Willie Koorts - 2018-09-01
Ensure you enable subtitles (click on "CC" bottom-right) to help where the sound is not audible. 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.
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.
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!
We used to metalize lighting reflectors using the same process. The diffusion pump always fascinated me. It makes such a tight vacuum.
Wow, I had no idea it needed that high of a vacuum.
Thank you for sharing this amazing process.
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
Fantastic video Willis! thanks for taking us through the process with this much detail :)
Really fantastic explanation. Thank you for taking the time to share this with your sensitive attention to detail
Thoroughly enjoyed every minute of this video, brilliant explanation of every step. I was astonished at the mirrors when they came out.
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?
How and where does one get that job?
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
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
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.
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 :-)
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 https://youtu.be/hxb-KOqaCYQ
Awesome video! Thanks for showing us with soo much details how you proceed! Is the vacuum tank DIY made? Great work!
Amazing! Thanks for showing us such a great work!
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!
Very interesting process! You learn new things everyday. Thanks for sharing Willie.
Thank you for posting Willie. I had a reflecting telescope when I was a lad and always wondered how the mirror finish was managed. Cheers
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.
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.
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.
Thank you very much for such an interesting and clean job and very good technical explanation.
Coisa linda de serviço 👍👌✌
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!
@Willie Koorts 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.
@chichi 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
you should do this out in space
@chichi " 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.
My first 6" was aluminised at Sutherland, and was very reasonable cost.
A newly coated mirror is a thing of beauty. Thanks for sharing this process, very interesting.
It was a wonderful learning experience, thank you for sharing !
This explains why telescope mirrors are so expensive. Thanks for teaching me something new ♡
Congratulations from Brazil. Thank you for showing us so interesting process, so clearly, step by step!!!
👏👏👏👏👏👏👏👏👏👏👏👏👏
Extremely educational. Thanks for sharing.
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.
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 ?
Wouldn't the results be better if the mirrors were in the bottom of the chamber facing up? I would assume that in the absence of atmosphere, gravity would tend to pull the aluminum vapor downward, would it not?
great video willie! how is insulation of the coils from the tank maintained to prevent making the tank electrically live, given that everything including any insulators gets aluminized?
Good point, I never thought about that, and it makes perfect sense. The drive-throughs are some unsulating material indeed with the one side of every coil grounded to the tank, so that side obviously does not matter. Seeing that the aluminium layer is very thin compared to the current we put through, so all I can think of is that it simply burns away - almost like a fuse that blows.
Your videos rule! Very well spoken. Thank you :)
Thanks! Glad to hear you enjoy them.
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
i remember speaking to you a long time ago about redoing my 10"...glad i found your videos, and subscribed ! great stuff
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?
Simplemente genial el proceso!!!
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?
Awesome, makes me want to build my own
Very informative. Thanks for sharing. I'm curious to why the glass gets mirrored with aluminium but the other surfaces in the chamber don't ?
Probably does. the interior space looks very aluminum in color. The glass port probably does also, but the layer is so thin that it still permits some light to pass.
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.
Nice to see someone skilled keeping these classic Edwards pumps working at peak performance!
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!!!
Thank you so much for sharing! Very cool stuff!
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?
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.
Nice video, very interesting! Some questions: 11:25 Where is the aluminum? Is it coated on to the tungsten coils beforehand? 12:48 When the aluminum is vaporized, wouldn't it fall to the bottom of the chamber? I'm surprised that the mirrors are hung veritcally rather than having them at the bottom and vaporizing the aluminum above them. Since the mirrors are hung vertically, the aluminum must make a cloud that fills the chamber when it is heated? But then, wouldn't it coat everything inside the chamber, not just the mirrors? How come the glass porthole in the chamber door doesn't get aluminized, along with the mirrors?
Indeed everything does get coated with Al; but in a high vacuum the collisions are few and the Al atoms travel almost horizontally (like the beam in a torchlight) and most land on the opposite side of the filament.
The aluminium hooks does not show well in the wide angle GoPro view at 11:25, hence the close-up inserts at 11:34 and 11:43 onward. You can also see in the close-ups how the aluminium form balls on the tungsten filament and do not fall to the bottom (the bottom of the tank gets inspected for this after every run - there are indeed occasional "drips", but these are minimal). Gravity plays no role in the process, so aluminising chambers come in any orientation. No aluminium "cloud" forms, but rather aluminium "bullets" flying at high speed in all directions from the source and do indeed coat everything else as well, hence the grey colour of the tank's inside. The porthole windows do indeed get aluminised, hence the protective glass needs to be cleaned after every run as can be seen from minute 14:38 to 16:14.
@Willie Koorts Ah, I see at 17:10 how you hang the Al wire on the coils, missed that before. And 14:44 where you clean the porthole glass that has also become aluminized. Thanks for the explanation! I have a small diffusion pump and am thinking of making a small chamber to re-aluminize my 14-inch telescope mirror.
Rickie Odem - 2019-10-18
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!
Say Rock - 2021-10-21
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.