Using Polishing Pads to Clear the Back of a Telescope Secondary Mirror
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- เผยแพร่เมื่อ 11 ม.ค. 2017
- Gordon Waite is making a batch of big secondary mirrors for some Renegade telescopes that are being built. To speed up the job, he shows the technique of using optical polishing pads on a separate flat to do the work. Gordon demonstrates both grinding and polishing the backs of the mirror blanks by hand on the turntable of one of his polishing machines.
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*Good **MyBest.Tools** power and rpm control.*
Hello Gordon.
I´m not from the field. I dont even have a telescope. I´m just a curious guy looking for good content.
Your channel is excellent and you are extremely didactic.
Your job is amazing.
Thanks for sharing your knowledge.
Seen all your programs from start to finish! Love them!!
Gordon thank you on behalf of myself and others for taking the time from your busy schedule to answer my questions.
You are all very welcome! I try to help as much as I can. I learned mirror making all by myself, by reading books. It was a long, hard journey, and I would have benefited greatly from having an experienced hand lend some advice! Mirror making is one of the least "intuitive" things you can attempt, so a helping hand can really shorten the learning time. Good luck with all your projects!
Thank you very much Gordon. I needed this.
No problem! You are welcome. Thanks for watching!
Thank you Gordon.
thank you gordon, very nice
Looks beautiful!
Thanks!
Very intresting videos! 👍😀.
Parabéns pelo vídeo, eu tenho todos os seus vídeos baixados em meu computador, uso eles como aulas sobre telescópios, aprendo muito com eles, muito obrigado! Artt. Honorio
Thanks! I also used to help run a class on mirror making, maybe 10 or 15 years ago. It's fun and satisfying to help people. If you have an idea for a video, please let me know, especially if it would help your class.
Make a video about polishing the secondary mirror I thank you. att Honorio
Gordon. I'm in love with your channel and just how awesome it is that you're freely sharing your expertise. Is there a reason you used those 120v florescent bulbs? Would something like a strip of red LED's work?
Red LED's will not work. The light source needs to be monochromatic. The florescent bulbs have well-defined spectral lines, particularly one red line. By using the red filter, I isolate one single wavelength and thus get a monochromatic source.
A laser diode or rcled would work, but you need a monochromatic source
Another plus I gleaned from your video: No need for a two way mirror or beam-splitter set at 45 degrees, a regular plate glass will do, as long as the inside walls of the testing box are blackened under the diffuser, but lined with foil above it. The disadvantage is that I need a reference flat. That is the reason I am intrigued with the water test. No need for a flat with that one.
Get yourself three smallish blanks, maybe 4" or 6" and make yourself three flats using the ABC method. Use one of them for your reference flat, and now you are in business! After you get some experience, make a bigger flat if you need to test larger flats. But even a 6" reference flat is big enough for the testing most people would need to do.
Alert! - Perfectionist at work! :-D
Hi, Brian. That's kind of the problem with mirror making. There's not a lot of room for error, so you get a little fussy about things sometimes. Thanks!
@@GordonWaite how hard would it be to make a odd shaped telescope mirror, like a square or hexagon? and would it be to hard to make a single main reflective surface out of multiple mirrors?
@@fallinbeef You can certainly make a square or hexagon mirror. AAMOF, I am currently making nine hexagonal mirrors for JPL for a multi-mirror instrument. For non-round mirrors, it is hard to get a good figure on the edges. And making your won multi-mirror telescope requires very demanding mechanical devices to keep the mirrors in alignment. You would be talking about WAY big bucks and JPL resources.
hi gordon whats up? love your work man very professional!!!! waht kind of motor are you using in the grinding tables? did you built them yourself? i would liketo build one for me if i can get a list of components to build a table like that it will be great. tnx for the vide thay are greatttttttt! all the best bro!
The motors are mostly 3-phase around 1/2 to 3/4 hp. I use an inverter so I power them from 120 vac, and the inverter gives me very good speed control. I get most of that equipment from automationdirect.com
Hi Gordon... Thanks for video
Plz make a video about microscope oil immersion 100x lens
That's a little out of my experience, but I'm always open to new hobbies. I've wanted to acquire a good microscope since I was young. Maybe now is the time!
