How to Aluminize Telescope Mirrors
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- เผยแพร่เมื่อ 31 ส.ค. 2018
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Modern optical telescope use mirrors and very few lenses.
Unlike bathroom mirrors, where the shiny layer is sealed in to the back of the glass, telescopes use so-called “first surface” mirrors, with the reflective layer on top. This makes them difficult to clean.
When they get dirty or as they loose reflectivity over time, they need to be re-aluminised. Professional telescopes in constant use, need their mirrors to be re-aluminised roughly every two years.
The word “aluminising” suggests that aluminium is involved - which is indeed the case. A very thin layer of very pure aluminium gets deposited onto the mirror’s surface in a special vacuum deposition process. This video shows this process in detail, from beginning to end.
Because aluminising is a fairly unique process, but since it is needed at every observatory, it is therefore commonly found at observatories. The South Africa Astronomical Observatory is no exception, and has a total of four aluminising tanks of different sizes, ranging from 330mm (13 inches) to 2 metre diameter.
In this video we will be using the tank capable of handling up to 40-inch or 1 metre diameter mirrors in which we will be aluminising a mixture of professional and amateur mirrors - 5 mirrors in total. - วิทยาศาสตร์และเทคโนโลยี
I use a 50+ year old vacuum pump regularly at work. Old cast iron pump oil lubricated has a little messy exhaust but it works as well today as when I bought it used 40 years ago. Amazing how old school tech can work so well and last so long.
The secret is to purchase things built before "optimization" occurs in the design cycle; i.e. before the accountants and managers get a go at it.
indeed! Typical of stings that were built before the disposable era! They were not only built to last, but also to be maintained and repaired.
well, remember to not buy the cheapest tools you can find next time you go shopping
Fascinating video to watch. An old friend, now deceased used to grind and aluminize custom mirrors in his shop. He made them up to 48 inches in diameter for various users from amateur to professional observatories. He used to say that creating a high vacuum was more like black magic than engineering. RIP David Sinden.
Wow! 48 inches! That is something to do by hand! David will live on for a long time in the mirrors he made! I have aluminised two mirrors that were older than 100 years already!
We used to metalize lighting reflectors using the same process. The diffusion pump always fascinated me. It makes such a tight vacuum.
Hey thanks a lot for sharing this Willie, that was really fascinating to see the process. It really gives a new appreciation for the work that goes in to producing these mirrors! You did a really good job editing this and explaining it in a way that could be understood too. Great stuff.
The Al melting is a clear and excellent example of a solid-to-liquid phase change and enthalpy. Since the heat ~= brightness due to blackbody radiation, the clear, discrete dimming on the heating filament you see each hook melt. I often joke that the Mirror Lab at the University of Arizona is actually a giant glass lab, since the coating is done in situ. Great stuff!
This video in conjunction with others I've seen on how to grind the mirrors. Shows how skilled these people are and how much time and passion they have for there trade.
Very interesting process! You learn new things everyday. Thanks for sharing Willie.
Thank you for posting Willie. I had a reflecting telescope when I was a lad and always wondered how the mirror finish was managed. Cheers
Wow, I had no idea it needed that high of a vacuum.
Thank you for sharing this amazing process.
You have an amazing shop. I would love to spend my working days here. absolutely beautiful. thank you for the video
This was a wonderful video, it helps us to understand all the hard work and dedication it takes behind the scenes. So often we forget the engineering and technical marvels that make modern scientific research possible. Also your video is excellently made to tell us all about the process as you are moving through it. Many thanks!
This is not at all what John described as the process he used to aluminize the mirrors we ground in his class. He describe a process, from what I can recall, where the glass is suspended a certain distance from the source of the machine that shot out the aluminizer like a few electrons thick across the entire surface of the glass. Of course we never got to witness this process, but we did soon receive our perfect reflector mirrors.
