12 inch f4.9 Telescope Mirror Part 2
Вставка
- Опубліковано 16 лип 2024
- Gordon starts to grind the 12" mirror by hand, on the powered turntable, starting with 25-micron Microgrit abrasive. He shows how to center the ceramic tile tool on the turntable, and shows the marks left on the mirror blank from the diamond-tool generation of the curve. The grinding strategy is discussed, with hints about deciding how long to grind at this 25-micron level.
- Наука та технологія
I am so happy not having to use all kinds of body parts for measures. The metric system is wonderfully easy.
You buy mirror blanks by the inch, and abrasives by the micron. You take Foucault readings in 1/1000ths of an inch, and then calculate surface accuracy in nanometers. Go figure...
Me: "Gosh this house is a pig sty, I really should clean it."
UA-cam: "Here's a dozen videos of a guy talking about grinding optical mirrors."
Me: "I'm not at all interested in this...."
3 hours later "Hey hun, I'm gonna build a telescope!"
lol... so true
Have you built that telescope yet? Me either! 😂
Hi Gordon, really like your videos. This summer (like every summer) I plan to make a telescope, your videos are so clear and inciteful, they are really useful. Strangely they are also very therapeutic to watch.
Joshua Mcateer Thanks, Joshua. I think that mirror making is something like therapy for me! You get long periods where you can just push glass and think about things. Let me know if I can help in any way with your telescope project. Send me an email and let me know what you are planning.
Thanks Gordon, I really enjoy your videos, writing the spherometer numbers on the mirror helped me understand how that works. I have a 10" that I've been working for a year or so, at the polish stage now, your videos have been a great help to me.
Brian Reid Hi, Brian.... glad you're pushing glass. What's the focal length on your mirror?
GordonWaite The focal length is about 61.5", its my first try at mirror making, I got discouraged about 6 months ago and put it in a drawer, it has a pretty good tde. Watching your videos gives me inspiration to try it again.
I never knew I needed or wanted this but now both are true
Hello Mr. Gordon beautiful work
honorio sergio Thanks! I like working the pre-generated blanks. You get down to business quickly, and they are fast to get through grinding and on into polishing.
Hi Gordon. Love the videos. Its really fascinating to watch.
***** Thanks, Tom. Are you thinking about making a mirror? -Gordon
GordonWaite I would love to. But. ☺ . Other thing are keeping me busy. I made a cylinder using Cedar recency. and I wondered about telescopes and found your channel. Happy days. I am amazed at the equipment that you use. Aside from the radius measuring machine etc. It all seems doable. The skill in the grinding is practice and following your instructions very carefully. I also finds your videos very relaxing. ☺Keep up the good work.
Just a FYI I accidentally hit the thumbs down but gave it a thumbs up to correct it. Love your videos
Just ran across this video again. I love watching you grind mirrors, you make it look easy. I have a 7" f7 project that has been sitting for a few years now. Over a decade ago I was given a 2' square piece of 3/4" plate glass and I managed to treepan two 7" disks of glass out of them with a homemade tool on my drill press. Grinding glass on glass (old school, no ceramic tool!) I managed to get to an F7 curve rather easily, but polishing it eludes me. I made a plaster cast of the mirror to build a polishing tool. I used the old school method of casting strips of pitch and fixing squares of them to the tool, followed by a hot press and a cold press. Hand polishing the thing is awfully hard to move, and I only manged to slightly polish it before giving up. The tool is a mess. I'm wondering if I can chip the lap off and remelt the pitch (maybe add some lindseed oil to soften it) and try again with the pour, and channel method you use. Yeah it's plate glass, but in this size I'd hope it would serve.
You can always chip off the pitch and re-use it. I put the tool in the freezer, and then when it is frozen, the pitch just falls off with a tap. If the tool is hard to move, it means you don't have good contact between the pitch and the mirror. Using your method of sticking on the pitch squares, it is quite hard to get good contact. Much better to mold the pitch against the mirror, and then cut channels with a razor blade. Best of luck!
