Great series Gordon, thanks or sharing! I made a 6" f8 many years ago when I was a teenager. Back then, about the only information on the subject was a three-part volume written in the 1920s (called Amateur Telescope Making if my memory serves me well). Making those pitch laps was a nightmare, it's horrible stuff and that smell has never left me... my parents must have been very understanding! I wish I had known about your trick to put them in the freezer before trying to get the stuff off!
OK, It's 10 years after the video was made but some things stay constant. The few professionals using the Foucault place the knife edge on the left so that the apparent direction of illumination is from right, just as R W Porter did. Either way is OK as long as the atm standardizes on the setup. Lunar observers first establish the direction the sun is shining on the Moon so that mountains, domes, and craters don't get reversed. The same for the mirror surface. Once a hole is established in the central zone of the surface, then parabolizing is on the way. One advantage of the Couder Mask or Everest Pins, is that the null zone is unambiguous. If the video had used a longer exposure per frame, the diffraction ring on the right would be visible. The angle of wrap of a belly band, whether the mirror is thick or thin is to be about 160 to 170 degrees. If it wraps around 180 degrees, it will squeeze the mirror at the 3 o'clock and 9 o'clock positions, introducing astigmatism into the surface. To avoid this, the posts that hang the band should be slightly wider than the diameter of the mirror. Any retaining clamps must also not pinch the mirror. The band should be about 90% of the edge thickness of the mirror. I prefer using steel packing band from shipping containers but it must be able to flex. I line the inner edge with thin rubber strips, glued in place. Airplane cable is also OK and if a slight groove is ground in the perimeter of the mirror, it will not tend to let the mirror fall out of the stand. Another was is to slightly tilt the stand and mirror top back away from the tester and raise the tester higher than the mirror center. Don't tilt more than a few degrees unless the stand has kinematic pads supporting the mirror like a flotation mount. Paul V.
Hey Gordon, thanks for posting this. I'm currently working on a 12" f/6 and am about to start the 25 micron grind. I still have a lot to learn about interpreting the tests and I just wanted to say your many videos have helped me a lot. Keep it up, we need it. Ray
I had my 150mm mirror tested and refigured it was 1/12th wave and a Strehl of .986 and I went for the enhanced aluminize of 96% reflectivity which is what I sent it in for. I must say it was definitely a better view. I bought the scope and the mirrors were scratched up from improper cleaning. A C6-N f5 Newtonian I enjoy it quite a bit.
Thanks Gordon. You are a pro, not me :-) So probably you are right. I wanted to post quite "tough message" to be sure to have a reaction. I can delet it. I am sure of a TDE, we can see in ronchi as well, look attentively, but for sure not 3mm :-) And I am sure all mirrors polished with standard method has small tde. The flexible pitch, as soon as the tool has overhang, the tool will "bend" a few nanometers and make a TDE as soon as the strokes comes back to the mirror, on the first tenth of mm at least. But for sure this mirror is excellent (second note: look 5:45, it is as well a bit astigmat. The external dark part is not touching the internal dark part the same way up and down, it is a sign. It could be that you knife is not aligned correctly with the light-line, but other pictures it is well adjusted. I am really sorry but this is even not lambda/15, astigmatismus is higher, but impossible to see lambda/10 astigmatis in ronchi or foucault. Lets forget about this "lambda-disease", this is for sure an amazing mirror and I am jaelous :-). The point is is, I cannot understand why a diffraction ring in only on one side. This is still strange for me. If your knife is really at the radius of the extreme edge (approx 1mm further then the last window center), the diffraction ring should be the same left and right, there is no reason why some more light is received from the left side than from the right side. That is why I am pretty sure that all mirrors have a very small (1mm?) TDE... my mirrors at least...
Hi Gordon... Great video, as always! Can you make a video showing your Foucault test apparatus? Your images are o steady and clear. Do you use an eyepiece or other optical aid at the knife edge??? I have some difficulty with my tester because at the knife edge there are always frustrating shadows next to the blade that look something like floaters, and they interfere with the shadow images coming from the mirror. Not sure what this is from, but I would love to see your setup and see if I can determine what the issue with mine might be. I'm sure there are others who would like to see your setup as well!