Thoroughly enjoyed every minute of this video, brilliant explanation of every step. I was astonished at the mirrors when they came out.
i remember speaking to you a long time ago about redoing my 10"...glad i found your videos, and subscribed ! great stuff
Really fantastic explanation. Thank you for taking the time to share this with your sensitive attention to detail
Dankie Willie! This was now very interesting to see! Thanks for going to the trouble of making the video
Congratulations from Brazil. Thank you for showing us so interesting process, so clearly, step by step!!!
👏👏👏👏👏👏👏👏👏👏👏👏👏
Fantastic video Willis! thanks for taking us through the process with this much detail :)
Great video! Wish you health! Crossing the fingers your profession should not be forgotten
It's increadible to see how high of a vacuum this process really needs :o
Indeed! It has all to do with "mean free path" to make sure the aluminium does not meat any oxygen molecules on their way to the mirror, resulting in a black coating!
@@SterremanWillie This is indeed not true. As soon as you open the tank, the Al surface comes in contact with oxygen and forms a layer of Al2O3. It does not become black. High vacuum is needed so that the Al atoms go straight to the surface and deposit there. If ANY gas is present, the Al atoms will hit them and lose energy and direction and result in a poor coating that can be peeled off. Because you are using a simple diffusion pump, you will get a pressure of 10-3 mbar (at best) and the mean free path will be around a couple of cms. So each Al atom will hit a couple of O2 molecules on the way but will not lose much energy and the deposit will be good.
@@janami-dharmam I have not had the (bad) experience of a black coating, but I'm told that that is indeed what happens for the reason explained. As you can see at minute 09:50, the final pressure was better than 2 times 10^-5 mbar
@@janami-dharmam " _As soon as you open the tank, the Al surface comes in contact with oxygen and forms a layer of Al2O3. It does not become black._ " Forming thin layer of Al2O3 is completely different from mixing the Al crystal with a huge amount of random oxygene atoms making it barely metallic. A thin layer of insulator is transparent, while strongly irregular crystal structure in a metal increases its resistivity, which reduces reflectivity.
@@PafiTheOne You are right; Why it should become black at all? It becomes black when it does not reflect any of the visible light back to the observer. As you correctly say a thin layer of Al2O3 is transparent and a thick layer of Al2O3 is white for visible wavelengths.
I am a millwright and I had a job maintaining the equipment used in vacuum deposition of metal on flat glass. We coated glass not only with aluminum but copper, bronze, silver, titanium, zinc, and stainless steel. Maintaining the vac pumps, including the diffusion pumps was an interesting job.
Ah, were you a member of the Aluminati?
It is one thing to read about the process on Wikipedia , it was quite another to watch it being done with clear accompanying explanation. Fascinating ! I thoroughly enjoyed your video, thank you Mr koorts :-)
Nice to see someone skilled keeping these classic Edwards pumps working at peak performance!
That was beautiful to watch... thank you Willie.
It was a wonderful learning experience, thank you for sharing !
Thank you so much! For caring enough to share your knowledge.
Thank you so much for showing how the magic is done! To answer a previous question asked, yes, the aluminum oxidizes into a clear Sapphire coating. AlO basic chemistry! And a wonderful byproduct! Self sealing and protective!
Most educative video. Anyone can now set-up a mirrorizing business!
Great to hear you enjoyed the video. Good luck to anyone trying to set up a plant based on my video. There is an amazing amount of institutional knowledge not shown!
Yeah, that it will be profitable.😂😂😂😂😂😂😂
Excellent video, Willie! I remember seeing this apparatus in one of the telescope domes, the 74" I think. Very cool to finally see it in use.
Glad you enjoyed this video. Yes, there are a total of three aluminising tanks in Sutherland, SALT, 74-inch and 40-inch. This was done on the latter. I also have a video showing the 74-inch's primary mirror being done - see th-cam.com/video/hxb-KOqaCYQ/w-d-xo.html
Thank you very much for such an interesting and clean job and very good technical explanation.
Greetings from New Zealand. Simply amazing.
Extremely educational. Thanks for sharing.
Wow. Thanks for this video. It is amazing. Thanks for sharing this.