Wonderful vid, Thankyou! David
+david duffy No problem! If you have an idea for a new video you would like to see, just email and let me know. Always happy to get new ideas. Thanks again! -Gordon
great stuff. I wonder how these lumps of glass are made. What are they called even?
Can i ask what position of tool impacts what part of the mirror? I am grinding a 12inch Glass with 8 inch glass tool. Tool at the bottom gave me a little circular curve in the centre, i changed the position with mirror in the bottom and grinded the edges and now i have two curves with a continuous curve with a little deeper circle in the centre ..
You are very clever
still an interesting video
You could gently tap it with a leather faced mallet while rotating to centre quickly
You might find a felt tip marker on the top of your tool edge will help you get the adjustment at fine adjustments
That's a good idea. Thanks!
GordonWaite I figured and you grind and polish the line would be worn off. Kind of like bluing steel parts for machining
bom dia amigo!
preciso de sua ajuda, não estou conseguindo calcular o tamanho do espalho secundario, como posso eata calculando essas medidas? sei q e cortado em 45º,mais o diametro e a distancia focal nao estou conceguindo.
obrigado!!
Have you ever used diamond paste?
Talc and Al2O3 aren't water soluble so how do you ensure an even distribution of particulate within the mixture's composition? As the moisture content of the plaster changes with the water, how do you compensate for the variance of water content? Is there any reason why you chose water as your cutting lubricant rather than a standard machinists oil (I'm not a machinist and a poor chemist so pardon me if these questions are dumb)
To ensure "even distribution" I just shake the bottle before I squirt it on. Even distribution isn't a goal, as long as the mixture isn't clumpy. I keep the plaster saturated with water, and don't worry much about variance. It has never been a consideration, as long as the plaster is saturated. Using oil would be an unholy mess. Everybody in the industry uses water for hand work. If you are doing long-session with a CerOx recycling setup, then you use a more sophisticated mixture to ensure it doesn't get foul, that it doesn't get foamy, and that the pH stays within bounds.
Where buy?
Hi again, Gordon! I will never hide my passion in telescope mirrors making although I bought my first telescope a few weeks back - 10'' Dobsonian. I have a childhood dream to make a mirror myself. Your videos are great, you're a true master but can I ask you - Is the tool curved or flat (based on the videos I assume it's curved)? You're making a lot of mirrors and I'm sure that you have a lot of old tools but if a simple novice like me want to build that kind of a tool, how can I make it curved if my mirror blank is flat? Also - if (let's assume) that I have a 12 inch mirror, how can I make 9 inch plaster tool if I can't stick tape across its perimeter (obviously if I do it, I'll make a 12 inch tool). And last - (for the 4-th part of these series) is the pitch tool change it's shape through time? Somewhere I read that if you don't use the pitch tool (let's say 2 weeks) - you'll need to keep it on top of the mirror to be in the proper shape.
I really want to do it but I think I don't have the courage to try yet. Maybe when I buy some books about it I'll be more ready...
Please, excuse my ignorance but I try to understand the whole process...
+magdanoz88 Hello! The shape of the grinding tool has to match the beginning shape of the mirror blank. If you mirror blank is flat, then your tool must be flat. If you have a curved (pre-generated) blank, then you need to start with a curved grinding tool. To make a "sub-diameter" tool, I use a strip of plastic as a mold. I make a hoop out of wood that is the desired diameter, and then wrap a plastic band around it, thus making a mold. The plastic is thin and flexible, and it must have one very straight edge. When that straight edge is made into a circular mold, it will touch all the way around on the surface of the mirror. I normally weight it down a little, and the dental plaster dries so fast that it doesn't run out under the plaster dam. The pitch tool changes shape all the time. Before every work session, you must ensure that it matches the mirror shape perfectly. If it doesn't you need to either cold-press or warm-press the polisher on the mirror to mold it back into the correct shape. I often keep a working lap on top of the mirror for storage, but I put a layer of "oven paper" in between so the pitch won't adhere to the tool. It's not necessary, though, to store the polisher on the mirror. When I make a big mirror, I might have 4 or 5 polishers, and I just store them in plastic containers in my refrigerator. You will have to press them before use, but they will keep their shapes just fine in the 'fridge. -Gordon
+GordonWaite Thank you very much for the info! It's great as always. I already saw your videos about the laps and their maintenance so my question looks kind of stupid, sorry about that. I've learned so much from you! I also bought 2 books on telescope making and on understanding the foucault tests - can't wait to read them. :) I think I'm ready to try to grind an 8 inch mirror...
