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Foucault test stage

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#1 Astro Daddy

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Posted 23 April 2018 - 03:13 PM

Greetings all

I am having a difficult time getting repeatable measurements using the wood stage I built. Actually I tried two different designs the one from Stellafane's website and one using draw rollers but they just don't seem to be accurate enough . Most likely my craftsmanship. Just wanting to know if anyone is using a store bought stage and if so what size you are using and brand and such.

Thanks

John



#2 MKV

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Posted 23 April 2018 - 03:26 PM

Use any x-y stage with 0.5 to 1 inch travel, with micrometers. I would avoid miniature stages because they are light. You want something heavy that doesn't easily slide or vibrate. A milling machine cross slide work too. 



#3 TOMDEY

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Posted 23 April 2018 - 03:53 PM

This may be a Catch-22, but have you checked your stage against a good mechanical reference? Even under good conditions, Foucault patterns may be difficult to interpret and "noisy." So the longitudinal knife positions (zonal testing etc.) will have a lot of scatter. The toughest reading is trying to find the radius of the center zone. One zone in from the edge tends to be the most repeatable... and a good point to reference other readings to.  Tom



#4 Astro Daddy

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Posted 23 April 2018 - 04:11 PM

This may be a Catch-22, but have you checked your stage against a good mechanical reference? Even under good conditions, Foucault patterns may be difficult to interpret and "noisy." So the longitudinal knife positions (zonal testing etc.) will have a lot of scatter. The toughest reading is trying to find the radius of the center zone. One zone in from the edge tends to be the most repeatable... and a good point to reference other readings to.  Tom

Tomdey 

Not sure I know what you mean "checked your stage against a good mechanical reference". I have a dial indicator for reading the zones.

Thanks

John



#5 TOMDEY

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Posted 23 April 2018 - 07:36 PM

Dial indicator is good to have, makes life a lot easier than e.g. trying to read a home-made Vernier. What is it that is not repeatable... the indicator readings for repeat measurements of (fluffed) zones? If so, the noise in that data has two components: 1) pattern-interpretation variation 2) test-set hardware variation. One wants to build a setup where #2 is stabilized, so that only #1 dominates. I believe you are saying that #2 is dominating... and that can have a lot of contributors. One way to get a handle on the balance between #1 and #2 is to take many data sets, and tabulate the means and Standard Deviations of the gauge readings, for each zone. Get out a calculator and add a column tabulating this: RxSD^2/r, where r is the radius of the zone(s) and R is the half-diameter of the mirror. Plot that vs r. If it forms a reasonably straight line, you are dominated by interpretation noise... and that is as good as it can get. I hope I haven't confused things further...  Tom



#6 Astro Daddy

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Posted 24 April 2018 - 05:46 AM

Thanks Tom for your input. I will need to think about what you suggested, what that equates to is trying to remember the math that has been lost in the last 45 years. Don't use it lose it is true. I also am reaching out to a friend who may be able to rework my homemade stage to improve upon what I have put together. If it would be okay I would like to touch base again when I accomplish these goals. I also noticed what I see and what a camera sees is different, less noise or scatter with the camera. This may be another tool I will try and use to help determine my zones.

Thanks again

John



#7 Spectral Joe

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Posted 24 April 2018 - 08:51 AM

Try a microscope mechanical stage, smooth motion and verniers. This one is doing Ronchi at the moment, but also does Foucault when asked.

 

 

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#8 Astro Daddy

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Posted 24 April 2018 - 12:09 PM

Try a microscope mechanical stage, smooth motion and verniers. This one is doing Ronchi at the moment, but also does Foucault when asked.

Nice! Does it have the resolution needed for reading zones? The calculations that I have made for a 10" mirror would require .001 resolution.

John



#9 DAVIDG

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Posted 24 April 2018 - 01:12 PM

 Just understand that the resolution on the micrometer doesn't mean your getting that accuracy.  First you need to understand the difference between precision and accuracy. Precision is getting the same result, accuracy is getting the correct result, they are not the same. There are sources of errors  in doing Foucault readings so those readings are not absolute with no errors. Some  errors can be systematic, as in unknown errors in fabrication of tester that adds an offset to all  the readings  like not having the micrometer mounted so it is 90.00 degree to the stage, or it can be a random errors like reading a digit wrong in one of the readings. One of largest sources is judging were each zone nulls.  

   I have been teaching optical fabrication for close to 40 years now I and have tested many mirrors in which the makers  used reading entered in to computer programs that said "1/20" wave but they never crossed checked their results against other methods to determine what the errors were in their results. Many of those mirrors when  checked against  other test methods were closer to 1/2 wave then a 1/20 wave. 