David, I always use a video camera to view the test, whether Foucault or Ronchi. You need to use different lenses, depending on the focal ratio of the mirror, although a good f/2 lens would do for most mirrors. Getting a short zoom lens, maybe 50 mm to 135 mm would be good, too. I'm going to upgrade my tester soon, so we'll try to do a video of it. Thanks!
Dear Gordan Waite, You are really good at explaining things in your videos. I really wish you would create a video explaining how to make a Foucault - Knife Edge Tester. I’m sure you have probably designed and built your own version. I loved this video on how to actually use a Couder Mask. When I was building my first telescope mirror 46 years ago, the making or finding a pre-made Foucault tester was the one thing that stumped me and of course using one even confused me more because I didn’t have one to play with. I finally found one for sale at Edmund Scientific, but it was useless because it was not really like they showed in the books and no instruction came on how to use it. There are a couple of visual how to build a Foucault tester on TH-cam, but none have any narration of what or why they are doing things and no measurements. So, if you could please create a video showing us how to make a good easy to use yet simple Foucault tester with a list of supplies that are readily available today. Thanks.
I should have studied up first.. I'm not privy to the "theory of operation" regarding R&F test.. I'll do better next time. But it was, none the less.. FASCINATING!!! Thank you. p.s. Is it to late to purchase this "exact" mirror? ;O)
Thanks! It's nice working with quartz... not so much waiting between sessions, so you can spend a little extra time where it's needed. Thanks for checking out the mirror! -Gordon
Thank you so much for your videos. Is this your primary means of testing your mirrors? I was under the impression that to reliably get lambda/10 required a fizeau interferometer.
Gordon, great series of videos! Could you do a video of your KE setup? Your images are so crisp and your shadow edges are clearly defined. Are you using a Ross Null lens, such as Ceravolo's Ross Null tester? I have not seen any videos of a KE set up anywhere. The way your deliver your information is so easy to understand. I, and many viewers I am sure, would love to watch a video of your demonstrating the setup. Thank you for all your videos Gordon! Amazing!
The surface is amazing well polished, wouahh!! But sorry, there is a terrible TDE (Turn Down Edge). Look at 5:15. At least the last 3mm near edge, totally turned down, probably several mm of radius different. This is extremely difficult to avoid, but it is good to have less than 2mm, here this is more. We sometimes think this is "a reflexion, or diffraction", but now, there is no one reason to have such an "arc of light" when we make the last window", this is absolutely a TDE, believe me… Strehl 0.98 yes, but turned edge, sorry. the 1/30 wavelength is probably, as usual, on 95 or 98% of the diameter, but the edge is maybe 2x lambda wrong. If you disagree, please tell me, really, i would be happy to be wrong… because my last mirror (a quite big one lets say) had the same, on 1-1.5mm near edge… Thanks
Hi, Christophe. Thanks for the compliment on the polish on the mirror. But there is no "terrible TDE" on this mirror. What your eye cannot see on the video is what I mention at 6:25, "has a diffraction ring on the RIGHT side." You cannot have a 360-degree diffraction ring and say there is a 3mm TDE. And the Ronchi certainly doesn't show a 3mm TDE. You have to be very careful when evaluating video of Foucault tests, as I've never seen a camera that didn't have problems with contrast on these tests.
@skyscraperjim Have fun with the 6" mirror. Are you making an f/6 or f/8 or what? I've been thinking about building a short 8" f/4.0 just to have something short and easy to throw in the car. Enjoy your project!
Hi sir, greetings of the day! Sir I have a doubt. If the KE reading of a particular zone is higher than the ideal reading for that zone (as calculated by figure xp), the zone is higher than ideal or lower than ideal? I mean how do I interpret and understand if that zone is raised or depressed than ideal.
I'm familiar with looking at interferometer data, and unfamiliar with Ronchi tests. I see a ton of coma in your "Ronchi-gram". Can you comment on why the mirror is still good? Very interesting video!
You are not seeing coma. Coma is a defect that every parabola has where off-axis points of light become distorted. A Ronchi test shows the actual shape of the mirror that you are testing. With a perfect paraboloidal mirror, if you set up a Ronchi grating behind the center of curvature, you will see bands that are themselves perfect parabolas. You are seeing the parabolic bands that show the mirror is parabolic, as long as the grating isn't too far from the center of curvature. If you are interested, Google "matching Ronchi test" for a better explanation.
