1 00:00:05,000 --> 00:00:11,000 Every night, all year round, the ESO Very Large Telescope, or VLT, 2 00:00:11,000 --> 00:00:16,000 opens its four giant eyes to scrutinise the beautiful southern skies. 3 00:00:16,000 --> 00:00:23,000 Each eye is a huge mirror, 8.2 metres in diameter that gathers the light of the night sky, 4 00:00:23,000 --> 00:00:29,000 and reflects it into optical systems that form ultra-sharp images of the Universe. 5 00:00:29,000 --> 00:00:35,000 But keeping the VLT´s eyes clear requires each mirror to be cleaned and recoated occasionally, 6 00:00:35,000 --> 00:00:38,000 a delicate and complex procedure. 7 00:00:42,000 --> 00:00:44,000 This is the ESOcast! 8 00:00:44,000 --> 00:00:49,000 Cutting-edge science and life behind the scenes of ESO, the European Southern Observatory. 9 00:00:49,000 --> 00:00:55,000 Exploring the ultimate frontier with our host Dr J, a.k.a. Dr Joe Liske. 10 00:00:58,000 --> 00:01:00,000 Hello and welcome to the ESOcast. 11 00:01:00,000 --> 00:01:05,000 In this episode, we’re going to follow the recoating of one of the VLT’s primary mirrors. 12 00:01:05,000 --> 00:01:10,000 Although this happens quite regularly, this is far from being a routine procedure. 13 00:01:10,000 --> 00:01:14,000 Under the open sky, dust collects on the surface of the mirror. 14 00:01:14,000 --> 00:01:17,000 And although this layer of dust is very thin, 15 00:01:17,000 --> 00:01:20,000 it nevertheless degrades the quality of the astronomical observations. 16 00:01:20,000 --> 00:01:23,000 And so the mirror must be cleaned regularly. 17 00:01:23,000 --> 00:01:26,000 In fact, it must not only be cleaned, it must be recoated, 18 00:01:26,000 --> 00:01:31,000 meaning that its thin layer of reflective aluminium must be replaced. 19 00:01:31,000 --> 00:01:35,000 The ESO technicians and engineers must work both quickly and carefully 20 00:01:35,000 --> 00:01:39,000 to complete this operation on the fragile 17‑cm‑thick mirror 21 00:01:39,000 --> 00:01:42,000 in order to limit the telescope’s downtime. 22 00:01:45,000 --> 00:01:50,000 First, the telescope must be prepared so that the mirror and its cell can be taken out. 23 00:01:50,000 --> 00:01:54,000 The team is removing an instrument attached to the mirror cell. 24 00:01:57,000 --> 00:02:01,000 Outside, a custom-built carriage is moved into position. 25 00:02:01,000 --> 00:02:05,000 This carriage will support the mirror and the cell, cushioning the load during transport. 26 00:02:06,000 --> 00:02:09,000 A dedicated platform lifts the carriage to the mirror deck. 27 00:02:09,000 --> 00:02:13,000 When it reaches the deck, the carriage slides into the building 28 00:02:13,000 --> 00:02:18,000 on a stream of pressurised air, ending up just below the mirror and the cell. 29 00:02:18,000 --> 00:02:21,000 The carriage is raised to receive the cell and mirror. 30 00:02:23,000 --> 00:02:27,000 Once the cell is safely on the carriage it can be disconnected from the rest of the telescope. 31 00:02:27,000 --> 00:02:31,000 All the bolts are released and the data and control cables are removed. 32 00:02:31,000 --> 00:02:35,000 Now this is always a pretty tense moment and the team is working very carefully 33 00:02:35,000 --> 00:02:38,000 to make sure that they don’t damage the fragile mirror. 34 00:02:38,000 --> 00:02:43,000 After all, there is no spare at ESO, and making a new one would take years. 35 00:02:45,000 --> 00:02:48,000 The surface of the mirror is extremely vulnerable. 36 00:02:48,000 --> 00:02:51,000 It must be covered before it is taken out of the building 37 00:02:51,000 --> 00:02:59,000 The cover not only protects the mirror but also prevents it from concentrating sunlight and burning anything in its path. 38 00:03:08,000 --> 00:03:12,000 Before the mirror can be covered completely, a smaller mirror must be dismounted. 39 00:03:12,000 --> 00:03:17,000 This mirror sends the light gathered by the main mirror to different instruments at the telescope. 