  So don't get a false sense of security that just because you have a  well built tester, that the measurements you are getting   are  to the accuracy required to finish the mirror to the wave rating you want. You need to cross check your results and find the errors in the total procedure of measuring the zones to have confidence in your results.

  Measuring zones is like playing a music instrument. You can have the very best instrument but the lack the skill to play it. So it is combination of both the tester and skill, to do the measurement correctly.

 

                    - Dave 


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#10 Astro Daddy

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Posted 24 April 2018 - 02:18 PM

 Just understand that the resolution on the micrometer doesn't mean your getting that accuracy.  First you need to understand the difference between precision and accuracy. Precision is getting the same result, accuracy is getting the correct result, they are not the same. There are sources of errors  in doing Foucault readings so those readings are not absolute with no errors. Some  errors can be systematic, as in unknown errors in fabrication of tester that adds an offset to all  the readings  like not having the micrometer mounted so it is 90.00 degree to the stage, or it can be a random errors like reading a digit wrong in one of the readings. One of largest sources is judging were each zone nulls.  

   I have been teaching optical fabrication for close to 40 years now I and have tested many mirrors in which the makers  used reading entered in to computer programs that said "1/20" wave but they never crossed checked their results against other methods to determine what the errors were in their results. Many of those mirrors when  checked against  other test methods were closer to 1/2 wave then a 1/20 wave. 

  So don't get a false sense of security that just because you have a  well built tester, that the measurements you are getting   are  to the accuracy required to finish the mirror to the wave rating you want. You need to cross check your results and find the errors in the total procedure of measuring the zones to have confidence in your results.

  Measuring zones is like playing a music instrument. You can have the very best instrument but the lack the skill to play it. So it is combination of both the tester and skill, to do the measurement correctly.

 

                    - Dave 

Thanks Dave

The judgement of null zones is for me an area that I am having a hard time trying to learn. I have been using the stage as it is and studying the zones and incorporating a mask. I find it hard with a mask and want to try the beam and pin method. I think that that may help me more. At this point I need to tackle the precision aspect too. One of the many books that I have been reading tells of someone who could achieve great accuracy using the Foucault test, but that person had many years of experience. When I first set down with the tester I had a hard time even seeing the effect but as I have been putting time in at the instrument that has become much better, I was told that it was like riding a bicycle. That has proven to be true. Dave what do you mean by cross checking your results to find errors? Use a different type of test? As far as playing an instrument I am still trying to learn where to put my fingers and I hope to have enough years left to make good music.smile.gif

Thanks again Dave

John



#11 ckh

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Posted 24 April 2018 - 02:34 PM

You can avoid of the subjective determination of zones by using a camera (without a mask) in place of your eye. The images can be analyzed using software such a Foucault Unmasked (the latest version can downloaded from http://ge.tt/7GmbYIo2). Use a camera with a sizeable aperture (avoid cell phone cameras as their small apertures are difficult to align).  Place the camera within an inch or so of the KE on a fixture such as a tripod.

 

For best results the top, bottom, left and right edges of the mirror should be visible in the images. It will still work if either the left of right edge is not visible (e.g. when the diffraction ring is absent).

 

Carl


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#12 Astro Daddy

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Posted 24 April 2018 - 03:54 PM

Thanks Carl

I have been able to get some pretty good photos here lately using the timer function on my camera. I will see what I can do when I get my Diy stage back from my friend. He is trying to make it a bit better.

John



#13 IvorB

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Posted 24 April 2018 - 05:48 PM

This may be a Catch-22, but have you checked your stage against a good mechanical reference? Even under good conditions, Foucault patterns may be difficult to interpret and "noisy." So the longitudinal knife positions (zonal testing etc.) will have a lot of scatter. The toughest reading is trying to find the radius of the center zone. One zone in from the edge tends to be the most repeatable... and a good point to reference other readings to.  Tom

I will second that.  The depth of focus at the center is many times that of the outer zone.  I learned years ago to never ever use the center as the base reading.  Also using micrometers to take the readings is early 20th century technology.  For Foucault readings, they suffer from what mechanical engineers call stick slip which tends to make the reading over correct if there is any slide friction at all.  But I see you are using a dial indicator which should take care of that problem.  The best thing I ever did for my tester was to take the  plunge and buy a dial indicator that measures to four decimal places.  It is very easy to read and has greatly improved my ability to reduce scatter.