magdanoz88 I love all of the old books on mirror- and telescope-making. I can read them over and over and never get tired of the stories. Those were generous people that shared a lot of knowledge.
I do not understend a piece. The disk tool ceramic is totally plane? Or need to be little convex form to make concave disks? Tks
The surface of the mirror and tool must match. If the mirror starts flat, you start with a flat tool. If the mirror is pre-generated with a curve, the tool must start with a matching curve. After that, the curve of both the mirror and the tool change together as grinding commences.
Sir if I use 12inch diameter 2000 focal length Plano concave lens for reflection telescope which length and diameter of secondary mirror will I used?
1.52" to 1.83" minor axis elliptical, someplace in there, I expect. Assuming you mean 2000 mm focal length.
Is the tool dome or flat?
and can i work with flat one?
In this case, I bought the mirror blank with a pre-generated curve already on it. So the tool that I am starting with also needs to have the curve on it at the beginning. So to start here, the tool does have a dome shape. If you want to start with a flat tool, then you have to start with a flat mirror blank.
thanks
Hi Gordon, how can I contact you direct please. Regards
John
+John Welsh gordon@waiteresearch.com
Where would we mere mortals be able to purchase Microgrit???
Thank you!
he shows you the box of grit and if you search it is the very first hit, 15 bucks 1kg: lmgtfy.app/#gsc.tab=0&gsc.q=microgrit%20wca%2025t
I'm surprised you don't use a trimmer router with a diamond round over bit. That is what they are made for.
Usually when you are putting a bevel on an edge, the mirror is already concave, and sometimes quite a bit concave. That means there is no flat surface on top for a router to travel on for stability. The rotary tool is much lighter and easier to control than a router, and just feels a lot safer and more controllable.
Why not mark circles on your table for centering the tool? That would work faster than iterative tweaking
Or just drop a plumb bob vertically from a ceiling beam, moving it along the beam axis until you touch-off. You'd probably be able to true within a thou or two.
His life would be a lot easier using 3 jaws instead of 4 too, since it's self-centering. (And since it looks like he's just using Perspex or some polymer, you don't have to worry about the traditional burden of grinding jaws, just chuck them out and make a new one).
I don't use circles on the table, because the mirror sits on a piece of artificial turf. On my current machine, I use a fixed ring to hold the mirror, which automatically centers it. You don't need the mirror centered all that accurately. If it runs true to .02 or .03, that is plenty good enough. Most people don't use a "chuck" mechanism where the hold-downs (jaws) all move together. Instead, every one is hand-adjusted. With that kind of system, it is easier to use four than three. With four hold-downs, you align in one axis and lock those two. Then you align in the other axis and you are done. If you try to use three hold-downs, you will go round-robin forever.
GordonWaite
I'm familiar with the 4 jaw vs 3 jaw situation from my lathe work. What you say about it is very true (no pun intended).
@@GordonWaite What is the error that crops up in the glass shape the farther out you get from .02-.03 (inches I guess) from center? Is it just that the center of the mirror is not at the center of the glass...and the entire telescope optics will have to adjust to line up with that? Or would you then get some weird surface shapes in figuring, zones or whatever they might be called?
