Grinding a 25-Inch F3 Telescope Mirror: Thinning and Flattening the Back
Vložit
- čas přidán 15. 12. 2014
- Gordon started with a 2" thick telescope mirror that weighed about 70 pounds. By thinning the blank, he's lightened the mirror by about 20 pounds! This video explains how he ground off the excess glass and flattened the back to begin fabrication of this F3 mirror for one of Waite's "feet on the ground" Renegade 25" telescopes!
- Věda a technologie
Makes me miss my dad, he made a 6" mirror and reflecting telescope by hand back in the late 60's and took photos of the moon. He then made his own machine to grind a 12" lens, and together we made the telescope out of an old x-ray machine mount and mounted that to a hand made stand designed for our latitude. I learned a great deal from it, and love science because of it. Great father son project.
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.
When a person shares their expertise, they make the world a better place.
+Dink Martini Thanks... glad you enjoyed the videos!
This is what I love about CZcams. So much craft to be learned.
Teach a man how to fish :)
Ah I see you are australian
Im genuinely amazed at the techniques that create these optics. These videos are fascinating.
Awesome youtube suggestion, glad i checked this out. I always wanted to know more about mirror and lens making!
Fascinating. My dad and I hand ground a 6" mirror back in the late 60s. I still have it mounted in my telescope. :)
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?
That was great! Thank you for sharing your system with us!
What a noble and ancient art. This must be so satisfying.
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
That's just plain old good work Brother. Thanks for taking the time. I found The Plaster Guys a good source for all types of plaster including Dental Plaster.
PPL that make scope mirrors are just masters at theyr "game" so precise and so pacient...
This is the first video I have seen of yours, and six minutes in I have an urge to become a telescope mirror polisher.
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.
Of my 220 subscriptions on CZcams I have NOTHING like this in my repertoire but I watched the whole thing and will check out more of your videos! 😎👍
Absolutely fascinating!
Thanks yt recommendations for this gem
Gordon I've watched a lot of your videos. I hope to own one of your scopes like a 25" one day. Continue making videos I enjoy watching them. You definitely do great work!
+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..
fantastic information gordon i have a process question if you have 2 mirrors that have the same sagita but are say a 12 inch and a 24 inch and you used a tool say 70 percent of their diameters would it take about the same amount of time to hog them out?
Oh god, that is gorgeously huge.
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 would have loved to see the finished product.
In the year 1970 I began constructing a Newtonian telescope following "How to make a telescope" instructions. From Jean Texereau. The first challenge was to get the blank. I had to go to request melting the glass at a local factory in Bogota Colombia. I finished grinding about one year long. Finaly I finished my telescope exactly as shown on the book. Now this telescope is shown on an university museum.
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
Hey Gordon! Wish you could publish more videos more frequently.
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?
If you contact on of the large telescopes in the world, they should get you in touch with someone that can show you how to make radial and circumferential cuts and remove a lot more material.
I used to polish 4" silicon wafers back in college days.
This would have been the one and only time I would have liked to hear some of that royalty free youtube music lol
NOOOOOO!!!!!
very nice job!
Thanks! -Gordon
I have a 12" plate glass porthole that I want to flatten the back of. It has some frosting and unevenness I want to remove. Could I do this with a tile tool? Maybe switching the tile tool between top and bottom?
Home Depot has a better grinding wheel - intended for leveling concrete. Fits 4" grinders, and has 2 rows of diamond blocks on the flat of a dished wheel. Not cheap (about $45), but far more controllable than an edge grit wheel. Made By Ridgid HD-AWD40
Good idea. Thanks!
is it feasible to re use that spent abrasive? As in putting it thorough a mesh so particles of only one size are present then use it as a finer abrasive.
This is really awesome. I'm really curious though how do you get dips and valleys out if the grinding stone isn't fixed to something that will not grind Valleys. you can see it going up and down has its grinding... will it eventually flatten it out or will it just keep those dips and valleys in there?
It evens out quite quickly. The surface averages out against the grinding tool, and contact becomes closer and closer as you grind.
the moment i hear his voice. thought i am watching Cody'sLab
I know right!
How? Doesnt sound anything like him imo
@@brandonbaker5792 simmilar accent.
Have you thought about making an arm for the grinder to attach to which permits it to slide in an out to get a good starting flat finish from the grinder?
I would if I had to do it more than once.
Amazing.
Fantastic!! thank you so much for the lesson , subscribing to your channel.
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!
Hi ,I am from Poland. I started my journey with mirrors for teleskops. I have a question....do you still nade teleskops mirrors?
I'm a bit confused ... How do you get the exact amount of concave (1/8" depression) to be perfectly equal all around the mirror? I don't see how this is achieved - Does the difference in size between the tool vs the mirror blank make this process automatic? Obviously I'm a beginner and sorry if my questions sound dumb ...
How good performs Your glasses once they are finished? Worth the money investment for the blanks (thousands dollars)?