#14 TOMDEY

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Posted 24 April 2018 - 07:58 PM

 Just understand that the resolution on the micrometer doesn't mean your getting that accuracy.  First you need to understand the difference between precision and accuracy. Precision is getting the same result, accuracy is getting the correct result, they are not the same. There are sources of errors  in doing Foucault readings so those readings are not absolute with no errors. Some  errors can be systematic, as in unknown errors in fabrication of tester that adds an offset to all  the readings  like not having the micrometer mounted so it is 90.00 degree to the stage, or it can be a random errors like reading a digit wrong in one of the readings. One of largest sources is judging were each zone nulls.  

   I have been teaching optical fabrication for close to 40 years now I and have tested many mirrors in which the makers  used reading entered in to computer programs that said "1/20" wave but they never crossed checked their results against other methods to determine what the errors were in their results. Many of those mirrors when  checked against  other test methods were closer to 1/2 wave then a 1/20 wave. 

  So don't get a false sense of security that just because you have a  well built tester, that the measurements you are getting   are  to the accuracy required to finish the mirror to the wave rating you want. You need to cross check your results and find the errors in the total procedure of measuring the zones to have confidence in your results.

  Measuring zones is like playing a music instrument. You can have the very best instrument but the lack the skill to play it. So it is combination of both the tester and skill, to do the measurement correctly.

 

                    - Dave 

I like your consul there, Dave. Especially precision vs accuracy (relative vs absolute). And systematic vs random sources of error. And doing a "sanity check" using another tester, preferably another test Method! I take it that we agree the intent of Foucault set-up/execution is to control to the point where the accuracy is substantially-limited by pattern recognition, and little else.

 

 

You would appreciate this related anecdote: I worked at EK FSD, where the Hubble "Backup PM" was fabricated. EK proposed to P&E that both mirrors be redundantly-tested at the other's facility, just to assure that one or both were near perfect, and (Bonus!) to get a handle on how close our wavefront mappings would agree. That idea was rejected as too risky. As we now know, the flight mirror's flaw was not detected until... First Light... on orbit! Yikes! The perfectly-good EK PM is on display at the Air & Space museum, on the Mall.  Tom

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#15 Astro Daddy

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Posted 25 April 2018 - 05:45 AM

I will second that.  The depth of focus at the center is many times that of the outer zone.  I learned years ago to never ever use the center as the base reading.  Also using micrometers to take the readings is early 20th century technology.  For Foucault readings, they suffer from what mechanical engineers call stick slip which tends to make the reading over correct if there is any slide friction at all.  But I see you are using a dial indicator which should take care of that problem.  The best thing I ever did for my tester was to take the  plunge and buy a dial indicator that measures to four decimal places.  It is very easy to read and has greatly improved my ability to reduce scatter.

In one of my books " Understanding Foucault" the author seems to suggest starting at center and moving away from the mirror. So real world is the other way? Start at the zone further from center then work in. How does one establish a zero position using that method or am I just miss understanding what you guys are saying. I also was thinking that micrometers was the way to go thanks for that pointer. I have been truly amazed at the sensitivity of the Foucault test. One more question if I may. What is the other test you use to confirm the output of the Foucault test? I also have a 133 line Ronchi screen and was graciously given some software to show what the view should look like at different positions along the optical axis. Would this meet the requirement?

 

Thanks John



#16 IvorB

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Posted 25 April 2018 - 08:16 AM

In one of my books " Understanding Foucault" the author seems to suggest starting at center and moving away from the mirror. So real world is the other way? Start at the zone further from center then work in. How does one establish a zero position using that method or am I just miss understanding what you guys are saying. I also was thinking that micrometers was the way to go thanks for that pointer. I have been truly amazed at the sensitivity of the Foucault test. One more question if I may. What is the other test you use to confirm the output of the Foucault test? I also have a 133 line Ronchi screen and was graciously given some software to show what the view should look like at different positions along the optical axis. Would this meet the requirement?

 

Thanks John

To start your readings from the outer zone, calculate the zone readings from the center then subtract the outer zone reading from the results.  You will end up with negative numbers but just ignore the negative sign.  You can then zero your dial indicator on the outer zone.  I assume your indicator is located on the mirror side of the tester.

 

It sounds like you have some backlash or side play in your tester.  Drawer rollers should work if you can eliminate any side wobble.  I use caged ball bearings running in a vee groove (aluminum angle).



#17 DAVIDG

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Posted 25 April 2018 - 09:14 AM

 You can start at any zone you want as the zero point since it is the delta between the zone readings that counts.