40 00:03:17,000 --> 00:03:22,000 During the coming days, the optical engineers will inspect and clean this mirror as well. 41 00:03:24,000 --> 00:03:28,000 Eventually, the larger mirror is completely covered. 42 00:03:29,000 --> 00:03:33,000 The carriage slowly slides out of the telescope building, 43 00:03:33,000 --> 00:03:38,000 supporting the full weight of the mirror and the cell, a substantial 50 tons. 44 00:03:38,000 --> 00:03:43,000 Once again, the lifting platform is used, carefully lowering the carriage to ground level. 45 00:03:46,000 --> 00:03:52,000 A truck brings in the hydraulic trailer. The trailer is carefully positioned to take the precious load. 46 00:03:56,000 --> 00:04:05,000 The load is secured with chains to prevent it from sliding off the trailer in case of an earthquake or a sudden stop. 47 00:04:11,000 --> 00:04:14,000 The team is now ready to start the trip down the mountain to the recoating plant. 48 00:04:14,000 --> 00:04:18,000 This journey is a crucial stage in the whole process. 49 00:04:18,000 --> 00:04:21,000 Before the mirror is brought out, the weather must be checked thoroughly: 50 00:04:21,000 --> 00:04:24,000 high winds must be avoided at all costs during the transit 51 00:04:24,000 --> 00:04:28,000 because any flapping of the lightweight mirror cover might scratch the mirror. 52 00:04:28,000 --> 00:04:35,000 Furthermore, the truck driver is supposed to never apply the brakes, because an abrupt halt might be dangerous for the mirror. 53 00:04:35,000 --> 00:04:39,000 And so the truck moves along at a leisurely 5 km/h. 54 00:04:39,000 --> 00:04:44,000 But even at this walking pace, it eventually arrives safely at the mirror maintenance building. 55 00:04:52,000 --> 00:04:58,000 The first task at the mirror maintenance building is to remove the mirror from the carriage and its cell. 56 00:04:58,000 --> 00:05:01,000 The carriage is positioned under a custom-made handling tool. 57 00:05:02,000 --> 00:05:08,000 Brackets connect the mirror and the cell. Engineers have to release each one manually. 58 00:05:08,000 --> 00:05:14,000 This may sound like a routine job, but, with the sensitive mirror surface only centimetres away, 59 00:05:14,000 --> 00:05:18,000 it requires the team’s undivided attention. 60 00:05:18,000 --> 00:05:24,000 Forgetting to take off even one bracket could damage the mirror severely during the lifting operation. 61 00:05:24,000 --> 00:05:30,000 The team is only ready to lift the mirror off its cell after a triple check of every bracket. 62 00:05:32,000 --> 00:05:39,000 Hooks are lowered to bear against the back of the mirror and the handling tool lifts the mirror up and off the cell. 63 00:05:40,000 --> 00:05:42,000 Now the cell can be removed. 64 00:05:51,000 --> 00:05:57,000 Subsequently, the mirror is lowered again to allow some work to be carried out on its back surface. 65 00:05:59,000 --> 00:06:07,000 Before the optical engineers start work on recoating the optical side of the mirror they must clean its back surface by hand. 66 00:06:07,000 --> 00:06:15,000 This cleaning is essential as it removes oils, particles and other contaminants that could damage the mirror in the vacuum chamber. 67 00:06:19,000 --> 00:06:26,000 Afterwards the bottom half of the vacuum chamber is brought in and the mirror is carefully lowered into it. 68 00:06:29,000 --> 00:06:32,000 The mirror is next taken into a cleanroom. 69 00:06:41,000 --> 00:06:46,000 The first action in the cleanroom is to treat any badly polluted areas on the mirror manually. 70 00:06:46,000 --> 00:06:51,000 This requires a steady and skillful hand in order to avoid scratching the surface. 71 00:06:51,000 --> 00:06:57,000 The point of doing this is to remove any particles that might not be easily flushed away during the subsequent washing. 72 00:06:57,000 --> 00:07:02,000 Afterwards, the vessel holding the mirror is positioned in the washing chamber 73 00:07:02,000 --> 00:07:06,000 where the old reflective coating of the mirror is washed off. 74 00:07:09,000 --> 00:07:17,000 The washing process begins and a giant, purpose-built sprayer arm rotates just above the surface of the mirror. 