How to Aluminize Telescope Mirrors

[@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!

[@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!

[@jimawhitaker]

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

[@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?

[@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.

[@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.

[@tracylemme1375]

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

[@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.

[@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!

[@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.

[@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.

[@shmutalov]

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

[@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.

[@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.

[@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.😂😂😂😂😂😂😂

[@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.

[@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.

[@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

[@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

[@michaelaldan6969]

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

[@peterspencer6442]

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

[@AirCommandRockets]

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

[@richardhead8264]

I see a smudge on the large mirror at its 2 O'clock position, at time-stamp 13:00. Did you have to redo that mirror?

[@AzimuthAviation]

It was in several frames from different perspectives....

[@SterremanWillie]

This was damage caused where the telescope's secondary fell onto the primary. The secondary also got badly chipped. I was asked to just aluminise everything and they will assess how the damage will affect performance.

[@richardhead8264]

@@SterremanWillie Thank you for your reply! I am relieved to hear that the damage was already there, and that you were not stuck having to redo everything. 😄👍

[@wannahockaloogiewannahocka1040]

@@richardhead8264 love the name 😄

[@JasperJanssen]

Willie Koorts ouch. That’s gotta be a heart stopping clatter to hear.

[@Your_Daily_Scroll]

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

[@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 :-)

[@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.

[@6rw1b]

Fascinating. So that's how dichroic glass is made too I guess, but supplementing different metals for the desired effect. Thanks for sharing. Much respect for your skill and knowledge.

[@SterremanWillie]

@@6rw1b Thanks for the compliment. For dichroic you need very fine control of the layer thickness. I once wanted one, and sort off got it right by stretching a silk stocking over a piece of glass!

[@6rw1b]

@SterremanWillie Sure man no worries. I've used gold and silver to fume borosilicate glass in the flame, hence my interest. It popped into my head to make some kaleidescopes for my grandkids hence I stumbled on your vid. Thanks again for sharing, how else will our civilisation develop. Well done that man 🙂👍🏻🌟

[@guilhermetorresj]

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

[@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

[@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.

[@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?

[@everythingquads]

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

[@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.

[@abdelrahmansayed8593]

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

[@gowdsake7103]

Is it an expensive process ?

[@JAKOB1977]

no, not really, recall the price on this alu-vapor-spotting on the big one was around 85 USD.. though its the shipping that really takes a knock on your purse with the big fragile glass-mirror like this that needs baby-care-shipping-channels.. 13.765 USD in shipping cost, though that also include return shipping.-

[@dogodogo5891]

@@JAKOB1977 mind if i like to ask you some question if something like this used for solar concentrator is it bit overkill or not? What category of telescope mirror an optic imaging or not imaging? Thank you

[@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

[@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

[@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!

[@rosco4659]

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

[@theelliotwoods]

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

[@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

[@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.

[@adiisthere94]

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

[@SterremanWillie]

Thanks, great to hear.

[@julioduque1967]

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

[@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.

[@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.

[@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!

[@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.

[@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 ?

[@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 ?

[@SunilSharma-wl5op]

Interesting work👍👍👍👍👍👍

[@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.

[@narexfu]

Where You Buy SCT Or Other Cassegrain Mirror Set?

[@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.

[@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.

[@johnnycash4034]

What kind of seal is on that chamber? What is the thickness of the chamber walls and is it just steel? Also does it really need that large size diffusion pump? What's the flow specs on that size pump? Thanks if you antwoord please. Regards

[@SterremanWillie]

Hi Johnny. Are you not supposed to be singing about steam trains or something? ;-) ;-) ;-) Here are some antwoorde: The seal is rubber - we prefer Viton, where possible. The wall thickness of the chamber is about 1/2-inch steel. It could actually do with a bigger diff pump since it takes so long to pump down. Modern pumps are more efficient, so may not need to be this big. Sorry, I have no idea what the pumping speed is. The pump is about 60 years old, so the spec-sheet is lost by now!

[@johnnycash4034]

@@SterremanWillie thanks for the answer. Do you still work there? Seems like a great place to be. On the viton O-ring - - is the groove just a machined recess into the flange of the chamber?

[@SterremanWillie]

@@johnnycash4034 yes, I have two years to go to retirement. Yes, just in a groove. A few years ago they replaced the o-ring on the other, bigger tank, capable if taking a 2-m diameter mirror (see kzbin.info/www/bejne/nqnFXn6FpsZ2j7M).

[@philliphaasbroek]

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

[@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.

[@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?

[@h.cedric8157]

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

[@prabhakarrao4922]

Greetings from New Zealand. Simply amazing.

[@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.

[@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?