Why do you grind spherical mirrors? Since you have a curve meter you can with a bit of math determine the correct parabolic shape for that focal length. Are you removing the spherical aberrations with lenses otherwise? and if you use lenses are you going to use triplets to get it apochromatic?
+murdakah Hello! All of our mirrors that are intended for telescopes are finished as parabolic mirrors. During the early grinding phase, all mirrors finish the grind as spheres. During rough polishing, they stay spherical, mostly. After the mirror is completely polished out, that is when we start to "figure" the mirrors, taking the surface from spherical to parabolic. -Gordon
+GordonWaite Ooh I see. I didn't think the final polishing would have such a drastic effect. Thanks.
Can make the primary mirror with epoxy resin?
No, the mirror needs accuracy to maybe one millionth of an inch. Epoxy expands and contracts as it cures, and varies with temperature too much.
Quite a long time ago, a gentleman came to my house selling tapered cylinders that he called "funnels"... they might be useful to you.
They are a pain to clean and the AlOx doesn't flow smoothly through them. The spoon is faster and I can keep it right in the container.
@@GordonWaite lol ok.
Is there a convex radius on the back or is it flat?
It's flat. Standard mirrors have flat backs. Some slumped mirrors or meniscus mirrors are curved, but that's not the norm.
radious? i thought that the measurements were depths. its not clear
The spherometer takes depth readings, but it converts them to the radius of the sphere.
This might help:
ua-cam.com/video/snz7JJlSZvw/v-deo.html
Investing in a little plastic funnel might save you 30 seconds of abrasive mixing time 😁
You would think so, but funnels just don't work well. The AlOx doesn't pour smoothly, and I only need a little bit. And then I would have to clean the funnel and try to keep it fairly clean, where I can just toss the spoon back in the abrasive container. Funnels work fine for SiC, but are not much good for AlOx.
@@GordonWaite maybe you could heat the spoon till you can deform it enough that the tip fits mostly into the water bottle, so sort of mini spoon/scoop-funnel, one for each grit size since they're cheap. Like this, I just did one now, heating a polystyrene spoon a bit with a heat gun: pixeldrain.com/u/FFaqFfyD (link is on a public file share and disappears in 30days). You could also cut/shape the tip to improve dispensing quality (e.g. straight cut or maybe central v-cut, groove, etc.) or cut the sides of the spoon off before bending up if it's too big to fit the bottle mouth.
I'm thinking, (Yes, I know I'm about
4 years late to the party) How about
using a soft rubber (mouse) pad, with
concentric circles on it, for under the
mirror blank?
One sixteenth would be difficult to see,
but 1/2 inch would be EASY.
(Sorry, been watching too many 4 jaw
chuck centering competitions.
ua-cam.com/video/ArXor2FhfDw/v-deo.html
steve
Good idea. I think the bottom disc is called the tool.
Where can one buy Microgrit?
I buy it directly from the company: microgrit.com
There are two ways to get expert advice on any topic. You can pay an expert to teach you or you can just make a video about the topic you're exploring then post it online and wait two years.
Experts from across the globe will come to tell you what you did wrong. Internet. ..ain't it great?
18:43 Ok this irked me.
Who is this video aimed at.
Surely not a DIY telescope maker?
We don't have access to a spherical surface measuring tool nor a ceramic special grinding tool.
Hi, Peter. While I have a professional spherometer for measuring the radii of mirrors, I made a couple of videos for my channel which show how anybody can make a great mechanical spherometer for just a few dollars. Basically it is just a cheap pulley wheel and a cheap dial indicator, probably $20 or less in total cost. And there is nothing special about the ceramic grinding tools. I have posted several videos showing how anybody can make one out of plaster and ceramic tiles. The ceramic tiles can be purchased at places like Home Depot or Lowes for less than $10.
You can get a spherometer for under $100. Tile tools are nothing special.
reality is poison and we´re all lambs to the cosmic slaughter!
You take far, far too long to explain even simple concepts. And you explain the something far too many times.