Wow what an intering video, I have a lot to learn still - Dankie Willie!
Fascinating process. Many thanks.
very interesting video thank you so much for posting this i was very happy to see this process being done since im building my own telescope at the moment it was the perfect time to watch your video awesome work and a clear explanation i loved it
Wonderful video!
Hopefully I will start grinding my first mirror this year I also work on an ultra high vacuum chamber to coat that mirror later for myself.
One of the most interesting videos I've seen recently. Thanks for posting. 👍👍👍
Great to hear @ SCAPE0GOAT. Glad you enjoyed it.
I concur with that statement!!!
And no annoying commercials!!!
Amazing! Thanks for showing us such a great work!
This explains why telescope mirrors are so expensive. Thanks for teaching me something new ♡
We run our 48 inch primary at least 4 years before cleaning with some light 40 grit drywall sandpaper. Brings the sheen right back while avoiding the need of expensive aluminizing and down time.
Either "40-grit sandpaper" means something else to you than to me, or you are kidding - I suspect the latter!
@@SterremanWillie
Lol...great video Willie, I coated my first 3" mirror 40 years ago in a pickle jar with an Edwards diffstack and roughing pump I got from a pix tube rebuilder.
Kinda supprised you don't have preheat shutters on the evap coils.
@@sparkyy0007 Interesting! I have physically seen the insides of about half a dozen aluminising tanks in my life and pictures/videos of another half a dozen, but they were all similar to what we use. Tell me more about preheat shutters please?
@@SterremanWillie
There will always be some contaminants on anything you put into a vac chamber no matter how careful.
Micro-scratches in the Al wire will hide pull die oils that are impossible to remove with anything but heat.
Cleaning solvents, even reagent grade are never 100% pure, and anything that doesn't come off at 21 C will out-gas or boil off.
These contaminants will boil off (line of sight) onto the substrate just like the Al, and depending on the composition of the contaminants or their decomposition products when heated can cause surface reactions or adhesion problems with the Al coating.
All E-Beam evap systems use a mechanical shutter during warmup to prevent these volatiles from reaching the substrate.
The volatiles ( usually oils or hydrocarbons, but sometimes organics and dust) come off at a pretty low temperature, usually far below the melt point.
E-beam is extensively used in the semiconductor and optics industry where multiple evaporants are utilized in a single session used for multi-layer stacks, and where coating purity is paramount. No tungsten is heated during the process.
moorfield.co.uk/knowledge-base/electron-beam-evaporation/
www.nanomaster.com/images/deposition/e-beam/nee4000-dualclosed.jpg
Here is K Leskers units, they shutter their heater boats as well.
www.lesker.com/newweb/Vacuum_systems/thumbnail/Photo/Photo-SY-NANO36_04-THUMB.jpg
As well, with any tungsten filament evaporator, there is always a small amount of (W) boiled off depending on how hot you run the filaments, and this will reduce the reflectivity, W is really black. If there weren't, we would never need to replace the coils.
@@sparkyy0007 Interesting. This problem gets addressed by doing a "melting on" procedure. You may know this, but what you do is to load the coils and close the tank (without any mirrors) and pump to vacuum. Then you fire the coils only to the point where the aluminium hooks "melt on", i.e. form into bubbles on the coils and stop the current. This boils off all the bad stuff you described. You then release the vacuum, load the mirrors and continue the process as in my video. For some reason, our guys dropped this step, so I don't do this any more either.
WOW! thanks for showing this. truly fascinating.
I wish that fine gentleman could have heard my "wow" and seen my face on the reveal. I really appreciate this video, great insights.
Thank you for sharing the knowledge.
In no other place you find this explanation , thanks great video
Thanks very much. Yes, there are indeed not many aluminsing videos on TH-cam.
A really great informative video. Thankx a lot!
Thanks. Great to hear you liked it. Sorry about the sound in places, hope the subscripts helped.
I haven't seen an oil diffusion pump in ages. I would have thought a small turbomolecular pump would be used. Great video! Thank you.
