How to Aluminize Telescope Mirrors

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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.

[@wdwerker]

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.

[@jessielove1252]

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.

[@SterremanWillie]

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.

[@666Tomato666]

well, remember to not buy the cheapest tools you can find next time you go shopping

[@MrBanzoid]

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.

[@SterremanWillie]

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!

[@jimawhitaker]

This explains why telescope mirrors are so expensive. Thanks for teaching me something new ♡

[@josephalvin9282]

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!

[@millwrightrick1]

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.

[@ian_b]

Ah, were you a member of the Aluminati?

[@tracylemme1375]

We used to metalize lighting reflectors using the same process. The diffusion pump always fascinated me. It makes such a tight vacuum.

[@shmutalov]

Great video! Wish you health! Crossing the fingers your profession should not be forgotten

[@millenialfalcon8243]

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.

[@SterremanWillie]

Thanks for the nice feedback. Wow, it sounds really interesting what you guys do. It would be wonderful to see a video, hint, hint!

[@sparkyy0007]

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.

[@SterremanWillie]

Either "40-grit sandpaper" means something else to you than to me, or you are kidding - I suspect the latter!

[@sparkyy0007]

@@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.

[@SterremanWillie]

@@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?

[@sparkyy0007]

@@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.

[@SterremanWillie]

@@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.

[@TheControlPhilosopher]

Most educative video. Anyone can now set-up a mirrorizing business!

[@SterremanWillie]

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!

[@sharpthingsinspace9721]

Yeah, that it will be profitable.😂😂😂😂😂😂😂

[@nates8520]

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.

[@GravityBunk]

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

[@SterremanWillie]

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.

[@rickieodem488]

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!

[@sayrock6343]

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.

[@guilhermetorresj]

9:11 The guy who named the pressure gauge a "monitorr" deserves a raise...

[@franciscoosuna259]

@Willie Koorts I am amazed that the last thing to touch the glass surface before the aluminum coating was not the deionized water but a wad of cotton. Was that really necessary? I would think the cotton would risk leaving a microparticle that can not be detected by cursory inspection. Any deionized water still clinging to the glass would be boiled off by the vacuum pumps making wiping the surface unnecessary. Of course this does not even consider what is floating around in the air. I used to be involved in designing cleanroom environments. It appears that some attempts were taken toward building a clean space. But, I see dusty footprints on the floor. Apparently whoever walks into the room are allowed to wear street clothes and shoes. Transfer of particles from the tech to the glass by static electricity can occur without guidelines on proper clothing types or static dissipation. i.e. I am surprised there does not appear to be an interest in maintaining a cleanroom environment despite the obvious intent to build and equip one.

[@SterremanWillie]

Indeed, but seeing that water is the hardest thing to pump out of a vacuum chamber, you definitely do not want to start pumping with a wet mirror. We only use a very spesific brand and pureness of cotton wool and ensure that only "virgin" cotton comes into contact with the optical surface. We do this by never turning the wod over in your hand for fear of some from your glove contaminating the mirror. Particulate matter is actually less of a problem compared to oils and similar impurities.

[@franciscoosuna259]

@@SterremanWillie I did not think of the pump problem, a very helpful insight. It is a great video detailing the process. Thank you very much for your series.

[@bfarm44]

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

[@jimmcdonald9244]

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

[@GoldSrc_]

Wow, I had no idea it needed that high of a vacuum. Thank you for sharing this amazing process.

[@abdelrahmansayed8593]

Now I can see why I can't aluminize my home-made ATM mirror at home 😢

[@Mtaalas]

It's increadible to see how high of a vacuum this process really needs :o

[@SterremanWillie]

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!

[@janami-dharmam]

@@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.

[@SterremanWillie]

@@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

[@PafiTheOne]

@@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.

[@janami-dharmam]

@@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.

[@peterspencer6442]

Nice to see someone skilled keeping these classic Edwards pumps working at peak performance!

[@ukaszjozwiak9468]

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.

[@AirCommandRockets]

Very interesting process! You learn new things everyday. Thanks for sharing Willie.

[@eddiepires3998]

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 :-)

[@engineerstoolkit4900]

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.

[@All_Noing]

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.

[@SterremanWillie]

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.

[@das250250]

I still am amazed at how the pump works . I didn't really understand the processes completely ,Ill need to look deeper .

[@SterremanWillie]

Indeed, Google "oil diffusion pump". The roughing/backing pump is equally interesting - Google "rotary vane pump" - there are several intimations on the internet.