Thanks for the video! I managed to find a document that has images of surface errors in a Ronchi test. (astigmatism/coma/spherical/rolled edges). Do you use software to predict the ideal pattern?
Yes. Mel Bartels has software (matching ronchi test) that generates the theoretically perfect pattern for any mirror. Very easy and fast. This doesn't give high-accuracy numbers, but it is very useful to see how you are doing quickly as you work.
I am inspired to try making my firtst mirror. Problem is I dont want anything small LOL. I want to make something 10" or larger. Prob not the best thing to do as a beginner. Though I may be better off having mirrors made for me I don't know. I think I'd have more fun building my telescope and have the morors made. I'm thinking a 14" mirror set. Thank for the great video's. Cheers, IC.
I built my first scope (8") and just bought a Meade mirror. The rest of the scope was a pretty fun build but not grinding your own mirror does two things, 1) You save time and 2) You completely loose out on the aspect of perfecting and better understanding the optics. Waite's videos are a godsend in this respect. I watch them, repeatedly, and I have gotten a good basic grasp of the process of mirror grinding, figuring, polishing and coating. And as some might suspect, once you are really good at the science (and business) of mirrors, it can become your next career!
@@GordonWaite Already have it and it is quite good. it is actually free online and being that there is more area for diagrams and color, I tend to like the online version better. www.atm-workshop.com/foucault.html
6:12 I'm a bit confused by this pattern that I see a lot in these videos. Why is it desirable to have this raised doughnut area towards the edge rather than a perfect curve from the edges to the middle of the mirror?
If you saw a perfectly even curve of semi shadow, it would mean you had a spherical mirror. The classic doughnut shape at the 70% zone is indicative of an aspheric surface. The distance the knife edge nulls-out in front or behind that point let's you know just which one you have.
@@bscottlam Thank you! So when seeing that you have an aspheric surface like this, it would be preferable to try and flatten out the doughnut to make it spherical? Because my confusion is that light hitting the outside edges of the mirror will be reflected off to the sides rather then into the middle.
@@DidntKnowWhatToPut1 You want your mirror to have a parabolic shape. A parabola touches a sphere at the 70% zone. Both inside of that and outside of that the parabola is "below" the sphere. Thus the nature of the shadow that you see in the Foucault test. Check out the little book "Understanding Foucault" for a better and fuller explanation.
Thanks. I don't wear the cape in the grinding room, as it tends to get caught on the edge of the turntable. At 60 rpm's I get wound up pretty tightly when that happens!
The classic read is "How to Make a Telescope" by Jean Texereau. It's an old book... in his time, you did the algebra by hand. Today you just plug numbers into software.
This video was a support video for a long thread I did on Cloudy Nights, as mentioned in the video. If you look up "twin 20's" there, you will see the whole story.
![ง้อ (ALRIGHT) - FOURTH [ TEASER ]](http://i.ytimg.com/vi/ya6MBfo-Wvg/mqdefault.jpg)
Incredibly smooth! And 1/30th wave, wow! Congrats!
Great series Gordon, thanks or sharing! I made a 6" f8 many years ago when I was a teenager. Back then, about the only information on the subject was a three-part volume written in the 1920s (called Amateur Telescope Making if my memory serves me well). Making those pitch laps was a nightmare, it's horrible stuff and that smell has never left me... my parents must have been very understanding! I wish I had known about your trick to put them in the freezer before trying to get the stuff off!
OK, It's 10 years after the video was made but some things stay constant. The few professionals using the Foucault place the knife edge on the left so that the apparent direction of illumination is from right, just as R W Porter did. Either way is OK as long as the atm standardizes on the setup.
Lunar observers first establish the direction the sun is shining on the Moon so that mountains, domes, and craters don't get reversed. The same for the mirror surface. Once a hole is established in the central zone of the surface, then parabolizing is on the way. One advantage of the Couder Mask or Everest Pins, is that the null zone is unambiguous.