Awesome work. Might I recommend putting your different grits in cheap marked(per grit level) salt/pepper shakers. Makes distribution a little easier
I have used shakers in the past, and they work pretty well. Our newest machines all have automated dispensers now!
GordonWaite oh nice
F3!? Oh boy, the eyepieces
You used a bituminous pad once. I want to get into this but I have a hot pot 3D printer and silicone to make molds. I wouldn't mind making some to help others and finance my own telescope build.
Gordon what are you using as your testers?
is a mirror not concave to focus the image on the eyepiece in a reflecting telescope as opposed to a convex lens for a refracting one ? So are you grinding a lens ?
I am grinding a mirror. The optical face will be concave.
I love these people. I just love these people! Now when am I going to get my tacos!
Where do you buy the glass blanks?
Excuse me, I have a question. How do you center the glass blank before securing it in place? I just can't get the result I want, if I use my eye.
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
Hi! I want to know if the tool you use with a weight on top has some angle with the perpendicular? or if it works just because you use the grinding tool first? thank you
+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.
I'm still learning so I'm curious about using things . If u think about it using the carbide abrasive seems like it would imbed itself into the glass since it's a loose material . If the abrasive was glued to another object it would act like sandpaper an push the access glass away from the mirror . Am I thinking about this all wrong ? I'm guessing I'm thinking more about time . Using a sandpaper method would be quicker than putting abrasive on the glass I think
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.
These are all great videos. I was wondering...did you find the limits and/or optimum values for the various parameters? Like, will it grind faster if you keep adding weight (till just before the blank breaks)?
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.
Hi Gordon - you have the best scope making videos on youtube, imho - I have learned a lot from you... thank you. Just remember when you are hogging out your next blank with a diamond saw blade that telescope maker Robert Cox died of white lung disease from breathing glass dust. Can you devise a way to do this job fully wet so there is no glass dust created to wear a mask for - or with the disk underwater? I wouldn't touch that job without a full on paint-booth style breathing apparatus with remote air entry. Saying 'wear a mask' may not be enough for less well-informed viewers who may try this
This isn't meant to be negative in any way - I appreciate so much that you take the time to put these very informative and well-made videos up - amazing work, really :)
Keep them coming!
D.
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.
Can you show some pictures from the telescope of the moon etc
Why wasn't the blank cast to an approximate shape to reduce the grinding?
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?
How do you feel about using a Kiln to fuse plate glass to make a mirror blank? I know purchasing blanks can be rather expensive.
If you enjoy kiln work, more power to you! Enjoy yourself!
Mr Dobson himself would be proud of you!
I always thought the grinding tool had to be the same diameter as the mirror. Is there a rule of thumb as to how big it needs to be?
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.
I hollow grind clipper blades with aluminium oxide, never thought of doing glass. I'll have to come up with a floating holder that revolves as I drift across the hollow line. If I can do it, I should be able to produce 4" glass
F3.1... So, a salid Bowl... 🤣 Freakin awesome work...
Where i can buy a Ready glass like that ? Amazing Video ! Great Job !!
If you email me, we can talk about a finished mirror.
do i have to sent out the the mirror to get coating after I finished grinding it?
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.
Why do you have to grind the mirror's back to such an accurate flat finish? How does that effect the quality of the finished surface on the reverse side?
+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!
Hi I really love watching your videos. Would you please like to share with that is it possible to make perfect 40 inch parabolic primary mirror at home ...what would be the parameters for the same?
Все возможно, главное терпение
I didn't understand if the guy cleaning the face of the mirror or he making spherical shape on it? Please i need someone tell me what he doing.
Were do you find some glass to do this?
Just Google "telescope mirror blanks" online.
i wish my glasses were that thinn ...
How does glass get the desired curvature?
Where can i buy a Telescope Mirror like yours in the video?
I want to build it myself
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.
May i ask if there is any math underlying the grinding motion, maybe such that the relative speed between the two surfaces leads to a parabolic shape rather than just some randomly shaped valley?
I know that if you spin a liquid the surface will form a perfect parabolic shape and it is easy to derive the maths for that, so i was wondering if something similar exists for this grinding method...? Or how else do you end up with a perfect parabola?
Thank you very much, great video!
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.
Why is the mirror convex and not concave? Should light not converge rather than diverge?
The mirror is, indeed, concave. The tool is convex.
Where'd you get that mirror blank at?
United Lens, I believe.
Hi Gordon, What particular glass type you used for this kind of mirror/lens? Thanks
+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.
Hey Gordon!! Some months ago I came up with sn idea on how to make parabolic mirrors without grinding. Just a few days ago i finished my first 5" mirror using this method (spin casting). Could you please tell me whats the accuracy of this mirror? I know it is a bit difficult because you have not seen the mirror itself, but can you guess? Please! :)
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.
What is the difference with a glass for a telescope and a window glass ?. Just mention the basics if you would. Thanks
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.
what is the shape of that grinding tool? flat? concave? parabloidal? spherical? I can't see the top of the steel rod. Is it in some kind of pivot?