 As for other tests to double check your results I use double pass autocollimation as my main test and then star test to confirm the results. I also use the Ross Null when making other conics figures besides a parabola like a an ellipse or hyperbolas but again double check the results of the  complete optical system via double pass and star test.

   The Gremlins can creep in at any time for any test so double check your results. I had a 6" f/4 mirror made by a company who was  known for making good optics. It was sold mounted up as a  Newtonian so that was the configuration it was meant for. When I used the scope it showed a large amount of over correction. When I tested the mirror it was very smooth with a perfect edge, yet when I ran a set of Foucault measurement they were almost  exactly 2x larger then  what they were suppose to be. Then I realized what had  happened. When someone calculated the knife edge readings they used the focal length instead of the radius of curvature in the r^2/ R formula. This doubled the values they were shooting for. Gremlins can get it in all types of ways. So double check your results. Both independent test methods should agree, if not something is wrong since they both can't be right and don't just accept the better one.

  If you want  to get an understanding of your precision for  the total method of making Foucault readings. Make a set of reading and do it 7 times. Take the whole setup down and put all back up and do another set of readings 7 times. Take it all down and set it back up, do the take down and setup 7 time along with the 7 sets of reading per test session. Now take all those readings and look at the standard deviation to  see what the spread is.  Is the spread small enough so results are in the range of the  wave rating your going for ? Remember thou this is just a test for your precision ( ie repeatably) of making the measurements. It doesn't account for any systematic errors. So for example if your tester has a flaw and gives a constant off set or you made an error is measuring the radius of curvature, or your understanding of how to judge when a zone is nulled is wrong. It won't show up in this test. You need an independent test method  to confirm that your getting accurate results since both methods need to agree and to make sure a Gremlin hasn't got into the results. 

 

                  - Dave 


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#18 TOMDEY

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Posted 25 April 2018 - 09:39 AM

Nother (very nice) approach is to take that LA[r] raw data and fit a parabola to it. Most calculators will do that for you. Then delete the constant (aka set it to zero) and THAT adjusted data is the best value to use for the reference. There are other optimizations corresponding to best-fit area-weighted paraboloid, minimum removal, mini-max surface slope, tightest impulse response, mini-max PV, Strehl... but, regardless, as long as you have an Accurate test... you WILL eventually get to a glorious mirror! And that underscores Dave's recommendation.

 

At work, we had calibrated AC (null-test) interferometers all over the place. I would take my stuff into work and test (at lunch time). At home, the Star Test is the ultimate arbiter. If you can get a nice looking Airy Disc, you know you are there! And, no kidding, A textbook Airy Disc is breathtaking. Telescope and atmosphere cooperating to deliver the best allowed by Mother Nature.  Tom


Edited by TOMDEY, 25 April 2018 - 10:11 AM.

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#19 Pinbout

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Posted 25 April 2018 - 09:44 AM

 

when I ran a set of Foucault measurement they were almost  exactly 2x larger then  what they were suppose to be. Then I realized what had  happened. When someone calculated the knife edge readings they used the focal length instead of the radius of curvature in the r^2/ R formula.

we all do dumb things every once in a while. lol.gif



#20 ccaissie

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Posted 25 April 2018 - 01:09 PM

I sent a 12" f/3 to OWL for coating, and he came back with his declaration that it was way off.  I checked his math and he had entered 30" instead of 35" f.l.  When he put the right numbers in, it came to 1/5.8 wave p-v, agreeing with my numbers.

 

In XP, you'll find that the first reading or ZERO is actally an inch or 2 from the center.  It's easy to read that first crest, and almost impossible to read the exact center.  The amount of mirror surface inside that first zone is relatively small, and mostly obscured by the secondary.  

 

So start there, do your readings outward, 2-3 times, average them and analyze.  then Remeasure, starting from a different zone...one that you find easiest to read, and check the others...average and analyze. 

 

So you get your first collection of numbers and you get some idea of the shape and direction to go in.  Under/over, bad edge, etc.

 

NOW, use the Ronchi to make sure you have smooth transitions from zone to zone....there can be some surface information that hides in between Foucault readings...especially if the surface is not smooth, which easily shows up in Ronchi. 

 

Many tests make good mirrors.


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#21 MKV

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Posted 25 April 2018 - 02:24 PM

The judgement of null zones is for me an area that I am having a hard time trying to learn.

John, David ("One of largest sources is judging were each zone nulls"), and Ivor ("The depth of focus at the center is many times that of the outer zone.") hit the nail on the head. 