If the video had used a longer exposure per frame, the diffraction ring on the right would be visible. The angle of wrap of a belly band, whether the mirror is thick or thin is to be about 160 to 170 degrees. If it wraps around 180 degrees, it will squeeze the mirror at the 3 o'clock
and 9 o'clock positions, introducing astigmatism into the surface. To avoid this, the posts that hang the band should be slightly wider than the diameter of the mirror. Any retaining clamps must also not pinch the mirror. The band should be about 90% of the edge thickness of the mirror. I prefer using steel packing band from shipping containers but it must be able to flex. I line the inner edge with thin rubber strips, glued in place. Airplane cable is also OK
and if a slight groove is ground in the perimeter of the mirror, it will not tend to let the mirror
fall out of the stand. Another was is to slightly tilt the stand and mirror top back away from the tester and raise the tester higher than the mirror center. Don't tilt more than a few degrees unless the stand has kinematic pads supporting the mirror like a flotation mount.
Paul V.
Excellent videos Gordon. You're renewing that old interest in making my own mirror.
Hey Gordon, thanks for posting this. I'm currently working on a 12" f/6 and am about to start the 25 micron grind. I still have a lot to learn about interpreting the tests and I just wanted to say your many videos have helped me a lot. Keep it up, we need it.
Ray
You are the best home mirror guy ever
Thanks! Have you got a project going yourself?
Excellent mirror.
Very descriptive with good images on what a correct mirror should be
Great videos! I've been looking a long time for such descriptive videos on miror making and testing.
I had my 150mm mirror tested and refigured it was 1/12th wave and a Strehl of .986 and I went for the enhanced aluminize of 96% reflectivity which is what I sent it in for. I must say it was definitely a better view. I bought the scope and the mirrors were scratched up from improper cleaning. A C6-N f5 Newtonian I enjoy it quite a bit.
Very good video, descriptive comments, Gordon- thanks for going to the trouble to show it!
Wes James
Thanks Gordon. You are a pro, not me :-) So probably you are right. I wanted to post quite "tough message" to be sure to have a reaction. I can delet it. I am sure of a TDE, we can see in ronchi as well, look attentively, but for sure not 3mm :-) And I am sure all mirrors polished with standard method has small tde. The flexible pitch, as soon as the tool has overhang, the tool will "bend" a few nanometers and make a TDE as soon as the strokes comes back to the mirror, on the first tenth of mm at least. But for sure this mirror is excellent (second note: look 5:45, it is as well a bit astigmat. The external dark part is not touching the internal dark part the same way up and down, it is a sign. It could be that you knife is not aligned correctly with the light-line, but other pictures it is well adjusted. I am really sorry but this is even not lambda/15, astigmatismus is higher, but impossible to see lambda/10 astigmatis in ronchi or foucault. Lets forget about this "lambda-disease", this is for sure an amazing mirror and I am jaelous :-). The point is is, I cannot understand why a diffraction ring in only on one side. This is still strange for me. If your knife is really at the radius of the extreme edge (approx 1mm further then the last window center), the diffraction ring should be the same left and right, there is no reason why some more light is received from the left side than from the right side. That is why I am pretty sure that all mirrors have a very small (1mm?) TDE... my mirrors at least...
Beautiful shadows!
Hi Gordon... Great video, as always! Can you make a video showing your Foucault test apparatus? Your images are o steady and clear. Do you use an eyepiece or other optical aid at the knife edge??? I have some difficulty with my tester because at the knife edge there are always frustrating shadows next to the blade that look something like floaters, and they interfere with the shadow images coming from the mirror. Not sure what this is from, but I would love to see your setup and see if I can determine what the issue with mine might be. I'm sure there are others who would like to see your setup as well!
You will find this in the chapter about Foucalt Test in the the book of Daniel Malacara "Optical Shop Testing"
David, I always use a video camera to view the test, whether Foucault or Ronchi. You need to use different lenses, depending on the focal ratio of the mirror, although a good f/2 lens would do for most mirrors. Getting a short zoom lens, maybe 50 mm to 135 mm would be good, too. I'm going to upgrade my tester soon, so we'll try to do a video of it. Thanks!