[@das250250]

@@SterremanWillie ty I'll research it. Was that 10^-4 bar ? before Aluminum atomization?

[@SterremanWillie]

@@das250250 no, we pumped to 2x10^-5 mbar finally.

[@riaandewinnaar5040]

Kick start! Where can we send mirrors to recoat in South Africa?

[@balgopal100]

Send it to India..we will do

[@arindamghosh8517]

@@balgopal100 kindly share contact

[@jayaansh_1333]

@@balgopal100 pls send contact

[@Your_Daily_Scroll]

You have an amazing shop. I would love to spend my working days here. absolutely beautiful. thank you for the video

[@thecakeredux]

I wish that fine gentleman could have heard my "wow" and seen my face on the reveal. I really appreciate this video, great insights.

[@zbnmth]

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.

[@SterremanWillie]

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.

[@michaelaldan6969]

i remember speaking to you a long time ago about redoing my 10"...glad i found your videos, and subscribed ! great stuff

[@adiisthere94]

Sir, your technique of explanation is very nice. I understood very well.

[@SterremanWillie]

Thanks, great to hear.

[@theelliotwoods]

Really fantastic explanation. Thank you for taking the time to share this with your sensitive attention to detail

[@rosco4659]

Thoroughly enjoyed every minute of this video, brilliant explanation of every step. I was astonished at the mirrors when they came out.

[@AdrianWyngaard]

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.

[@SterremanWillie]

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 kzbin.info/www/bejne/nqnFXn6FpsZ2j7M

[@cofranariel]

In no other place you find this explanation , thanks great video

[@SterremanWillie]

Thanks very much. Yes, there are indeed not many aluminsing videos on KZbin.

[@keithjurena9319]

Old school ultra high vacuum. Huge diffusion pump needed as glass adsorbs a lot of water plus the aluminum inside also adsorbs some from the air. Water is slow to remove in vacuum systems. I use a few cycles of roughing pump and argon backfill but with turbo molecular pump.

[@SterremanWillie]

Yes, 60-plus-year-old technology, still in good working condition. Indeed, water is your greatest hurdle to get good vacuum! Interesting about the argon backfill at the roughing stage still! I guess it is like flushing out some water? Or is the backfill during the turbo-pumping stage?

[@keithjurena9319]

@@SterremanWillie Flushing with Ar is done during roughing to "sweep" out water vapor. This was my experience in UHV system for a electron spin resonance cryostat for liquid He service. Much smaller than NMR cryostat so superinsulation doesn't work..critical diameter etc.

[@SunilSharma-wl5op]

Interesting work👍👍👍👍👍👍

[@rkipl]

Fascinating video! Why it melts entire Al hook at once, not just burning through it in half? Is the current set in a way that the heat propagates through the entire hook?

[@SterremanWillie]

It does happen sometimes that the hook burns through and the two pieces fall to the bottom of the tank, but, probably thanks to surface tension, the ends get pulled up to the heater to form a bubble. We do turn up the current slowly to try prevent the hooks from burning through.

[@prabhakarrao4922]

Greetings from New Zealand. Simply amazing.

[@A.Netizen.Since.2010]

..That was a wonderful experience watching the whole process in a professional fashion of almininizing a telescope mirror. . .& thank you so very much for showing & describing to us everything in details........But sir, is there any simpler way in your knowledge. .to executed it by the amateur telescope [Newtonian/Dobsonian] builders at home?...If there's any, then please let us know....Best regards... . 👍🏼

[@SterremanWillie]

Thanks for the nice remarks for my video, glad to hear you liked it. The only homebrew method I know of, is the old silvering process, performed many years ago by amateurs. However this process is flawed with problems, compared to aluminising, so I would not recommend it! The best is to find someone who can aluminise your mirrors for you.

[@johnpelitidis6297]

That was beautiful to watch... thank you Willie.

[@louhenry3127]

Dit is baie interresant. Dankie dat jy die filmpie met ons gedeel het.

[@SterremanWillie]

Bly jy het dit geniet! Dankie vir jou terugvoer.

[@briantimar1105]

A wonderful explanation, thank you. Why do you hang the aluminum hooks off the tungsten, rather than running current through them directly?

[@SterremanWillie]

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.

[@MrMraza123]

Thank you for sharing the knowledge.