 

To get a "feel" of what they're saying, you need to be able to calculate what is know as the defocus tolerance

 

Defocus tolerance (s) for each zone (z) is  

 

s = λ/(2Nsin2Uz)

 

N = refractive index of the medium (in this case N = air, which RI is ≈ 1.00)

λ = wavelength of the light used

in this case sinUz = Dz/2R.

Dz = the mirror zonal aperture diameter

R is the radius of curvature of the mirror.

 

The defocus tolerance formula above gives you a focal range within which the error will not exceed 1/4 wavelength. It's intimately related to the f-ratio of the test beam. The slower the beam the greater the tolerance.

 

The best way to think of defocus tolerance is the case of a Foucault test is as an uncertainty range within which you can not know when the zone is nulled. 

 

As an example, let's look at the defocus range for a 222 mirror with  radius of curvature of 2000 mm. The mirror is tested at R. Let's say that you use a geren LED for light source, so λ ≈ 0.00055 mm.

 

Testing the 90% zone so D200 = 200, R = 2000, sinU = 0.1, sin2U = 0.01, N = 1.00, λ = 0.00055

 

defocus range  =  0.00055/(2*1*0.01) = ±0.0275 mm

 

For a 20 mm central zone D20 = 20, sinU = 20/2000 =0.01, sin2U =  0.0001

 

defocus range = 0.00055/(1*1*0.0001) = ±2.75 mm

 

What does this mean? This means that for the marginal zone your uncertainty will be ±0.03 m, but for the central zone your uncertainty will be ±3 mm -- a 100-fold increase in uncertainty where the null is! So, if you start nulling the zone near the center  you can be off by as much as 6 mm and not know it!  Clearly such a large error will throw everything off. 

 

This is one (of several) problems with the Foucault test. Sure, we can all make the numbers appear right, but unless we know exactly where the zonal nulls are we're creating a "fantasy mirror."  This is where the need for alternative tests comes in as a sanity check. Chances are that for slower mirrors the results will be better then for very large an fast ones. Much of it also depends on experience of the maker. 

 

Autocollimation with a full aperture flat mirror using a Ronchi test, and finishing with a knife-edge null is a better alternative then the Foucault because there are no zones to measure and the test is entirely qualitative.  

 

If you wish to know your mirror like with no other available test to ATMs,  then I strongly suggest you make a giant step and joint the interferometry group https://groups.io/g/Interferometry, build your own interferometer and get verifiable quantitative results.

 

Good luck,

Mladen


Edited by MKV, 25 April 2018 - 03:12 PM.


#22 mark cowan

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Posted 25 April 2018 - 02:27 PM

In one of my books " Understanding Foucault" the author seems to suggest starting at center and moving away from the mirror. So real world is the other way? Start at the zone further from center then work in. How does one establish a zero position using that method or am I just miss understanding what you guys are saying. I also was thinking that micrometers was the way to go thanks for that pointer. I have been truly amazed at the sensitivity of the Foucault test. One more question if I may. What is the other test you use to confirm the output of the Foucault test? I also have a 133 line Ronchi screen and was graciously given some software to show what the view should look like at different positions along the optical axis. Would this meet the requirement?

 

Thanks John

You can use micrometers (I have) but the key here or with a dial indicator is to always travel in the same direction (outside to inside if the stage components feel increased tension that way - spring resistance etc.).  When coming onto a zone same applies, never back out to the position, always approach it from the same direction.  This takes up any slop in the system automatically.

 

The zero position doesn't matter.  Just pick an arbitrary starting point where the inside zone (in the above out-in version) will be within the range of the measuring device and measure them all.  The analysis takes care of the rest.

 

Star testing will confirm the Foucault, Ronchi will give it to you in broader strokes but only if you know what you're doing.


Edited by mark cowan, 25 April 2018 - 04:09 PM.

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#23 Pinbout

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Posted 25 April 2018 - 03:27 PM

 

Star testing will confirm the Foucault, Ronchi will give it to you in broader strokes but only if you know what you're doing.

aint that always the way...but only if you know what you're doing.



#24 Astro Daddy

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Posted 25 April 2018 - 05:10 PM

Woww! Thank you all for all the guidance. It will take a bit to work through some of the suggestions and to really grasp others. A full aperture flat mirror would be the rub in doing a autocollimation test. The star test would be most likely be the one I could perform, if I could get some good seeing to do it.

John



#25 Pinbout

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Posted 25 April 2018 - 05:30 PM

Good seeing? Bla...

 

https://youtu.be/J6GUbBLXx70




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