The pumping speed of a small turbo pump is way too low for a tank of this size. Go check out how NASA also still use diff-pumps to evacuate their large vessels/chambers.
Awesome video! Thanks for showing us with soo much details how you proceed! Is the vacuum tank DIY made? Great work!
Dit is baie interresant. Dankie dat jy die filmpie met ons gedeel het.
Bly jy het dit geniet! Dankie vir jou terugvoer.
Worked in high vacuum metallization for 25 years. You ever burp a dp while standing anywhere near it when it’s hot you’re in for a merry surprise. This was fun for me to watch. Ours were chambered big enough to stand in but we weren’t doing mirrors. Just just 48ga pet and such at 86 inches wide, 120,000 feet per roll flying past at 13 mps also using plasma. Some fun
Thanks so much for the start-to-finish tutorial! Will give it a go if I ever get the chance 🤷♀
The observatory where I work used to be connected to the university. Not anymore since 2013ish. There is no aluminising capability around where I live, as far as I know.
That is a pity! You are welcome to contact me for help, if you ever want to get something going again. We have four aluminising tanks at our observatory, and I got involved at all of them.
Thanks for the interesting and informative video Willie 👊🏼😊
Sir, your technique of explanation is very nice. I understood very well.
Thanks, great to hear.
A newly coated mirror is a thing of beauty. Thanks for sharing this process, very interesting.
Hi Willie, very interesting and quite time consuming. I have two older telescopes (4" Meade SCT and 5 Celestron Comet Catcher from the 80's) that are getting to the point were the mirrors are needing re-coating. Who can I contact locally to do these? I am based in JHB. Thanks
Great video, thanks! I've always wondered how this is done. That unmistakable SA accent :-)
Dankie Johann
Thank you so much for sharing! Very cool stuff!
Another thing learnt today. Thank you.
Good information here, thanks for sharing.
I’m doing my masters in cryogenics and vacuum technology, we study in detail about the types of vacuum and types of vacuum pumps. Fascinating subject
Great! Then you must go watch my other videos on how I assemble our CCD cryostats, do leak-testing on them and cool them down with Liquid nitrogen.
amazing work...and finest skills of the professional....i would give more attention to my telescope mirror
Fascinating, thanks for sharing
Great video, too complicated to attempt at home probably but still very interesting! I'm trying to make some mirrors too!
Your videos rule! Very well spoken. Thank you :)
Thanks! Glad to hear you enjoy them.
Amazing video! Do you know if it's possible to source large mirrors in SA if you wanted to build your own Dob telescope?
I would love to go through this process but though it wasnt possible down here in SA.
Nice to know we have the technology right on our doorstep.
very good, thank you for filming this
I always wanted to understand how this beyond the theory. Thanks!
Very awesome video. Love it!!!
Also, you kinda look like the old guy from "Up"
THIS is a video to _reflect upon...😊_
RocKITEman _ 2001 I’m going to look into it on reflection though
Handsome looking mirror, indeed!
Great video - TY . @08:04 does the vapourised oil grab remaining gas molecules and attach to them and when cooled goes back into oil solution OR does vapourised oil get sucked out with molecules through the BACKING pump ? Also ,why do the remaining air molecules attach to the oil vapour ,what type of bond takes place ?
This is SO COOL!
Ok ! thank you Willie.
Outstanding!!!
Congatulations!!
9:11 The guy who named the pressure gauge a "monitorr" deserves a raise...
Simplemente genial el proceso!!!
I had a pleasure to work with such a process but we used cryo pump (2-nd stage) instead that diffusion and we had 3 stages pumping system (last was ionic pump). Also we (me and team) worked with tungsten-molybdenium boats instead of spiral because we coating also gold and silver half-transparent mirrors for such an applications like fabry-perrot interferometer. You are working clean, but we worked clean room standards. More or less ... nice work mister I am impressed. As you see it takes an ages to pump out 200 l chamber (by the way, nice chamber - simple and effective) by diffusion pump.