Hi Gordon I have three polished "flats" except they aren't flat yet. I will go ahead and make the box now. To test them I would wet their backs so that they were somewhat transparent. No I didn't have a box made nor did I shim them with parchment paper. I used the tedious ABC method and tested them with my spherometer before polishing. The polisher was a separate tool altogether. I used the hard pitch (I am not sure what Gugolz rating it is but it is black and quite hard), perhaps Gugolz #61
I can go ahead with my box and polish their backs with the pads. I suspect they may have astigmatism in them however until a proper interference tester is made the errors could have been due to a host of other factors.
That's great. Gugolz comes in #55, which is soft, through #64 (common medium hardness) to #73 which is harder and good for flats, through #82 which is harder than I ever use, even in Arizona! With flats, test often and learn how to take them concave or convex as you need. Enjoy!
It is Gugolz #73 1 kg. from Meller Optics. Black in colour.
Hello. Where can I buy these polishing pads online?
Can you do a follow up video explaining how the Newton Interfererometer tests works?
Will try to do that the next time I make a set of flats. Thanks!
Gordon did you post a video on making a flat? I don't recall seeing one if you did. I apologize for all these questions as you have a busy schedule. When you have time....not too many places where you can pick up these skills.
I'm trying to put one together. The next batch I do, I'll try to get some good video about the testing process.
О, Володя Ржавый из ред21!
Hi, Gordon.
Where can I purchase polishing pads and cerium oxide? I'll try to make a pair of lenses to learn the art
I usually get mine from a company called Salem Distributing. For smaller quantities, try gotgrit.com. Good luck and enjoy!
Michael Shearer
Hello Sir and thank you for your videos. If you have time, please do address the issue of extremely low audio in this video. I am straining to understand what you are saying and have to have the speaker from my iPad so close to my ear that I can barely manage to watch the video. Very awkward indeed! Thank you again for your time.
don't mind all the question, as I am 64 going on 5 when it comes to astronomy, I have been at it now for 2 months, and I got carried away because I already have 7 telescopes. 70x350mm, 70x400mm, 2-70x700mm and a 90x800mm and a 114mm x 1000mm newtonian celestron powerseeker. I went a little over board!, and I am getting ready to build my own 8" dobson. sorry I forgot about my 50mm x 700mm refractor
Mr Waite I am looking to up grade my telescope from 4.5 to at least 12 / 16” I can pay but I am on a fixed income where can I go to get or buy the mirrors
Hi Gordon, a noob question: Would it be an option to *not* polish the flat side at all, and instead:
- Use the reference surface upside-down and look for fringes through its back-side?
- Use a temporary medium like a puddle of water or "clear-coat" polyurethane to make the surface less hazy?
Thanks! (Great video, as always!)
Using the reference piece on top may or may not work. If the reference piece is much bigger than the flat you are working, it becomes difficult to balance the reference flat on top, and it may distort. I don't want to attempt to place the reference piece on top of the smaller mirror, have it tip or slide off, and then get a scratch. My test flat is 8" in diameter, and you will be commonly testing flats from 1" across and up. So putting the reference flat on top becomes a problem. Of course, your idea will work just fine, in theory, as long as nothing goes wrong and you are careful. Only one of the two pieces needs to be transparent. Thanks for watching the video!
Cool. Thanks! What about the other idea of using an intermediate material to coat the hazy surface?
Might work. I've tried wetting the surface with water, and that wasn't practical for my testing needs. Coating the back with some material might be workable. But you would probably want to strip that off before you went to mount the secondary in the telescope, especially if you are using some adhesive to mount the mirror to the secondary holder.
Good point :)
Hi Gordon,at the last of this vedio,is the bigger mirror on the bottom a normal mirror blank or a optical flat?
It is an optical flat. It was salvaged from a broken optical comparator. Not a great surface, but useful.
Your videos give me so many inspirations.Thanks for your answering.
Couple of questions: 1) Are you using regular paper as shims between the two flat sides? 2) Are these monochromatic CFL bulbs or did you have a coloured piece of plastic to give you a filtered monochromatic light to aid in seeing the fringes?