[@hblandim720]

Congratulations from Brazil. Thank you for showing us so interesting process, so clearly, step by step!!! 👏👏👏👏👏👏👏👏👏👏👏👏👏

[@philliphaasbroek]

Nice to know we have the technology right on our doorstep.

[@SakCyb]

Wow what an intering video, I have a lot to learn still - Dankie Willie!

[@rajab7995]

Thanks for the video. What are the materials used for cleaning the existing coating, you mentioned it as KOH and FeCl3, is it correct..?, By any chance do they etch the glass, if the solution ratio is not correct, or if I keep it for a longer period... If possible, please let us know how to prepare the KOH and FeCl3 solutions for cleaning

[@GodzillaGoesGaga]

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 ?

[@franciscoosuna259]

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.

[@dharmatal]

WOW! thanks for showing this. truly fascinating.

[@kenpremo56]

I have seen the results of a few large chamber vented while the diffusion pump was on and running. Shot the end of the chamber across the floor!

[@SterremanWillie]

Wow! Amazing! Luckily not happened on my watch (yet!).

[@rbettsx]

Having spent a life carefully cleaning optical glass with air or lint-free fabric .. very surprised by the cotton-wool... it obviously can't leave fibres, but I wouldn't that to a lens.. is there a difference?

[@cceciljr]

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!

[@saqibhussain7217]

Thanks, very big and professional Chamber for vacuum coating, I am a student of Physics and want to perform this aluminum coating on the mirror , what should be the design voltage applied to melt the aluminum for evaporation?

[@sebastianschmidt566]

The voltage is irrelevant here. You need Amps. But it's to dependent on your coil and the setup you use. How big the object would be you plan to aluminize ? Is it a little one time Experiment or is the plan to build something that lasts and is daily used. For a little one time Experiment maybe it's easier to use a big Akkupack like one from a car(or if you're uni has Akkupacks for Experimenting with electric vehicles) such Akkus could easily deliver 100 Amps

[@lambdasun4520]

Great video, too complicated to attempt at home probably but still very interesting! I'm trying to make some mirrors too!

[@glifencible]

Brings back memories. We used to use sodium hydroxide to take the aluminum off, and nitric acid to neutralize it. Good times!

[@水巷-i4l]

Is that chromium oxide liquid ?

[@anonymic79]

After cleaning, are you concerned about hydrocarbons contaminating the glass before it goes into the chamber or are these all volatile enough to be removed by the vacuum? Or is this not a concern given the exposure to atmosphere and water after plating, where the atmosphere is governing the life of the mirror?

[@abumoslemtamer7331]

you need to make plasma cleannig in the vacuum chamber before you applay AL coat.

[@bardicdad]

Handsome looking mirror, indeed!

[@FesixGermany]

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.

[@alaskajdw]

Thanks for the interesting and informative video Willie 👊🏼😊

[@oleguernogues]

Fantastic video Willis! thanks for taking us through the process with this much detail :)

[@HamishBarker]

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?

[@SterremanWillie]

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.

[@lukasskymuh5910]

I always wanted to understand how this beyond the theory. Thanks!

[@everythingquads]

A newly coated mirror is a thing of beauty. Thanks for sharing this process, very interesting.

[@Allan_aka_RocKITEman]

FWIW: Where I used to work, we used aluminum {aluminium} wire as "twist ties" to hold loose parts together, and to attach paperwork to parts. It is easy to twist, but holds things together quite well. Also easy to cut. How _pure_ the wire was, I do not know.

[@janami-dharmam]

most likely the wires came from parts of electrical cables (cut pieces that are often discarded) and they are highly pure (but I do not know the exact purity). Al is rather easy to purify.

[@WellingtonMoederkerk]

Trots op jou Willie! Well done

[@SterremanWillie]

Ahhaaa! Baie dankie.

[@richardd5009]

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?!

[@SterremanWillie]

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.

[@richardd5009]

@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...

[@SterremanWillie]

@@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.

[@richardd5009]

@@SterremanWillie incredible.

[@yurikhakhnazarian7664]

Thank you very much for such an interesting and clean job and very good technical explanation.

[@laura-ann.0726]

So, when amateurs grind their own DIY telescope mirrors at home, I guess that they have to send them to a facility like yours to be aluminized? I don't see how anyone could do this at home, the equipment would be far too bulky and expensive. Fascinating video! Do the world's largest telescope mirrors, like the 200" Hale, have to be re-aluminized at intervals also? That mirror is huge, and weighs 13 metric tonnes! A vacuum chamber big enough to hold it would have to be equally enormous.