Dankie Oom Nicely made and interesting video thanks
Kick start! Where can we send mirrors to recoat in South Africa?
Send it to India..we will do
@@balgopal100 kindly share contact
@@balgopal100 pls send contact
Always wanted to see that, thank you.
Great video! Thanks for sharing.
It is possible to take the same operating mechanics of a 127 mm Mak/Cass optical and convert the mirroring into a 1.25" telescope that can be screwed into the main telescope?
Would the image be enhanced to produce the same image twice but would have appear in the eyepiece as a stacked image?
Many thanks.
Hi Willie, have you a plans from your equipment that can be order?
Trots op jou Willie! Well done
Ahhaaa! Baie dankie.
wow, what a great video. Really interesting to see how this is done from start to finish, thanks.
Can I bring my tarnished old 12" mirror to you to realuminize?!
Thanks Richard. With Covid and then a severe drought in the Karoo, I have not done any aluminising for a long time, so there is a huge backlog. I'm also retiring in a year's time, so not sure if I will be able to make this up in time.
@Willie Koorts tough times. I have seen the night sky from your great country just once, from Franschoek. I can't imagine what the night sky at sutherland must be like. Maybe one day...
@@richardd5009 as you say, although Franschoek is not bad, Sutherland is something else. Once you are well dark adapted, you can see your own shadow in the Milky Way! I did not think this was possible, until experiencing it myself.
@@SterremanWillie incredible.
Superb ❤️🎉❤️ information 🙂 thanks..🙏❤️
incredible
great video willie! how is insulation of the coils from the tank maintained to prevent making the tank electrically live, given that everything including any insulators gets aluminized?
Good point, I never thought about that, and it makes perfect sense. The drive-throughs are some unsulating material indeed with the one side of every coil grounded to the tank, so that side obviously does not matter. Seeing that the aluminium layer is very thin compared to the current we put through, so all I can think of is that it simply burns away - almost like a fuse that blows.
Enjoyed that. thanks for posting. It is an interesting process. Why does the aluminum not tarnish once the mirrors are exposed to air?
Interesting question. The aluminium do actually oxidise quickly, but it hardens the layer and I don't think change the reflectivity much.
yes aluminium oxide is very very hard, commonly used as an abrasive as it is a 9 on the moh's hardness scale. It is also transparent, meaning as soon as the aluminium is oxidized it gets a skin of this tough clear oxide, and it doesn't corrode any further
Brings back memories. We used to use sodium hydroxide to take the aluminum off, and nitric acid to neutralize it. Good times!
Is that chromium oxide liquid ?
Very informative. Thanks for sharing. I'm curious to why the glass gets mirrored with aluminium but the other surfaces in the chamber don't ?
Probably does. the interior space looks very aluminum in color. The glass port probably does also, but the layer is so thin that it still permits some light to pass.
A wonderful explanation, thank you. Why do you hang the aluminum hooks off the tungsten, rather than running current through them directly?
The short answer is, it is the easiest way. As you saw, the wires "hoist" themselves up as they start to melt to form bubbles, before evaporating. One get different ways of holding the material you want to evaporate, some in the form of tungsten "boats" for different applications.
Great video! Thanks for posting!When is the SIO overcoat applied in the process?
Thank for the comment. Our system is not equipped to apply any overcoat unfortunately. I think it is also done under vacuum, but not sure, sorry.
@@SterremanWillie
So, what is the average lifespan of an aluminum coating without SIO overcoat ?
Some noted that the high vacuum was to eliminate any O2 and other gases from the process. I am wondering if the ultra vacuum has any role in lowering the temperature at which the aluminum vaporizes?
Awesome, makes me want to build my own
My first 6" was aluminised at Sutherland, and was very reasonable cost.
Tre instrua! Dankon!
hi do you offer this as a service to amature telescope builders? I am based in pietermaritzburg and am busy grinding my first 6" mirror and am looking for somwhere to get it coated also what is the aproximate cost
Fascinating!