Hi Gordon: Great video and enjoyed watching. I haven't made a mirror in years and would like to try it again. My first successful grind was done back in the day with Edmund Scientific telescope mirror kit. Can you please tell me where I can find a suitable starter kit to make an astronomical mirror 10" short f value for wide field viewing. Best regards. David
Hello Mr.Gordon thanks for posting wonderful videos. I need a clarification. Many say focault and ronchi tests are not accurate for f2 mirrors is it true. Then what is the optimal test method for fast mirrors. What's the maximum focal ration bath interferometer can test accurately. Once gain thanks for sharing knowledge earned over years of experience with everyone.
Nagarajan K Thanks for your kinds comments, Nagarajan K! You can use Ronchi without a problem, as Ronchi isn't really a quantitative test. It's very helpful for finding zones and checking the overall state of the mirror. Using the Foucault test on fast mirrors is okay, with a couple of comments. First, you will need to make very accurate measurements, far better than would be acceptable for an f/5 or longer mirror. The tolerances are much tighter on the fast mirrors. Also, there are correction factors that you might need to consider for Foucault testing fast mirrors. For details, search on the internet for "correction terms for the longitudinal aberration" and you will find a lot of math! Finally, make sure you are using a moving-light-source tester.
I'm curious, what sort of telescope is this for I'm guessing some sort of prime focus astrograph maybe? With a coma corrector + camera mounted on a spider maybe?
***** Hi, Derek... this particular mirror will be gold coated and goes into a telescope that feeds images into an infrared spectrometer. It's pretty much a standard Newtonian reflector, just very fast.
What is the focal mearsurement in grinding an f2 the curvature of the center of the mirror. This will be real shallow grinding without the precut fabrication
@@GordonWaite I know this is old, but on a bigger mirror, couldn't you "chunk out" a large block of the middle if you knew what the dimensions would need to come out? Like taking out 80% of what you need to, to cut that down? (like drilling a lot of holes etc)
***** Hi! It is possible to regrind to f/2, but if you want to use the mirror in an astronomical telescope, then f/2 is probably too fast. It is very difficult to figure an f/2 mirror, and the coma is very bad. If you wanted a shorter telescope, I would recommend a 10" f/3.5. That is a very wide-field telescope that you would enjoy very much. Or even better, I would probably go ahead an aluminize your 10" f/5 and then make a new mirror that is f/3.5. The 10" f/5 will give you excellent views of the moon and planets, and it would be a shame to throw away the hard work that you've already put into it. Best regards, Gordon
GordonWaite Thanks a lot, I know it will be hard to figuring, I just need it to be spheroid for a project in mind that's why it has to be an f/2 but again shpere not parabola, thanks for the information, best regards.
+Bill Schlosser Hi, Bill! This mirror isn't built for visual use. It is a "light collecting" telescope that is part of a spectrometer. So it is just collecting and directing light onto a sensor.
+Calvin Valencia For making telescope mirrors, there are three common types of glass. The first is regular plate glass, also known as soda-lime glass. This is very cheap, and is commonly available in 1" thicknesses. It works well for mirrors in the 12" range and smaller. It requires longer cooling at the beginning of observing sessions, but a plate-glass mirror can perform just a well as a mirror made on a more expensive substrate. Next would be a glass called BK-7. This is slightly better than plate, with a lower thermal expansion coefficient. This kind of glass is also used for making lenses. The most common glass for mirror making is a borosilicate glass made by Schott called Supremax-33. This is fairly expensive, but is has good thermal properties, and is really excellent, both in quality and in performance. There are more exotic and expensive substrates like fused silica, quartz, Zerodur and the like. These have very, very little thermal expansion, but are expensive and more difficult to work.