[@Metaldetectiontubeworldwide]

why isn't silver be used ? is it not even more refelecting ? I know the weight maybe an issue . althought its just a film like layer ? grtzz from the netherlands johny geertss

[@oleguernogues]

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

[@johnpelitidis6297]

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

[@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.

[@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.

[@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!

[@rock3tcatU233]

How is it that the shiny surface doesn't oxidize and become blurry?

[@DrysonBennington1]

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?

[@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 ?

[@poly_hexamethyl]

Just wondering about the roughing valve....would it be possible to rough evacuate the chamber through the diffusion pump before it is heated, to avoid the need for the roughing valve? Or is the roughing value just to save time if the pump is already hot from a previous cycle, so you don't have to wait for it to cool down?

[@SterremanWillie]

Good question. I'm not sure. For a short time we do pump through the diff pump. There must be a good reason because all the pumps I've seen, work this way.

[@briankarlsen6676]

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

[@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.

[@hblandim720]

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

[@thesarthakranjan3946]

sir, i have a 76mm reflector telescope and want to clean, how to clean it? like i dont have much stuff like chemicals and want to do at home only

[@benverdel3073]

I saw somebody (i guess it was a teacher of some sort) grinding 2 glass discs one over the other to get a convex and a concave (the mirror) surface down to a polished surface (focal lenght about 8 feet). But aren't these 2 surface circular in stead of hyperbolic?

[@bardicdad]

Handsome looking mirror, indeed!

[@superbiakugan]

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

[@lambdasun4520]

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

[@Cien_Swiatla]

what is the cost of covering, say, a 200mm lens?

[@MrMraza123]

Thank you for sharing the knowledge.

[@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!

[@JohannSwart_JWS]

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

[@SterremanWillie]

Dankie Johann

[@danc8278]

Do you have to do anything to ensure the aluminium coating is a uniform thickness across the mirror?

[@SterremanWillie]

With the system so old, there is nothing like that built in. It relies on the design to yield an even coat, i.e. even distribution of the coils, the sequence we fire them, distance between the coils and mirror, etc.

[@rkalle66]

Hi Willie, did you perform a glow discharge in the vacuum chamber before vaporizing the aluminium to get rid of remaining water molecules?

[@SterremanWillie]

Ahaa! I can see you know something about aluminising! Yes, the glow discharge is actually done at a higher pressure while there is still enough gas in the tank to ionise. Ever since I saw our process 30 years ago, we have not done the glow discharge, even though we are equipped to do it. I'm not sure why the guys stopped doing it here, but it is currently not in our procedure.

[@SterremanWillie]

I since enquired why we don't bother doing the glow discharge - see my reply to Kurt Schreckling.

[@maximilianlindner]

@@SterremanWillie I can't find a comment by Kurt Schreckling...

[@gertvbiljon]

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

[@psycronizer]

Just a thought, but, would this be a simpler process, with the possibility of not needing such a high vacuum if the tank was first purged with Nitrogen or Argon prior to setting up the vacuum ? I have a eight inch reflector and would love to make my own 14 inch mirror, I'm also a chemistry freak so have heaps of reagents like a lot of Silver nitrate, that stuff make very nice mirrors too, could that be used to coat a mirror ?

[@SterremanWillie]

The vacuum level is dictated by "mean free path" (google it) irrespective of the residual gasses - basically the statistical chance of the aluminium molecule meeting a gas molecule on its way to the mirror. Silver coatings were commonly used a century ago, before the aluminsing process was discovered. There are modern equivalents that yield a thin even coat, without distorting the mirror's accurate figure - aluminsing's claim to fame is being thin en even. Silver tarnishes quickly, so an overcoat is also required for longevity.

[@louhenry3127]

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

[@SterremanWillie]

Bly jy het dit geniet! Dankie vir jou terugvoer.

[@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.

[@brian.charlesworth]

I have an odd question....Can you aluminize objects that are not flat , let's say a sphere? Or would you need to rotate it during the aluminization process?

[@GravityBunk]

I saw a wasp that got coated in gold completely using the same vacuum deposition technique.

[@WellingtonMoederkerk]

Trots op jou Willie! Well done

[@SterremanWillie]

Ahhaaa! Baie dankie.

[@martinlagrange8821]

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

[@ManishFrenchStudio]

Superb ❤️🎉❤️ information 🙂 thanks..🙏❤️

[@dharmatal]

WOW! thanks for showing this. truly fascinating.

[@christopherhutchinson8043]

😎 , what stops the aluminium coating from oxidizing ?

[@SterremanWillie]

The way I understand, it does actually oxidize as soon as the tank is opened. The oxidization layer is very thin, is transparent and actually protects the auminium from the elements.

[@SakCyb]

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