Hi, Dan. The light bulbs are just screw-in flourescent bulbs that are more or less "painted" red by the manufacturer. If you go and look at Lowes or Home Depot, they have them in all colors. The trick is, the flourescent bulb actually emits a fairly small set of monochromatic bands, one of which is in the red. So the red coloring on the outside of the bulb actually filters out all of the other color bands in the spectrum, leaving pretty much only that red monochromatic band. So it's a very simple trick to getting a monochromatic light. And as you see, it works great! They cost $4.35 each at Home Depot: www.homedepot.com/p/CLI-Energetic-60W-Equivalent-Red-Spiral-CFL-Light-Bulb-FE15313SRVP1/205144263
Hi, Dan. For shims, I usually use what is called "parchment paper." This is a non-stick product that is very cheap, and is usually sold for baking, like in putting it under cookies in the oven so they don't stick to the pan. We always have some in the kitchen, and it is amazingly handy in many situations. Look for it in the supermarket with aluminum foil, and kitchen plastic wraps.
@@GordonWaite I LOVE IT!! Cheap and effective!! :-D
10:58 No one:
My OCD Brain: "Hehe. I'm in danger!"
Hi sir I want to primary and secondary mirror. Where to purchase. Or could you tell me how much of this yours mirror. And sorry for my english iam from india. What's the name of the primary mirror called.
If you are interested, please email direct to gordon@waiteresearch.com
Gordon why don't you use the water test for flatness?
The water test takes longer to set up, by far, and is more complex. I just clean my work piece, and sit it on my test flat and turn on the light. Don't have to wait for the waves to die out, and the test isn't ruined every time a car or truck drives by on the street. The water test needs some mechanism to level your mirror relative to the water. And you need a Fresnel lens as big as the flat under test as a collimator. Even a draft or breeze blowing over the water will ruin the water test.
bom dia amigo!
preciso de sua ajuda mais uma vez, não estou conseguindo calcular o tamanho do espalho secundario, como posso esata calculando essas medidas? sei q e cortado em 45º,mais o diametro e a distancia focal nao estou conceguindo.
obrigado!!
Perhaps try this calculator: www.bbastrodesigns.com/diagonal.htm
Thank you Gordon.
Would you say a mirror made of regular glass with a diameter of 23" will be useable? Is low thermal expansion coefficient a MUST? Also - I'm making great 23" spheres, but can't perfect polishing to make it a parabola. Can I do it by hand or will that take too many hours? Thank you! Love your videos!
When you say "usable", if the mirror has a good shape, the usability will depend on cooling time and the performance of your mirror cell. If you can wait for cooling, and you spent good money on a great cell, or built a great cell yourself, the mirror will be perfectly usable. I would be more worried about the ability to fabricate that good, thin mirror in the first place! How thick is your "regular" glass? If it is standard plate glass, maybe 1" thick, then making a quality 23" mirror of normal focal length will be quite a challenge. If you could make the mirror without 'stig, and you had a GREAT mirror cell, then the mirror would work fine, of course. But to be honest, unless you have made a lot of mirrors and you have maximum experience, your odds of success are quite low. At least if you are looking to make a good quality mirror. The amount of time it will take you to go from a sphere to a parabola will depend on what your focal ratio is. You can do the work by hand, but it will take a while. For an amateur maker, parabolizing a normal thickness 23" f/5 mirror from a sphere might take 50 to 100 hours of hand work on a turntable. But if you are trying to make an f/4 mirror, that time might easily double. An f/3.5 might double it again. The faster the mirror, the more glass you have to excavate, obviously. But the measurement accuracy becomes more difficult the faster the mirror. And the faster the mirror, the more closely it must approach "perfect" to get a good wave rating or Strehl number. That means a lot of extra time. Myself, working as a professional fabricator, my limit on 1" plate glass is around 16" diameter. At that point, for me, the cost of the additional time spent overcoming the thin blank would match the price of thicker borosilicate glass. And, of course, the thicker borosilicate mirror will hold its shape better in the field, and have better thermal performance while cooling down, and can accept a lower-quality cell. On a 16" blank, the difference in price might be only $500 or $600 between the two blanks. At 23", though, a 2" thick Supremax-33 blank might easily cost $2,500 more! So it's really about what is your time worth, and how much misery are you willing to accept during observing?
Okay, first of all thank you for your reply! You're my idol.
My "regular" glass 23" blanks are a little less than 2 inches thick. They are two 23" blanks 0.9" thick - fused together. I'm an amateur astronomer from Lithuania since 2006 (I'm 26 now) and started thinking of how cool it would be to make a telescope myself. Only in 2013 did I come to it. Started with 6", made over 10 pcs, only sold one, gave the rest away as charity to schools and children's homes. Carried on to 8", made one perfect one and moved on to 23" (600mm, sorry, metric system). The whole process took me 2 years to design and test 3 versions of grinding & polishing machines and learn how to make mirrors. Making F3 to F8 in rare cases spheres. Long focal length naturally perform great compared to horrible F3 spheres, but I'd love to learn how to parabolize 23-inchers properly. They're big and difficult for me.
