Modern optical telescopes 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 metres diameter. In this video we will be using the tank capable of handling up to a 40-inch or 1-metre diameter mirror in which we will be aluminising a mixture of professional and amateur mirrors – 5 mirrors in total. After preparing the aluminising lab, the mirrors are unpacking from their shipping containers. A suitable metal ring is then selected for each mirror in order to clamp them vertically in the tank. The mirrors are first rinsed with tap water to get rid of most of the dirt. Then their old reflective surfaces need to be stripped. Mirrors that were previously coated with aluminium only, are easily stripped within a few hours, using a mild solution of potassium hydroxide. . However, mirrors with protective overcoats or very old mirrors that were still silvered, can be very stubborn and need something more aggressive. In these cases Ferric Chloride works well, as will be seen. Ferric Chloride is Brown in colour and it attacks all metals. Since the large mirror had an aluminium ring attached to it, it was soaked, face down, in potassium hydroxide overnight. However, the coating would not budge, but at least the metal ring came undone, so it could be stripped using ferric chloried as can be seen in this time-lapse – it took about three hours to be completely clear. [music] OK, let's check the oil level .... ... it is fine. Let's start the mechanical pump ... warm it up. [Pump starting noises] It needs a bit of assistance ... but there its going. It is about 60 years old .... but still running fine. Since the stripping process started, the mirrors were not allowed to dry out. They are now given a thorough rinse of 20 minutes, to wash off all traces of the caustic of ferric chloried. An old garden sprinkler works very well for this. Next they are washed with a weak solution of sodium loral sulphate and given a light rubbing with pure cotton wool. [Water flowing sounds] This is followed by another 20 minute rinse with tap water. For the final rinse, de-ionised water is used liberally. [Noise of the pump in the background] 100% pure cotton wool is again used for drying. [Squeaking noises] Squeaking clean! [Squeaking noises] [Noise of the pump in the background] Next, we release the vacuum on the tank .... [Hissing sound] ... before we can open up. [Music] OK, now we start opening the roughing valve. You will see that when ..... the pump starts kicking into action ..... [Pump making a clacking noise] .... as we evacuate the air from the vessel. [Pump making a clacking noise] It takes about 20 minutes to open the valve fully. Now here's an amazing thing! You saw how I tightened these [bolts] with these massive spanners before .... So, the valve is'nt fully open yet and I can already move these bolts by hand. Because the air pressure is pushing so hard and sealing the tank further 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. [Slight change in pump noise] 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. [Screeching and pump noises] The 12 tungsten coils inside the tank are fired two at a time by changing connections at the back of the tank. Their glow can be seen in the small porthole at the back. Notice a double pulse each time as the aluminium melts and cools down the coils slightly. It looks even better with the lights dimmed. So we start by dialling up 8 Amps 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. GoPro footage looking through the porthole, with inset close-up video, taken with a cellphone. Not an easy thing to video! See how, when the aluminium hooks melt, they cool down the coils slightly. Tiny aluminium balls then form on the coil, which gradually evaporates. [Pump noises] 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. [Hissing sound] Now for the big reveal! [Crane and pump noises] NOT BAD! Looks pretty good! [Music] 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. To see how effective Ferric Chloride is on freshly aluminised mirrors ... We will do this in real time ... .... and .... .... I literally wipe away the aluminium. It it that quick. Let's try and put only a drop there .... It should literally eat a hole through there ... You can also see how directional aluminising is. See, this is where the bolts were ... ... and it literally makes a shadow .... ... as you can see there. [Background pump noises] We need about 60 of these hooks - pure aluminium. Using this nifty little machine .... Here's our wire .... That is 99.998% pure aluminium. [Machine noises] So, there we go. We need them to be clean. Any dirt will come off onto the mirrors. So we will just give them a bit of an alcohol bath. The inside of the tank is fitted with 12 of these pure tungsten coils. So next we put 5 of these onto each of the coils. [Music playing out]