Dear Gordan Waite, You are really good at explaining things in your videos. I really wish you would create a video explaining how to make a Foucault - Knife Edge Tester. I’m sure you have probably designed and built your own version. I loved this video on how to actually use a Couder Mask. When I was building my first telescope mirror 46 years ago, the making or finding a pre-made Foucault tester was the one thing that stumped me and of course using one even confused me more because I didn’t have one to play with. I finally found one for sale at Edmund Scientific, but it was useless because it was not really like they showed in the books and no instruction came on how to use it. There are a couple of visual how to build a Foucault tester on TH-cam, but none have any narration of what or why they are doing things and no measurements. So, if you could please create a video showing us how to make a good easy to use yet simple Foucault tester with a list of supplies that are readily available today.
Thanks.
I need to upgrade my Foucault tester, so I'll try to do a video. Thanks for the idea!
@@GordonWaite Dear Mr. Gordon, I would very much like to know more about your Foucault tester. Thanks!
I should have studied up first.. I'm not privy to the "theory of operation" regarding R&F test.. I'll do better next time. But it was, none the less.. FASCINATING!!! Thank you. p.s. Is it to late to purchase this "exact" mirror? ;O)
What a beauty!
Thanks! It's nice working with quartz... not so much waiting between sessions, so you can spend a little extra time where it's needed. Thanks for checking out the mirror! -Gordon
Thanks, mr Gordon
Thank you so much for your videos.
Is this your primary means of testing your mirrors? I was under the impression that to reliably get lambda/10 required a fizeau interferometer.
Gordon, great series of videos! Could you do a video of your KE setup? Your images are so crisp and your shadow edges are clearly defined. Are you using a Ross Null lens, such as Ceravolo's Ross Null tester? I have not seen any videos of a KE set up anywhere. The way your deliver your information is so easy to understand. I, and many viewers I am sure, would love to watch a video of your demonstrating the setup. Thank you for all your videos Gordon! Amazing!
Thank you very much for sharing these videos. Would you please show us a picture of your foucault tester?
I am rebuilding my tester right now. Should be done within a couple of weeks, and I will post a video on it then. Thanks!
The surface is amazing well polished, wouahh!! But sorry, there is a terrible TDE (Turn Down Edge). Look at 5:15. At least the last 3mm near edge, totally turned down, probably several mm of radius different. This is extremely difficult to avoid, but it is good to have less than 2mm, here this is more. We sometimes think this is "a reflexion, or diffraction", but now, there is no one reason to have such an "arc of light" when we make the last window", this is absolutely a TDE, believe me… Strehl 0.98 yes, but turned edge, sorry. the 1/30 wavelength is probably, as usual, on 95 or 98% of the diameter, but the edge is maybe 2x lambda wrong. If you disagree, please tell me, really, i would be happy to be wrong… because my last mirror (a quite big one lets say) had the same, on 1-1.5mm near edge… Thanks
Hi, Christophe. Thanks for the compliment on the polish on the mirror. But there is no "terrible TDE" on this mirror. What your eye cannot see on the video is what I mention at 6:25, "has a diffraction ring on the RIGHT side." You cannot have a 360-degree diffraction ring and say there is a 3mm TDE. And the Ronchi certainly doesn't show a 3mm TDE. You have to be very careful when evaluating video of Foucault tests, as I've never seen a camera that didn't have problems with contrast on these tests.
@skyscraperjim Have fun with the 6" mirror. Are you making an f/6 or f/8 or what? I've been thinking about building a short 8" f/4.0 just to have something short and easy to throw in the car. Enjoy your project!
Hi sir, greetings of the day!
Sir I have a doubt. If the KE reading of a particular zone is higher than the ideal reading for that zone (as calculated by figure xp), the zone is higher than ideal or lower than ideal? I mean how do I interpret and understand if that zone is raised or depressed than ideal.
I'm familiar with looking at interferometer data, and unfamiliar with Ronchi tests. I see a ton of coma in your "Ronchi-gram". Can you comment on why the mirror is still good? Very interesting video!
You are not seeing coma. Coma is a defect that every parabola has where off-axis points of light become distorted. A Ronchi test shows the actual shape of the mirror that you are testing. With a perfect paraboloidal mirror, if you set up a Ronchi grating behind the center of curvature, you will see bands that are themselves perfect parabolas. You are seeing the parabolic bands that show the mirror is parabolic, as long as the grating isn't too far from the center of curvature. If you are interested, Google "matching Ronchi test" for a better explanation.
Thanks for the video! I managed to find a document that has images of surface errors in a Ronchi test. (astigmatism/coma/spherical/rolled edges). Do you use software to predict the ideal pattern?