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.
where do u get this large glass?? Is it a 40mm glass?
Elétrons Lab I generally buy my glass from Newport Glassworks in CA. You should be able to find them on the Web.
if you took your time would it be possible for a total amateur to do this? assuming you've got right measuring tools and equipment.
Tomei um susto com o som no início do vídeo. Vou fazer um espelho também.
GordonWaite; Can you tell me how hard is to make an F1 parabola, and is there any
where or reason it can be used too,
and how accurate the focal length of your F - 3.4 ?
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.
What is the different between normal mirror and thick one isn't the same reflection!?
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.
when generating a curve, does the tool need to have a convex?
this is what I have been meaning to ask
Review video at 10:18 is the explanation
Would a water mister be useful? I've seen machinists use a mister with lubricant and water mixed.
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.
Hello sir
I am making a telescope and looking for hexagonal mirrors having a diameter of about 1 meter can you tell me where can i order them
We can make those for you. Please send a note to waiteresearch at gmail.com
How do check for any strain in the glass?
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.
Viry nice movie
Please i try to make mirrors for my telescope at home manually using epoxy < but I need to the A concave mirror measure to make a mold to put epoxy inside it
What is the degree of concave mirror?
Depends on focal length maybe?
Hi,
Why you want to reduce the thickness? Isn’t better if 5he mirror is thicker?
Thanks.
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.
not to be "that guy" but couldn't you essentially set up a coolant pump like most CNC machines have, and feed in some grit from a hopper in the same line, from the center of your grinding disk post? that seems like the best placement to keep an automated feed of that stuff going through since it would disperse itself.
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.
i can building a 20 inch mirror with 25mm common glass thickness, is possible, as John Dobson made? thanks.
+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!!
So does this produce a parabolic or a spherical mirror?
The first stages produce a sphere. Then you alter the sphere into a parabola.
@@GordonWaite Is that what is known as the 'figuring' stage?
Where can I get a large diameter glass like this?
United Lens will quote you a price on a pre-generated mirror blank. If you just want a round blank that is plano-plano (flat on both sides) I can do a quote for you if you want to buy one from me. If you are interested, email me direct: gordon@waiteresearch.com
Thanks for your speedy reply. I will email you shortly! Thanks for posting these videos!
Ahhhh. My new sleep white noise 6:00
any reason why you aren't using any stroke. i get that you are trying to take out a ring towards the middle and the lip around the edge but some stroke would help you keep from convexing that blank while trying to smooth out the back.
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.
Is the top abrasive tool flat or curved?
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.
Just curious as why does mirror have to be glass. If made of softer material such as acrylic Lucite , or Lexan, grinding time would be much much less.
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.
THE sound....
Good heavens have some Dimond grinding tools made, it's not at all as expensive as you might think.
Please note I have never made a mirror in my life but I do grind things and I may be totally off the mark.
I have created and used many diamond tools over the years. I use diamond rings to rough in curves. And I've often used diamond paste for fine grinding and rough polishing.
@@GordonWaite Dimond tools are certainly useful when well made, fortunately in my backwards south African home town we were lucky enough to have a company that did Dimond plating.
It's really useful when you can turn a tool on a lathe and have it grinding into hardened steel the next day.
But as I said I have zero knowledge on telescopes or making mirrors but I feel compelled to trying.
Where can I buy a blank this size?Did a 15 inch and now i am ready for a 25. :)
***** 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
You are doing a good job, but how to solve the atmospheric turbulence problems ...
cool
Why is this mirror convex and not concave as we always see in reflective telescope designs?
The back is flat, and the front concave, like all Newtonian designs.
@@GordonWaite But strikingly from the video it seems to be convex. o.O
@@vicentecamilo5636 The video only shows me working on the back of the blank, which I am flattening. Perhaps the wide angle of the camera lens makes it appear convex to you. On this mirror, the back you see being worked will be flat; the other side will be concave.
@@GordonWaite I got confused reversing both terms, I'm sorry. What I meant earlier is that the glass is convex, while the Newtonian telescopes the mirrors are concave.
Colimating and keeping colimation on this fast of a mirror must be a bear.
Sure wish you would show the final scope.
F 3.1?
25 Inches?
Nice!!
i want a big 20+ inch f3 mirror. how can i get one as cheap as possible? once you get up to 20" they seem to skyrocket. like 8000+ just for the mirror
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.
Muito bom ter alguém para ensinar mais algo aqui no CZcams!! Mas não entendi nada doq vc falou, poderia colocar uma legendo
Strangely I could watch 20 minutes of this too
Please tell me how to get my hands on a 25" Blank???? The world wide web won't cough it up!
Newport Glass is where Gordon gets most of his pieces.
Tim, if you email be I can perhaps help yo acquire a blank. -Gordon
wow To make a telescope or just buy one? 😎 very interesting, thank you.