+Dink Martini Thanks... glad you enjoyed the videos!

This is what I love about KZfaq. So much craft to be learned.

Teach a man how to fish :)

Ah I see you are australian

It used to be called an apprenticeship. Learning from a master optician here. I wonder if Zambuto does one?

Any father involving his children in walking the footsteps of human kinds most brilliant and philosophically/scientifically revolutionary minds by creating the tool that opened up both the largest and smallest aspects of the very universe we reside in and in turn kicking off the era of enlightenment is doing something right...

Thanks for sharing this memory with us. My hats off in respect to him, for his work and his inclusion of you in his pursuits.

THank you for sharing!

100th like!

My dad made his first mirror by hand, before is was born. Went on to make optical components for Martin Marietta. And eventually, opened his own optics shop were I worked with him after school. I will always remember those days.

We were likely grinding mirrors around the same time. I made a 6" long-focus mirror around '69 or '70. It's in a box in the shop. You've motivated me to get it out and put it in a telescope! Thanks!

Awesome! :)

LordCarpenter: Gordon... are you my dad?

Go for it! It's a very satisfying hobby.

I hear ya. I'm already mentally designing new telescope styles... ever heard of a right angle reflector? I guess there's probably a reason that nobody makes one like a sideways periscope, lol.

Best feeling is the first look through a telescope where you built the mirror yourself. Kind of magical! Always hard to believe it works so well.

@@GordonWaite Not really ancient though, is it?

Roots back to Galileo, I guess. Unless you want to count Stonehenge as an astronomical instrument. Stretching it, I know! ;)

@@GordonWaite Oh well, I did not think about it in such a perspective, but in that case I guess you can say it's an ancient art. I thought it was about high precison mirror manufacturing :D

NOOOOOO!!!!!

Its for a smart phone camera lens. So he's go a way to go.

Person with the largest telescope when he dies, wins.

I know right!

How? Doesnt sound anything like him imo

@@brandonbaker5792 simmilar accent.

+m326119 Thanks! I have enough video shot to make five or six more videos, if only I could find the time to edit them all!!

@@GordonWaitePlease do not think much for taking time for editing, We can happily digest raw files directly from you. Please do not stop the the knowledge supply..

David Paul Hi! Whenever I use high-speed diamond tooling, I always recommend wearing a quality dust mask. And I always keep the piece under running water. I never work a dry piece with high-speed diamond tooling. Thanks for reiterating the warning.

In the rock and slab world there are some very nice wet, handheld, grinders and diamond abrasives...friend has one to free form polish obsidian...still requires a filter mask and he works outside.

You need an inexpensive industrial respirator that takes the violet colored HEPA filter, High Efficiency Particulate Air, it will filter out microbes down to 0.3 micron, used by asbestos workers.

It evens out quite quickly. The surface averages out against the grinding tool, and contact becomes closer and closer as you grind.

Thanks! -Gordon

Good idea. Thanks!

Thanks! I've been mining some older viideo footage, so I'm hoping to get some more videos up. Any particular topics you would like to see?

I would if I had to do it more than once.

Thanks, Mars! Let me know what you would like to see!

GordonWaite. I'm trying to learn how to turn a flat k9 glass disk into a concave lens (sigh) no tools for glass here so I have to figure out how. I was thinking carbide sandpaper (finer grit every time) and a pad... Sorry to bother so much, any advice? Thank you very very much!

How big a lens, and what focal length?

GordonWaite : thanks for reply. Lens should be about 35 cm diameter. It's not for optical use so it doesn't need to be accurate. I just need it to be a little bit concave (very shallow) but it has to be smooth. THANK YOU!!

Well, that's interesting! You can take normal sand, and wash it. Then crush it in a ball mill. Then use the technique where you suspend the sand in water by swirling it up, then separate it into grades by how long it takes to drop out of the water. You should be able to get 4 or 5 grades this way. Use that for your rough grinding. For fine grinding, many toothpastes have aluminum oxide in them, which is a fine abrasive. Good luck!

We have explored most of the parameters. Mainly you run into safety problems, like you can only spin a mirror so fast. And you can only put so much weight on a stack until the turntable won't turn any more. Or you find that more weight polishes faster but leaves a rougher surface. So in the end you have to find practical ways to do everything, while trying to optimize factors like cost, time, profit, and so on.

​@@GordonWaite it would be interesting to have a 'little black book' of parameter boundary values or graphs of parameter variation vs. glass removal with markers on the graph indicating points where other phenomena intervene (like 'turntable stops spinning' or 'grit starts flying off the blank' or 'glass blank explodes and takes out a wall') although I guess there's a lot of interplay between the various factors like rpm, grit load, water, etc. and it wouldn't be that straightforward.

