting. It must be fully anealed or the cut will release the strain. As mentionned previously by JohnH it is better to cut the diagonal first, block it with other pieces of glass and grind/polish. 


Edited by Pierre Lemay, 27 April 2020 - 10:21 AM.

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#25 Mark Harry

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Posted 27 April 2020 - 12:10 PM

Ellipticals are polished all the time on round, or ring (planetary) polishers. Also, as completely finished, cored, beveled, etc.
Usually, they're dummy-shined, and then finished on pitch. The thin edge will do fine as long as they're done right.




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