I've seen this type of thing before, but I don't get it. On the biggest telescopes they talk about a billionth of an inch tolerance. Here you have a dude grinding away by hand with grit. How can this not distort the telescope image dramatically? I don't get it
You start with coarse grit, but you change grades and get finer and finer as you go. I end my "grinding" with grit that is 5 microns in diameter, which leaves surface defects in the .5-micron area. Then we switch from aluminum oxide to cerium oxide, and go to a polishing action instead of a grinding action. Cerium oxide is 1 to 3-micron size, but it embeds in pitch and shears away glass instead of grinding it away. With this process, we can get down to the 2 to 10 nanometer level of accuracy. Optical testing is really accurate, so you can produce a surface that is good to the 10 nanometer level, which is about .0000004 inches. As far as I know, a telescope mirror is the most accurate thing you can make with your two hands.
I use an instrument called a spherometer. It gives me a highly accurate measurement of the radius of the mirror, and it only takes a minute. You should see it in use at the 4:43 mark in my video: ua-cam.com/video/IZJp8xaG31s/v-deo.html
So, in order to get it right, you start with a rough parabolic shape, then you use the spherometer often in the process, with some kind of calculation for each circumference on the blank to ultimately achieve a precision parabolic shape. Is that basically the method? If so, it's still not a diy project.
Well, not quite. The mirror stays spherical all through the grinding process and through the rough polishing. If you are using a spherometer, the mirror is still spherical. A spherometer isn't accurate enough to help you put in the parabolic shape. For that the mirror has to be polished, and you use optical tests to measure the shape of the surface. The Foucault test has you examine the radius of a set of concentric zones on the mirror optically. Then you run the numbers into a software program like FigureXP to see what you have, and to see where you need to rub next.
Great to see! It's a long time I did not do this kind of exercise, very enjoyable contact with the material.
Looking forward to the figuring sessions on this mirror. Hope you will make a video of them.
Hi Gordon: Great video and enjoyed watching. I haven't made a mirror in years and would like to try it again. My first successful grind was done back in the day with Edmund Scientific telescope mirror kit. Can you please tell me where I can find a suitable starter kit to make an astronomical mirror 10" short f value for wide field viewing. Best regards. David
Does the tile side curv such as required sagitta?
Hello Mr.Gordon thanks for posting wonderful videos. I need a clarification. Many say focault and ronchi tests are not accurate for f2 mirrors is it true. Then what is the optimal test method for fast mirrors. What's the maximum focal ration bath interferometer can test accurately. Once gain thanks for sharing knowledge earned over years of experience with everyone.
Nagarajan K Thanks for your kinds comments, Nagarajan K! You can use Ronchi without a problem, as Ronchi isn't really a quantitative test. It's very helpful for finding zones and checking the overall state of the mirror. Using the Foucault test on fast mirrors is okay, with a couple of comments. First, you will need to make very accurate measurements, far better than would be acceptable for an f/5 or longer mirror. The tolerances are much tighter on the fast mirrors. Also, there are correction factors that you might need to consider for Foucault testing fast mirrors. For details, search on the internet for "correction terms for the longitudinal aberration" and you will find a lot of math! Finally, make sure you are using a moving-light-source tester.
GordonWaite Thankyou for your reply.
I'm curious, what sort of telescope is this for I'm guessing some sort of prime focus astrograph maybe? With a coma corrector + camera mounted on a spider maybe?
***** Hi, Derek... this particular mirror will be gold coated and goes into a telescope that feeds images into an infrared spectrometer. It's pretty much a standard Newtonian reflector, just very fast.
GordonWaite Thanks Gordon, very interesting. I guess a spectrometer does not need to worry about coma. Probably just uses a very narrow field.
f2 ?! Whoa. Coma correction?
How big could you go, looking for unique sized optical lenses opposed to mirrors
In theory, you can make a lens pretty big, using the same techniques as for a mirror. Making a 10" or 12" lens is possible. Glass could get expensive.
What is the focal mearsurement in grinding an f2 the curvature of the center of the mirror. This will be real shallow grinding without the precut fabrication
On that 10" f/2 mirror, it is .3125" deep at the center! That is quite deep. Glad it's not a bigger mirror, as it's a lot of work already.
@@GordonWaite I know this is old, but on a bigger mirror, couldn't you "chunk out" a large block of the middle if you knew what the dimensions would need to come out? Like taking out 80% of what you need to, to cut that down? (like drilling a lot of holes etc)
Hello Mr. Waite, I already have an 10" f/5 parabolized but not aluminized, my question is, is possible to grind it to f/2, thanks in advace.