I have used shakers in the past, and they work pretty well. Our newest machines all have automated dispensers now!

GordonWaite oh nice

Very cool! You and I were making mirrors around the same time back then. My first was a 6", also with the help of Texereau, and the other ATM books of the time.

I can send you photos of me and the finished telescope. My mail: jorge646@gmail.com

Hi! Thanks for your question. I'll see if I can get Emerson to help me shoot a short video showing how to center the mirror on the turntable.

That would be great! Thank you very much and have a nice day!

ParaglidingManiac Use a machinist dial indicator. $50-$80 bucks on amazon for a good Fowler. That will do well enough for this. I imagine this question is no longer relevant

It clamps into the turn table like a laythe

If you email me, we can talk about a finished mirror.

The abrasive doesn't embed in the glass. Instead, it chips off tiny pieces of glass as the grains roll around between the glass and the grinding tool. With loose abrasives, the two surfaces get into closer and closer contact as the grinding continues, and you use finer and finer abrasives. Over the course of the mirror, you will end up using several pounds of the various grades of abrasive.

Would it be because the blank could be intended for a variety of telescopes?

Casting takes extreme heat and risks bubbles and sticking and devitrification. Kilns are expensive and wear out faster at that temp. Slumping thin glass is a better idea.

Most blanks are cut from a standard sheet of glass, then ground. Much cheaper than building a custom mold and doing a casting.

Could you use the almost finished mirror as a mould to cast many new mirrors out of some material like epoxy then get these Aluminised or would the new moulded mirrors just not have the correct optics?

I would hesitate to guess. Professionals have been spin casting mirror blanks for a long time. Given that the surface of any spin-cast blank is still going to need grinding and polishing, the accuracy probably isn't at the "optical" level, but only at the "mechanical" level. You would have to figure out if spin casting saves any time or money over just generating a blank like usual.

Все возможно, главное терпение

Depends on focal length maybe?

If you enjoy kiln work, more power to you! Enjoy yourself!

I am grinding a mirror. The optical face will be concave.

I sometimes just run a water hose onto the surface. It gets pretty messy, as the water splashes around because of the high-speed grinder. Once you go to the loose abrasives, I do have an automatic water feed to keep it wet on the fixed-post grinding machine. I have an Arduino that controls how the abrasive and liquid is fed during grinding, as well.

+Johny Burbano The tool works quickly because the mirror is somewhat concave when you start. If you move the tool in toward the center, the mirror will get more concave. If you move it out away from the center, the mirror will get less concave.

ok, thank you.

No real rule. If you have some experience, the grinding tool can be quite a bit smaller, especially if you have a good spherometer and test often as you grind. The smaller tools, say 50% diameter or so, work pretty well. If it is your first attempt at grinding something flat, though, a 60% or 70% tool might be easier.

Tres Longwell Hi! The grinding tool is concave, spherical. It is made from dental plaster and ceramic tile, molded to the face of the mirror. The top of the steel rod just rides in a bronze sleeve bushing. It has a locking collar above the bronze bushing that keeps the steel rod from lowering any farther into the receptacle on the back of the mirror. But it is held in only by gravity. You can just pull it straight up out of the back of the mirror, and keep going up until it comes out of the sleeve bearing on the gantry arm. There is no pivot.

+walter volpato Hi, Walter! Making a 20" mirror from 25mm glass is possible, but that would be a very difficult build. I would not want to do a project like that professionally, as the extra time you would need to spend would overwhelm the cost savings on the cheaper glass. It is hard to grind such a large, thin blank without introducing 'stig, plus the support of the mirror during testing would be difficult. Possible, but difficult!!

Window glass is soda lime glass, or plate glass. Telescope mirrors are generally a different type of glass, called borosilicate glass. This kind of glass doesn't expand and contract as much as plate glass, so it holds its shape better when the temperature drops at night.

Thin mirrors cool faster, and are lighter to fabricate in the larger sizes. Thick mirrors hold their shape better so you don't need as good a mirror cell. Always a tradeoff.

Yes, you need to send the mirror to a commercial coating company to have the reflective aluminum coating evaporated onto it. We often recommend Jeff Decker at Majestic Coatings. He has reasonable prices and very fast turnaround times.

A thinner mirror weighs less and is easier to handle during fabrication, as long as it isn't too thin. Also, the thinner the mirror, the faster it cools, and thus the faster you can have good viewing after setting up the instrument. And a thinner mirror makes for a lighter telescope which is easier to transport and set up.

The mirror is, indeed, concave. The tool is convex.