75 00:07:17,000 --> 00:07:20,000 Ultra-pure water and chemicals are used. 76 00:07:20,000 --> 00:07:27,000 Under the critical eye of the optical engineers the machinery does a flawless job and the operation proceeds smoothly. 77 00:07:28,000 --> 00:07:33,000 In the next stage, acid is used to remove the old aluminium coating. 78 00:07:33,000 --> 00:07:36,000 The real amber colour of the dish appears. 79 00:07:36,000 --> 00:07:42,000 The mirror is made of special ceramics that don’t deform under temperature changes. 80 00:07:43,000 --> 00:07:48,000 Again, the mirror is rinsed intensively to provide as clean a surface as possible. 81 00:07:48,000 --> 00:07:54,000 Once done, all the liquid is pumped out and the mirror is ready for the vacuum chamber. 82 00:07:57,000 --> 00:08:01,000 The vacuum chamber lies next to the washing unit. 83 00:08:05,000 --> 00:08:11,000 The mirror is placed in the chamber immediately after cleaning to avoid contamination with new pollutants. 84 00:08:13,000 --> 00:08:17,000 The lower part of the vessel is lifted up and the chamber is closed. 85 00:08:17,000 --> 00:08:24,000 The air can now be pumped out to establish a nearly perfect vacuum, necessary for recoating. 86 00:08:25,000 --> 00:08:29,000 Once the vacuum is established, the recoating can begin. 87 00:08:29,000 --> 00:08:36,000 The optical engineers use an electrically stimulated plasma, for a molecular cleaning of the optical surface of the rotating mirror. 88 00:08:36,000 --> 00:08:43,000 Then, individual particles of aluminium are deposited on the mirror in a process known as sputtering deposition. 89 00:08:43,000 --> 00:08:45,000 This takes about half an hour. 90 00:08:47,000 --> 00:08:53,000 Only 12 grams of aluminium are used to create the new coating for the roughly 50 square metres of the primary mirror, 91 00:08:53,000 --> 00:08:56,000 and so the coating is only about 80 nanometres thick. 92 00:09:00,000 --> 00:09:02,000 It’s really not a lot is it? 93 00:09:03,000 --> 00:09:09,000 Finally, the moment of truth has come: Will the quality of the new coating meet the high standards of the VLT? 94 00:09:11,000 --> 00:09:14,000 Recoating a mirror is always a tricky business: 95 00:09:14,000 --> 00:09:20,000 if the mirror isn’t thoroughly clean or if the vacuum chamber is contaminated then the coating will be flawed. 96 00:09:20,000 --> 00:09:24,000 Tension is high as the chamber opens and the mirror is brought out. 97 00:09:25,000 --> 00:09:29,000 A first visual inspection suggests that the recoating has succeeded. 98 00:09:29,000 --> 00:09:37,000 But what really matters is how reflective the mirror is, so the percentage of light the mirror reflects and scatters is measured. 99 00:09:39,000 --> 00:09:45,000 The result confirms the high reflectivity of the mirror and the experts are fully satisfied. 100 00:09:46,000 --> 00:09:50,000 The recoating was successful and the mirror is all shiny and new again. 101 00:09:50,000 --> 00:09:55,000 Our team is now ready to take the mirror back up the mountain and reinstall it at the telescope. 102 00:09:55,000 --> 00:09:59,000 The VLT will be back in full operation exactly on schedule 103 00:09:59,000 --> 00:10:02,000 to provide astronomers with a superb view of our Universe once more. 104 00:10:03,000 --> 00:10:05,000 This is Dr J signing off for the ESOcast. 105 00:10:05,000 --> 00:10:09,000 Join me again next time for another cosmic adventure. 106 00:10:09,000 --> 00:10:12,000 The Recoating Team 107 00:10:17,000 --> 00:10:20,000 ESOcast is produced by ESO, the European Southern Observatory. www.eso.org 108 00:10:20,000 --> 00:10:23,000 ESO, the European Southern Observatory, is the pre-eminent intergovernmental science and technology organisation in astronomy 109 00:10:23,000 --> 00:10:26,000 designing, constructing and operating the world's most advanced ground-based telescopes. 110 00:10:27,000 --> 00:10:30,000 Transcription by ESO ; translation by —