***** Hi! It is possible to regrind to f/2, but if you want to use the mirror in an astronomical telescope, then f/2 is probably too fast. It is very difficult to figure an f/2 mirror, and the coma is very bad. If you wanted a shorter telescope, I would recommend a 10" f/3.5. That is a very wide-field telescope that you would enjoy very much. Or even better, I would probably go ahead an aluminize your 10" f/5 and then make a new mirror that is f/3.5. The 10" f/5 will give you excellent views of the moon and planets, and it would be a shame to throw away the hard work that you've already put into it. Best regards, Gordon
GordonWaite Thanks a lot, I know it will be hard to figuring, I just need it to be spheroid for a project in mind that's why it has to be an f/2 but again shpere not parabola, thanks for the information, best regards.
What Coma Corrector would you use with an f/2?
+Bill Schlosser Hi, Bill! This mirror isn't built for visual use. It is a "light collecting" telescope that is part of a spectrometer. So it is just collecting and directing light onto a sensor.
+GordonWaite Gotcha!
Hello Mr. Gordon Waite about where you are located thanks, I´m from Brasil
honorio sergio Hi! We're in New Jersey, about 30 miles south of New York City. Thanks!
What particular glass type people use for lens making?
+Calvin Valencia For making telescope mirrors, there are three common types of glass. The first is regular plate glass, also known as soda-lime glass. This is very cheap, and is commonly available in 1" thicknesses. It works well for mirrors in the 12" range and smaller. It requires longer cooling at the beginning of observing sessions, but a plate-glass mirror can perform just a well as a mirror made on a more expensive substrate. Next would be a glass called BK-7. This is slightly better than plate, with a lower thermal expansion coefficient. This kind of glass is also used for making lenses. The most common glass for mirror making is a borosilicate glass made by Schott called Supremax-33. This is fairly expensive, but is has good thermal properties, and is really excellent, both in quality and in performance. There are more exotic and expensive substrates like fused silica, quartz, Zerodur and the like. These have very, very little thermal expansion, but are expensive and more difficult to work.
If not perforated u have to start from scratch
holy hell I am dizzy now
I've seen this type of thing before, but I don't get it. On the biggest telescopes they talk about a billionth of an inch tolerance. Here you have a dude grinding away by hand with grit. How can this not distort the telescope image dramatically? I don't get it
You start with coarse grit, but you change grades and get finer and finer as you go. I end my "grinding" with grit that is 5 microns in diameter, which leaves surface defects in the .5-micron area. Then we switch from aluminum oxide to cerium oxide, and go to a polishing action instead of a grinding action. Cerium oxide is 1 to 3-micron size, but it embeds in pitch and shears away glass instead of grinding it away. With this process, we can get down to the 2 to 10 nanometer level of accuracy. Optical testing is really accurate, so you can produce a surface that is good to the 10 nanometer level, which is about .0000004 inches. As far as I know, a telescope mirror is the most accurate thing you can make with your two hands.
So you're constantly measuring the parabolic shape with some kind of precision instrument?
I use an instrument called a spherometer. It gives me a highly accurate measurement of the radius of the mirror, and it only takes a minute. You should see it in use at the 4:43 mark in my video: ua-cam.com/video/IZJp8xaG31s/v-deo.html
So, in order to get it right, you start with a rough parabolic shape, then you use the spherometer often in the process, with some kind of calculation for each circumference on the blank to ultimately achieve a precision parabolic shape. Is that basically the method? If so, it's still not a diy project.
Well, not quite. The mirror stays spherical all through the grinding process and through the rough polishing. If you are using a spherometer, the mirror is still spherical. A spherometer isn't accurate enough to help you put in the parabolic shape. For that the mirror has to be polished, and you use optical tests to measure the shape of the surface. The Foucault test has you examine the radius of a set of concentric zones on the mirror optically. Then you run the numbers into a software program like FigureXP to see what you have, and to see where you need to rub next.
В инглише не шарю, но видос понравился. )
F2?! Thats such a waste. How mu...er...little magnification does that get?