+Joseph Dragan he's grinding what will become the front. That's what I got from 3:15 to 3:35 "This for example is GOING to be an F3.1 mirror..." That also makes sense from thinking about how crazy it would be to invest that effort on the wrong side.

+Joseph Dragan Hi, Joseph! In this video, I am grinding on the back of the mirror blank. This was originally a 25" blank that was over 2 inches thick. That is thicker than I like for a modern mirror. Mirrors that are this thick take a long time to cool down, and they are very heavy, making for a mirror blank that is difficult to fabricate, and a telescope that is very heavy and difficult to move. So I decided to take about a half inch of glass off the back of the blank. This removed about twenty pounds of glass, and results in a far more observer-friendly telescope. The work takes quite a while because you actually want to get the back quite flat. This is necessary to make it easier to control astigmatism, which you don't want to see during fabrication, and for SURE you don't want to see in the eyepiece!

The grinding motion naturally tends toward producing a sphere (or a flat), when using a simple fixed-post grinding machine. You control the shape of the mirror by changing the pivot point on the tool on top of the mirror. Inner positions produce a deeper curve in the center. Outer positions produce a shallower curve toward the edge. You manipulate this position during grinding until you arrive at something close to the target radius. By then you should have found the "neutral" position that does not change the radius. By grinding through the finer grades of abrasives at the neutral position, you produce a very accurate spherical surface. Producing a paraboloidal surface cannot be done geometrically. At every distance from the center of the parabola, the radius of that zone will be unique. So the radius constantly changes from center to edge. To produce a parabola you traditionally use deformable tools. As you work a given position, you measure the radius to match a target. In practical terms, you define anywhere from 3 to 12 zones on your mirror and measure them all as you work the surface. Each zone has a target radius, and you work to deform the spherical surface of the glass to hit that target number. Doing spherical surfaces is easy, while making paraboloidal surface is much more time-consuming, and thus more expensive.

@@GordonWaite I see, thanks for explaining, so you have to do alot of measuring and adjusting to aproximate the parabola, have you ever thought about using a centrifugal oven/furnace? - I suppose it wouldnt be too hard to build an insulated chamber on a bearing, driven by a servo or stepper motor to keep a constant frequency/rpm and automate the whole thing with an arduino-microcontroller (popular and super easy to use/program component that alot of hobby DIY'ers use). - Everything the microcontroller would have to do is keep the motor at a constant speed, and measure the temperature and keep the chamber at the desired temperature to very slowly let the glass cool down... If cooled down slowly enough to avoid tension and stress in the glass, that will yield a perfect paraboloid of any desired focal length (given by the rpm) and the size is only limited by the size of the furnace... Maybe, once set up, that would be an easier and cheaper way to do it?? In any case it would be a fun DIY project i think =) , if i lived near you, we could team up and build it hehe

@@GordonWaite Do you reckon that that furnace apporach would work or not? You obviously are the expert, maybe im wrong to think that this would work... thx.

@@jeffreylebowski4927 They have been making very large mirrors using this technique for quite a while, specifically at the Caris Mirror Lab in Arizona. First big problem is the glass takes days to melt, then many days to anneal and then cool slowly to normal temperature. Everything about this idea is expensive and time consuming, and thus it is only used for mirrors in the 3-meter+ range, or so. You're not going to be making $10,000 mirrors with $10,000,000 worth of equipment. The other problem is the word "perfect." The mirrors that are made with spin casting are nowhere close to perfect paraboloids. They will still have to be rough ground, fine ground and polished, and then figured to final shape. The closest the industry comes to this is you can have sub-meter mirrors molded to approximately the right radius. Molded blanks are about 50% more money than plano/plano, and they still require all the grinding, polishing and figuring operations. The way this is done today is with diamond generation, which we do in shop. We can diamond generate a 20" mirror in an hour or so, with maybe $20 in consumable diamond products. Impossible to beat those economics!

@@gordonwaite1697 Yeah you are right, while it wouldnt take days for the glass of a 'small' 20" mirror to melt and anneal, or cost 10 million$ - thats only true for the huge, professional funaces - as the glass cools down it would contract and maybe change shape even if done perfectly homogenously/infinitly slowly, i had not thought of that, that means my idea wouldnt make sense... Thanks for your insights, maybe one day ill ask you for a mirror =), right now im still happy with my 8" skywatcher.

For telescope mirrors, the problem is expansion and contraction due to temperature change. Glass is quite stable, and borosilicate glass is very stable. That allows you to form good images even if the mirror is heating or cooling because of being taken out of a garage or house and being exposed to temperatures that vary by 20 or 30 degrees. Accuracy of a mirror's surface is measured in nanometers, so thermal effects are very important.