Yes. Mel Bartels has software (matching ronchi test) that generates the theoretically perfect pattern for any mirror. Very easy and fast. This doesn't give high-accuracy numbers, but it is very useful to see how you are doing quickly as you work.
Scotty, You may read the chapter about Ronchi Test in the the book of Daniel Malacara "Optical Shop Testing"
I am inspired to try making my firtst mirror. Problem is I dont want anything small LOL. I want to make something 10" or larger. Prob not the best thing to do as a beginner. Though I may be better off having mirrors made for me I don't know. I think I'd have more fun building my telescope and have the morors made. I'm thinking a 14" mirror set. Thank for the great video's. Cheers, IC.
I built my first scope (8") and just bought a Meade mirror.
The rest of the scope was a pretty fun build but not grinding your own mirror does two things, 1) You save time and 2) You completely loose out on the aspect of perfecting and better understanding the optics.
Waite's videos are a godsend in this respect.
I watch them, repeatedly, and I have gotten a good basic grasp of the process of mirror grinding, figuring, polishing and coating.
And as some might suspect, once you are really good at the science (and business) of mirrors, it can become your next career!
It's all Geek to me Gordon. Got a Foucault for Dummies reference ?
You’re finding the focal length at the various parts of the mirror, and then you can calculate whether the mirror is correct.
@@JasperJanssen OK I thought it was for testing the parabolic curvature.
Check out the book Understanding Foucault by David A. Harbour. Likely under $10 on Amazon.
@@GordonWaite Already have it and it is quite good. it is actually free online and being that there is more area for diagrams and color, I tend to like the online version better. www.atm-workshop.com/foucault.html
Can you tell us how and with what equipment you were able to video this ? trying to do same.
6:12 I'm a bit confused by this pattern that I see a lot in these videos. Why is it desirable to have this raised doughnut area towards the edge rather than a perfect curve from the edges to the middle of the mirror?
If you saw a perfectly even curve of semi shadow, it would mean you had a spherical mirror. The classic doughnut shape at the 70% zone is indicative of an aspheric surface. The distance the knife edge nulls-out in front or behind that point let's you know just which one you have.
@@bscottlam Thank you! So when seeing that you have an aspheric surface like this, it would be preferable to try and flatten out the doughnut to make it spherical? Because my confusion is that light hitting the outside edges of the mirror will be reflected off to the sides rather then into the middle.
@@DidntKnowWhatToPut1 You want your mirror to have a parabolic shape. A parabola touches a sphere at the 70% zone. Both inside of that and outside of that the parabola is "below" the sphere. Thus the nature of the shadow that you see in the Foucault test. Check out the little book "Understanding Foucault" for a better and fuller explanation.
@@GordonWaite Thank you so much
I'm not quite grasping this. Can you recommend a book for newbies to understand this ? Alternatively, a website for us slow folks ?
Try "Understanding Foucault" by David A. Harbour, available on Amazon, I think.
@@GordonWaite Already bought and read most of it. I'm working through a Stellafane tutorial now.
¿Is the mirror parabolic?
Yes!
GordonWaite Not all heroes wear capes! Unless you wear one whilst making mirrors!
Thanks. I don't wear the cape in the grinding room, as it tends to get caught on the edge of the turntable. At 60 rpm's I get wound up pretty tightly when that happens!
How much, 25 inche mirror?
Around $12,000 to $15,000 depending on focal length.
Hey why is there a hole in the center?
Because it is for a Cassegrain type telescope.
جميل جدا
What do you use to generate your Caude screens?
This would have been a good one to answer!
4 years and waiting...
@@dantyler1558
10 years and waiting...
You lost me after one minute. I guess this video is meant for 'insiders' only. I may be needing some reading about the subject.
The classic read is "How to Make a Telescope" by Jean Texereau. It's an old book... in his time, you did the algebra by hand. Today you just plug numbers into software.
You must read the chapter about Ronchi Test in the the book of Daniel Malacara "Optical Shop Testing"
No clue what was going on in this film. Mention of a knife edge, but I didn't see a knife. Nothing was explained.
This video was a support video for a long thread I did on Cloudy Nights, as mentioned in the video. If you look up "twin 20's" there, you will see the whole story.