[@SterremanWillie]

Yes, we do amateur mirrors as well, in fact, most of the mirrors in that run were amateur mirrors. Yes, the mirrors of most huge telescopes are aluminsed. Some are removed (search KZbin for AAT (Anglo Australian Telescope) and 200-inch Hale) while others are done in-situ! Also see kzbin.info/www/bejne/nqnFXn6FpsZ2j7M

[@laura-ann.0726]

@@SterremanWillie - I have just watched the two videos you recommended, on the Radcliffe and Hale telescopes. The most surprising fact I gleaned was how long it took to fabricate the Hale's 200" mirror: according to the documentary, from the day the mirror blank was delivered from the Corning Glass works in New York to Cal-Tech, the grinding and polishing took 11-1/2 years, and fully 5 tons of glass were removed from the blank during grinding. The film doesn't state this, but I assume work on the observatory and the mirror was slowed during WWII, because it wasn't completed until 1948. The mirror was then trucked from Cal-Tech to the observatory, and aluminized in a facility there, I assume very much like what I saw in the Radcliffe observatory film. I assume the Hale mirror is lowered vertically from the instrument onto an elevator that drops it into a basement lab built below the dome, where a custom built vacuum chamber was installed for aluminizing it. Are all large observatory mirrors maintained in facilities similar to what's shown for the Radcliffe facility?

[@yqh8247]

A very informative video!

[@solarflare4259]

Great video! Thanks for posting!When is the SIO overcoat applied in the process?

[@SterremanWillie]

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.

[@abumoslemtamer7331]

@@SterremanWillie So, what is the average lifespan of an aluminum coating without SIO overcoat ?

[@TheDagda1000]

Fascinating process. Many thanks.

[@markspc1]

It was a wonderful learning experience, thank you for sharing !

[@iamtheman7018]

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?

[@cavinrauch]

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.

[@gyulawendler]

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

[@gertvbiljon]

Dankie Willie! This was now very interesting to see! Thanks for going to the trouble of making the video

[@ManishFrenchStudio]

Superb ❤️🎉❤️ information 🙂 thanks..🙏❤️

[@karim1485]

So if I understand correctly; the surface tension of the created liquid during the melting of the Aluminium hooks on the tungsten filamtent, pull the remaining substance up and onto the filament? The melting doesn't cut the hook into half? Thank you for this great video

[@SterremanWillie]

Yes, exactly. It sometimes happens that small pieces of a few hooks are found at the bottom of the tank when we open up, but mostly the surface tension does its thing as you described and can be seen in my video.

[@karim1485]

@@SterremanWillie Awesome, thanks for the quick reply. All the best to you!

[@bloodyfluffybunny7411]

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

[@MountainFisher]

I had my 6" mirror re-aluminized for $88, but they tested its parabola to see how good it was, for a mass produced mirror that is. They said it wasn't bad, probably because it was an older scope.

[@SterremanWillie]

Interesting. Great to have mirror testing included in aluminising service. Its quite an easy test, I seem te recall.

[@MountainFisher]

@@SterremanWillie They had a special on a refigure guaranteeing .95 Strehl and 1/10th wave so I went for it. .98 Strehl and 1/12th wave and I could tell, problem was I needed a new focuser.😆

[@das250250]

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 ?

[@Psycandy]

Dankie Oom Nicely made and interesting video thanks

[@billsimpson604]

Unless you are in an area with a lot of air pollution, are a scientist using the telescope for scientific purposes, or are cleaning your telescope mirror incorrectly by rubbing it, or using anything other than distilled water with a single drop of Dawn detergent and cotton dragged across the mirror with NO pressure whatsoever to occasionally clean your mirror, you shouldn't have to have this done for decades unless the coating was defective from day 1. Letting dew form on the mirror in areas with polluted air will accelerate the deterioration of the aluminum coating. Always use distilled water to wash a telescope mirror. A little dust on a mirror won't affect the view. Never rub a telescope mirror, even when wet. Frequent cleaning can do more harm than good.

[@SterremanWillie]

The reflectivity is measured for the mirrors of the professional telescopes, which decides when they need to be re-aluminised. These are in constant use every clear night, hence need to be re-done more frequently than amateur telescopes.

[@sharpthingsinspace9721]

@@SterremanWillie so this video is useless for 99% of viewers, GOT IT!!!!!!

