How to Aluminize Telescope Mirrors
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- čas přidán 31. 08. 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. - Věda a technologie
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!
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!
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.
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.
Very interesting process! You learn new things everyday. Thanks for sharing Willie.
Great video! Wish you health! Crossing the fingers your profession should not be forgotten
Thoroughly enjoyed every minute of this video, brilliant explanation of every step. I was astonished at the mirrors when they came out.
You have an amazing shop. I would love to spend my working days here. absolutely beautiful. thank you for the video
We used to metalize lighting reflectors using the same process. The diffusion pump always fascinated me. It makes such a tight vacuum.
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.
Congratulations from Brazil. Thank you for showing us so interesting process, so clearly, step by step!!!
👏👏👏👏👏👏👏👏👏👏👏👏👏
Wow, I had no idea it needed that high of a vacuum.
Thank you for sharing this amazing process.
Really fantastic explanation. Thank you for taking the time to share this with your sensitive attention to detail
i remember speaking to you a long time ago about redoing my 10"...glad i found your videos, and subscribed ! great stuff
Fantastic video Willis! thanks for taking us through the process with this much detail :)
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
That was beautiful to watch... thank you Willie.
This explains why telescope mirrors are so expensive. Thanks for teaching me something new ♡
Dankie Willie! This was now very interesting to see! Thanks for going to the trouble of making the video
It was a wonderful learning experience, thank you for sharing !
Fascinating process. Many thanks.
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?
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.😂😂😂😂😂😂😂
Thank you so much! For caring enough to share your knowledge.
Great video. I work for a company that does vacuum brazing using similar equipment, except the chambers are much smaller. We place a clamshell heater on the outside, and the chamber is heated to about 1000*C while under vacuum (1e-5 mbar) with parts inside.
Thanks for the nice feedback. Wow, it sounds really interesting what you guys do. It would be wonderful to see a video, hint, hint!
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.
Extremely educational. Thanks for sharing.
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 :-)
Thank you very much for such an interesting and clean job and very good technical explanation.
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 czcams.com/video/hxb-KOqaCYQ/video.html
Greetings from New Zealand. Simply amazing.
Amazing! Thanks for showing us such a great work!
Nice to see someone skilled keeping these classic Edwards pumps working at peak performance!
Wow. Thanks for this video. It is amazing. Thanks for sharing this.
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
Thank you for sharing the knowledge.
Wow what an intering video, I have a lot to learn still - Dankie Willie!
WOW! thanks for showing this. truly fascinating.
Awesome video! Thanks for showing us with soo much details how you proceed! Is the vacuum tank DIY made? Great work!
Handsome looking mirror, indeed!
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!!!
Another thing learnt today. Thank you.
Good information here, thanks for sharing.
Thanks for the interesting and informative video Willie 👊🏼😊
Fascinating, thanks for sharing
In no other place you find this explanation , thanks great video
Thanks very much. Yes, there are indeed not many aluminsing videos on CZcams.
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!
Thank you so much for sharing! Very cool stuff!
I wish that fine gentleman could have heard my "wow" and seen my face on the reveal. I really appreciate this video, great insights.
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.
Sir, your technique of explanation is very nice. I understood very well.
Thanks, great to hear.
Dit is baie interresant. Dankie dat jy die filmpie met ons gedeel het.
Bly jy het dit geniet! Dankie vir jou terugvoer.
Ok ! thank you Willie.
very good, thank you for filming this
amazing work...and finest skills of the professional....i would give more attention to my telescope mirror
Your videos rule! Very well spoken. Thank you :)
Thanks! Glad to hear you enjoy them.
Superb ❤️🎉❤️ information 🙂 thanks..🙏❤️
Simplemente genial el proceso!!!
Great video! Thanks for sharing.
Outstanding!!!
Congatulations!!
Great work... thanks..
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.
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.
Always wanted to see that, thank you.
Great video, thanks! I've always wondered how this is done. That unmistakable SA accent :-)
Dankie Johann
Many thanks.
Trots op jou Willie! Well done
Ahhaaa! Baie dankie.
Great video, Thanks :)
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
Nice to know we have the technology right on our doorstep.
Great video, too complicated to attempt at home probably but still very interesting! I'm trying to make some mirrors too!
THIS is a video to _reflect upon...😊_
RocKITEman _ 2001 I’m going to look into it on reflection though
Interesting work👍👍👍👍👍👍
Dankie Oom Nicely made and interesting video thanks
This is SO COOL!
Awesome, makes me want to build my own
I always wanted to understand how this beyond the theory. Thanks!
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
Very awesome video. Love it!!!
Also, you kinda look like the old guy from "Up"
Awesome video
Dankie Danie
Wow! That's surprise Video!!👍👍👍
Fascinating!
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.
Fascinating.
Glad you liked the video.
incredible
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.
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.
Thank you
Impressive!!
Tre instrua! Dankon!
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 ?
A newly coated mirror is a thing of beauty. Thanks for sharing this process, very interesting.
Very cool.
Спасибо! Очень Интересно!
Hola bien por su trabajo y preguntarle si puedo usar un cilindro de hierro para realizar una alumininizadora de vació.
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?
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.
Coisa linda de serviço 👍👌✌