How to Aluminize Telescope Mirrors

How to Aluminize Telescope Mirrors

Squeaking clean! OK, now we start opening the roughing valve. You will see when the pump starts kicking into action as we evacuate the air from the vessel. It takes about 20 minutes to open the valve fully. We have two pumps. The roughing pump is at the back there. This is the line evacuating the tank and the diffusion-pump will come in later to get good vacuum. Right, 40 minutes later, and let’s see what the pressure is. Right, 10-to-the-minus-one [Torr]. That’s actually where we can start diff-pumping. Now we close the roughing valve and open the backing valve. You can hear a slight change [in the sound from the backing pump] as we are now evacuating the diff-pump itself. But there is still a valve in here that prevents … it from working this way. OK, so now we have evacuated the diff-pump enough. and we can now open the butterfly valve. Now we are roughing through the diff-pump but we haven’t started the diff-pump yet – that’s the next step. What we do now is open the water flow. We run the water through all these cooling pipes. There’s a heater down below that heats some oil The oil boils and the vapours of the oil reaches the speed of sound in a funny funnel system and that gets directed down onto the sides of the pump which is now water-cooled. The oil re-condense and taking molecules with it. This is the way the pump works [without any moving parts] Right, 15 minutes later and now you can see the diff-pump really kicking in, moving to 10-to-the-minus-two So if we switch to the other … range you can see the vacuum drop quite dramatically now. You can actually see it moving! [speeded up] We can now also switch on this gauge and it should start to register soon as the vacuum reaches 10-to-the-minus-2 it should start to come up on this gauge [speeded up] OK, its about two hours later and the vacuum has moved … from there … to there, one decade. Tomorrow morning we’d hopefully be ending up there somewhere. OK, another 4 hours later about and you can see it moved to nearly 2-to-the-minus-5. Right, overnight … next morning now. and you can see way down! exactly were we wanted it, so we can now go over to aluminising. So we start by dialling up 8 Amps and as the coils warm up, you can see the current fall. and as the coils warm up, you can see the current fall. As the aluminium melts on, on each coil, you will see the current go up Yes, there’s the one [coil] firing – and there’s the other one firing [too] And as the aluminium melts off you see the current return again. The aluminium adds to the conductivity of the tungsten And now its all clean … and we can turn it back. This is a view into the side porthole. The mirrors can be seen on the left. All valves closed. Closed … diff-pump is off …. and we can open up! Now we can release the vacuum on the tank. Slowly open this valve to let the air back in. Also make sure all the bolts are loose. Now for the big reveal! NOT BAD! Looks pretty good! The tank has two porthole windows which we look through. These will of course also aluminise. So these can be replaced – it is removable. You can take out the glass and take off the alumimium.


  • Air Command Rockets

    September 1, 2018

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

  • chich

    September 1, 2018

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

  • Reyak

    September 1, 2018

    Amazing! Thanks for showing us such a great work!

  • rkalle66

    September 2, 2018

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

  • Kurt S

    September 3, 2018

    Hi Willie,
    thanks for your very interesting video. But I´m
    surprised you didn´t mention something about final cleaning by glow
    discharge. From my own experience this additional cleaning procedure
    seems to be a must. Examples of my DIY vakuum equipment:
    Best regards

  • Gert van Biljon

    September 3, 2018

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

  • Bryan Geerts

    September 25, 2018

    Awesome video! Thanks for showing us with soo much details how you proceed! Is the vacuum tank DIY made? Great work!

  • Paul Zacharias

    December 13, 2018 

    and… Spectrum Coatings Facebook page — https://www.facebook…mirrorcoatings/ 

    Ion Assisted Deposition. I have heard nothing but great things about the coatings this company offers. Check them out.

  • dtiydr

    January 20, 2019

    1. I'm really surprised it took this long to get down to vacuum with a huge diffusionpump like this as they pumps huge amount of air when they upp to full speed, any small leaks perhaps or to big flow of water or to low power to it?
    2 10-E5 Torr is quite high for this chamber since the mean free path at this vacuum level is a about only a meter and it seem like the coils are farther away from the mirrors then this? If so the surface quality can be affected in a way not seen by they eye.
    3. I assume a re coat of the big one due to what seemed to be smudge top right that wasnt removed before coating?

  • Midnight Customz

    February 7, 2019

    CAN SOMEONE HELP ME PLEASE ?……I'm unable to find where to buy a thick piece of glass so I can start my telescope mirror I'm wanting to make a 30in. Primary mirror an I don't know what kind of glass to use . I found a place online who sells 30in, tempered glass clear thats 1/2" thick . Would that work ? It's a $180.00 for the glass

  • ⵉⵜⵔⵓⵏⴰⵓⵜ

    February 12, 2019

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

  • Chip Cecil

    February 16, 2019

    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!

  • Andre XX

    February 19, 2019

    Baie mooi, dankie.

  • R Carroll

    February 21, 2019

    Thanks Willie! for that video… and ALSO for your old 9 inch "Telescope Rebuild" (full story) + other stories & the funny road sighs over at the SAAO (site has a few missing bits, but seems better again). I'm Glad you are still staying busy and having fun. Best wishes, from Arkansas USA.

  • parçala behçet

    March 12, 2019

    Thank you

  • Deepak Singh

    March 19, 2019

    Nice video sir

  • Gyula Wendler

    April 19, 2019

    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

  • paul30003

    April 28, 2019

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

  • chris takeltin

    June 11, 2019

    could someone please inform me how can paravolised one mirror ,how we can get paravolic shape of them ?

  • mostafa saadinasab

    June 28, 2019

    #Awards Mr koorts


    June 30, 2019


  • Łukasz Jóźwiak

    July 3, 2019

    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.

  • Alperen Atalay

    July 12, 2019

    Do you sale 20” mirrors ?

  • Steven Davis

    July 12, 2019

    Wow. Thanks for this video. It is amazing. Thanks for sharing this.

  • Dik Hed

    August 16, 2019

    what is the name of that music?

  • Geoff Rose

    September 10, 2019

    Fascinating process. Many thanks.

  • Dryson Bennington

    September 15, 2019

    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?

  • vijay sir

    October 10, 2019

    Is there any spray for my mirror , I can't buy vacuum pump sir.

  • Tim Chemaly

    October 11, 2019

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

  • martin aakervik

    October 12, 2019

    To make my simple mind understand this, someone need to explaine to me more detal about what happening in the magic yello vacume machin. I saw some pips, some bolts, some hot spirals and could not understand how not everything inside the machin become a mirro.

  • Bill Simpson

    October 13, 2019

    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.


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