There are.

You can order glass from places like Swift Glass, who will cut discs for you from sheet glass. Depending on what you order, you may need to have it Blanchard ground, edged and beveled. A ballpark estimate on the cost of a 25" blank like in this video would be something between $1,500 and $2,000. Get Supremax-33 for best-quality borosilicate glass for astronomical mirrors.

That video was done in 2014. These days we have computer-controlled grinding and polishing machines and computer-controlled abrasive applicators, slurry pumps and water supply. We had to design and build these ourselves. The market is so small that commercial machines didn't exist earlier, and even today they are WAY expensive and not economical for smaller fabricators.

Louis Barbisan Hi! Making an F1 parabola would be a very big challenge. And using it would be a big challenge. I doubt that any of the commercial coma correctors could do the job. The faster the parabola, the more precise the surface must be measured, and the longer it takes to excavate the glass to parabolize. I have made mirrors down to F2, but for practical astronomical work, mirrors around F2.7 or F2.8 would be a good practical limit.

GordonWaite What you'r telling me that it's very hard to develop, due to the always changing curvature line from the center out, and not been spherical at all ?

Louis Barbisan Yes, the difference between a sphere and a parabola are huge when you get down to F1. For example, the sagitta on a 12.5" F1 sphere is .7813 inches, while the sagitta on the paraboloid is .7939 inches, a difference of .0126 inches. But on an F5 mirror, the difference would be .000098 of an inch. So you can see that you have to remove 130 times more glass to parabolize an F1 mirror over an F5 mirror that size.

***** Hi, Tim... The first place I would get a price from would be Newport Glass Works in CA. Their Web site is at newportglass.com. Prepare for sticker shock! The price is likely to be north of $2,500. If you go for it, I highly recommend that you have them generate to blank to your required radius. This saves hours of time for you, and pounds of abrasive. As an alternative to Newport, you could try United Lens in NY. Likely to be even more expensive, especially for a one-off. Expect a delivery time of around a month to six weeks on a blank that size. Let me know what you decide to do, and I'll be happy to help in any way I can. -Gordon

The technique I use is called "fixed post grinding", and there is no stroking. With the fixed position of the tool, the mirror quickly takes on a spherical shape. By moving the tool position toward or away from the center of the mirror, you adjust the radius of the sphere. For a flat back, you are looking for the neutral point that gives you a sphere of infinite radius. Many, if not most, opticians are now using this technique, so there is no stroking during grinding or rough polishing of most mirrors. The only stroking comes at the very end of the process for the figuring of the final paraboloidal surface.

You are lucky to see the Milky Way! Try making a small mirror, maybe a 6", which isn't too expensive. It's very satisfying!

this is what I have been meaning to ask

Review video at 10:18 is the explanation

We can make those for you. Please send a note to waiteresearch at gmail.com

If you can find a 20" f3 for $8,000 you should get it. That would be cheap for a premium-quality mirror. They are difficult to make.

+Ferris Wheeler The particular mirror in this video is an old-school Pyrex substrate. You cannot buy Pyrex any more. Now almost everybody uses a borosilicate glass made by Schott called Supremax-33. This is a really excellent glass, and I have made many, many mirrors and am very happy with it.

ER, Pyrex is a brand name by Corning Glass, and it is a Borosilicate.

@@paulmoffat9306 Today's "Pyrex" isn't the same as the Pyrex of 30 years ago. It's an entirely different formula, and you really cannot make mirrors with it any more. Supremax-33 is the way everybody goes today.

Or a template.

You can only check if the blank is still clear and hasn't been ground. Then you use a big TV behind the mirror to evenly illuminate it, and observe through the blank with a polarizer. This will show the strain.

Just Google "telescope mirror blanks" online.

Only the table is being driven. The friction between the two causes the top disk to spin. The differential rotation causes the grinding action. Very simple and easy to build!

Elétrons Lab I generally buy my glass from Newport Glassworks in CA. You should be able to find them on the Web.

Hi, Carlos! I can supply you with a 20" blank myself, about 41mm thick? I can go thicker if you need it. Please contact me at gordon@waiteresearch.com with your thickness requirements and I can quote you a price. Thanks! Gordon Waite

United Lens, I believe.

It starts with a shape dictated by your mirror blank. If you have a flat blank, start with a flat tool. If you have a pre-generated concave blank, you start with a convex tool molded against that blank.

Both to remove weight and to make it thinner. The biggest problem with a thick mirror is heat retention. When darkness sets in the temperature starts dropping, and the thick and heavy mirror can't keep up. This sets up differential contraction in the mirror and ruins the images as the surface changes shape with the temperature.