[@YouTubeStat]

​@@sharpthingsinspace9721​ really... What's your problem? It was a good video, why you wanna be a jerk?

[@sharpthingsinspace9721]

You are correct I was thinking of cleaning aluminum mirrors.

[@YouTubeStat]

@@sharpthingsinspace9721 😂 ok then! ... That's pretty funny! God bless you!

[@das250250]

After seeing this several times over the years , I would imagine there is a thick coating of AL on the inside of the chamber if it is used regularly ? Has anyone measured how deep the coating is ?

[@SterremanWillie]

Not really, you must remember the layer per coating is about 10 microns, hence after 100 firings, its only 0.1mm thick!

[@h.cedric8157]

I live in the Philippines. I have yet to find a Philippine establishment that does optical mirror re-aluminizing.

[@martinlagrange8821]

My first 6" was aluminised at Sutherland, and was very reasonable cost.

[@DR.ELEKTRIK]

Now I'm curious about the techniques used on earlier telescope production.

[@SterremanWillie]

Silver coatings were used before aluminising. A silver salts mixture was pored onto the mirror, but one problem is that it was hard to get an even thickness layer. Also, silver tarnishes very quickly, so it had to be physically polished regularly. This has the risk of grime scratching the mirror (and the coating of course). Even older techniques (back in Herschel's time) had the mirrors made from "speculum metal", two parts copper to one part tin and a small amount of arsenic. This produced a soft metal which could be shaped into a parabola and then polished .. and frequently re-polished because it tarnished quickly.

[@JohannSwart_JWS]

Great video, thanks! I've always wondered how this is done. That unmistakable SA accent :-)

[@SterremanWillie]

Dankie Johann

[@SOUZAELS]

amazing work...and finest skills of the professional....i would give more attention to my telescope mirror

[@chichimus]

Enjoyed that. thanks for posting. It is an interesting process. Why does the aluminum not tarnish once the mirrors are exposed to air?

[@SterremanWillie]

Interesting question. The aluminium do actually oxidise quickly, but it hardens the layer and I don't think change the reflectivity much.

[@gabewrsewell]

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

[@hughezzell10000]

I believe this process is called sputtering, isn't it? My question has to do with the travel direction of the aluminum - how is it that the aluminum travels horizontally towards the mirrors? I've never seen this done but was under the mistaken impression that the sputters would be mounted vertically above the mirrors so gravity would pull the evaporated aluminum towards the mirrors. If the aluminum travels in all directions, does your tank have a coating of aluminum that you have to clean off sometimes? This process, by the way, isn't confined to aluminum - you can sputter just about any metallic or semi-metallic substance you wish. Sputtering is a common industrial process wherein very thin coatings can be deposited on just about any clean surface - even plastic. Thank you for showing me this.

[@SterremanWillie]

No, sputtering is a completely different process/technique to aluminising - see en.wikipedia.org/wiki/Sputter_deposition

[@Kpv_777]

Спасибо! Очень Интересно!

[@timchemaly5800]

Baie cool Willie! Are you always guaranteed of a perfect alumanized surface after the vacuum process?

[@SterremanWillie]

Very good question! No, there are many things that can cause a bad coating. E.g., if the vacuum was not good enough, the alumimium reacts with oxygen, causing a black coating! If the cleaning and particularly the rinsing was not good enough, hazy streaks appear in the coating or it does not stick properly to the mirror and starts to flake off after a while. If you don't follow the instructions properly, oil backflow from the pumps can contaminate the chamber, also preventing the coating to stick properly. Attention to detail and sticking to the recipe is very important.

[@gg5115]

@@SterremanWillie It seems that by hanging the aluminum stock over the heating coils like that, it boils the blank in two and most of it falls to the bottom. Very little of the stock is actually vaporized. I guess the wire is cheap enough, and you don't have to do it that often, but it just seems a curious delivery method. Are there units that boil the aluminum in a little tungsten pot?

[@julioduque1967]

Hi Willie, have you a plans from your equipment that can be order?

[@SCAPE0GOAT]

One of the most interesting videos I've seen recently. Thanks for posting. 👍👍👍

[@SterremanWillie]

Great to hear @ SCAPE0GOAT. Glad you enjoyed it.

[@rockerpat1085]

I concur with that statement!!! And no annoying commercials!!!

[@gazzabro55]

Awesome, makes me want to build my own

[@superbiakugan]

Hola bien por su trabajo y preguntarle si puedo usar un cilindro de hierro para realizar una alumininizadora de vació.