My vision for this whole idea of making mirrors, is to skip on an idea of making them great or perfect quality. What I remember clearly, is a real struggle in getting of these big mirrors for yourself. My price for 23" F5 is 600 euros (~638 USD). That's barely over it's material and production cost. The view isn't amazing, but an amateur must know clearly what is it that he want. If it's high magnification for planetary - F6-8 or whatever he desires me to do (good quality for long focal ratio mirrors). If it's wide field of view - then it's F3 or somewhere in that are. The quality will suffer if you use an eyepiece of 25 or less, but it will offer a great view and the DSOs will "burn your retina out" because it's 23". As simple as that of an ideology. A lot more I wanted to say, but I'm trying to make it brief:) Thank you!
Are you testing the 23" blank for strain? When you start fusing together pieces of glass, you are introducing even more variables into the equation. Strain in the glass would be the first thing I would worry about. The 2" thickness isn't a problem, really, as long as you work carefully to lower the chance of putting in 'stig. Rotate a lot, and do short sessions and test a lot and you should be fine. I would want to grind the edge to make sure it is square to the face and provides good edge support. You need to be able to tell true 'stig from test-stand-induced 'stig. What is the focal length you are shooting for on the 23" this time? Where do you get hung up during the parabolization process?
It sounds like you have realistic expectations. When your cost is so low, it's great to make your own mirror, even if it isn't perfect. The process of grinding, polishing and figuring is enjoyable in itself. And if you stick with low-power, wide-field views, you will probably be very happy. There is a lot of satisfaction in looking at the sky with a mirror that you made yourself. I would suggest always having a smaller mirror underway. The skill at parabolizing comes from doing it over and over, and learning how it works. Your craft skills will improve over time, with repetition. Enjoy everything about it!
13:37 Waite. Carnt you just wet the surface when doing the Newton air wedge inferometer test?
Yes, but it's messy. I like to lay a measuring device on the back of the piece, and getting it wet would ruin it.
is there a reason both sides of the mirror has to be polished, presumably both sides of your reference flat are already polished? you _can_ use an optical flat to inspect an opaque surface, after all
The reference flat is considerably larger than the elliptical flat. So it is far easier to put the smaller flat on top of the larger one. Thus, the back of the elliptical flat needs to be polished as well. You don't have to polish it well, nor worry about the shape of the back surface. So it's fast to get it done.
@@GordonWaite oh yes that makes sense, I'm so used to my flats being so small
You may not be able to put 6 on 3 without "rocking" but you can damn sure put 3 on 6 and accomplish the same thing.
if the surface is what gets the reflective coating then why do the mirrors need to be that thick?
They need to be fairly thick so they don't bend in the mirror mounts. The ratio of width to thickness used to be 6:1, but these days people make mirrors to 10:1 or 12:1. But 20:1 would be pushing it. It takes good technique to avoid 'stig at 10:1.
@@GordonWaite I know this is a stupid question because of my lack of knowledge in this field, but why don't the really big telescopes use a large sheet of aluminum pressed into the right shape and polished?. it would take away a lot of the weight and wouldn't need to be handled with kid gloves as much as glass needs and would be easy to polish if and when it turns dull or gets scratched. and to tell the truth it probably would be a hell of a lot cheaper than every thing they have to go through with a glass mirror.
@@davidbrandenburg8029 Aluminum expands far too much with changes in temperature.
I suppose you are using green LEDs today, or even a green LASER
hey gordon don't you do videos anymore?, I was wondering because your newest video is 2 years old.
Hi, David. I've been incredibly busy the last couple of years. But Emerson and I were just talking about doing another series of videos. What would you like to see?
Hello, do you understand in Russian? I would like to talk with you on Skype if this is possible. I admire your work. I also made the telescope myself. I really like to study astronomy.
Sorry, no Russian. Email is the easiest way to communicate! Thanks!
Добрый день! Сделайте пожалуйста субтитры, перевод на русский язык! За ранее спасибо!!!
youtube can do it automatically in many cases: click the gear/settings icon, then "subtitles" then "auto-translate" and choose Russian. It works, or at least provides Russian subtitles for this video but I can't say how accurate the translation is.