Timeline for When did "resilvering" large telescope mirrors actually refer to aluminization, and why was it necessary?
Current License: CC BY-SA 4.0
11 events
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| Apr 18, 2019 at 8:33 | history | edited | Florin Andrei | CC BY-SA 4.0 |
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| Dec 20, 2018 at 6:54 | comment | added | uhoh | I've added an answer, please feel free to comment, thanks! | |
| Jul 17, 2016 at 1:25 | comment | added | uhoh | I don't own a couch! Is there some nice way I can ask "how do you know this is true?" that you are comfortable with? Science is not just emphatic assertion. Can you show any scientific evidence to back up the assertion that the aluminum degrades? I'm trying to understand if the aluminum actually degrades and must be removed and replaced, and if so, why. But I need more than "because I say so! that's why!" type of support for the answer. | |
| Jul 17, 2016 at 1:03 | comment | added | Florin Andrei | One last time: oxygen is not the only tarnishing agent, and pure chemistry is not the only tarnishing process. A simple examination of dozens or hundreds of mirrors actually used out in the field for years should suffice to dispel this notion. Furthermore, reading a paper here and there from the comfort of your couch does not equate with doing science. Finally, and I feel this is the salient point here, you may want to examine whether it's the true desire for understanding that drives your argumentation, or it's something else. | |
| Jul 16, 2016 at 7:32 | comment | added | uhoh | However, the data is certainly consistent with the idea that the native oxide which quickly appears on a smooth, pure aluminum surface exposed to an oxygen containing environment will protect it from further reactions. | |
| Jul 16, 2016 at 7:20 | comment | added | uhoh | For example if you look at this paper they seem to report that both the control mirror (Figure 1. protected from the environment) and environmentally exposed mirror that was washed once a month (Figure 4.) remained at about 90% reflectivity for 32 months. But the mirror that was exposed but not washed (Figure 2.) dropped to about 80% over 32 months. There's no theory here, it's data. | |
| Jul 16, 2016 at 7:05 | comment | added | uhoh | Is there any data to back this up? Maybe a plot of reflectivity vs years, for an aluminum astronomical mirror, with some discussion of why its known to be degradation of the aluminum, and not just a build-up of dirt? You seem pretty sure, so there must be some scientific evidence that convinced you. | |
| Jul 16, 2016 at 5:38 | comment | added | Florin Andrei | Like I said, it's pretty complex. Your theory is wrong. There are definite cleaning procedures for telescope mirrors that don't involve re-coating. Regardless of the nature of the process, the fact remains that any reflective metallic layer working in conditions other than vacuum will, in time, become less and less reflective. There are many things in the air, not just oxygen, that will attack metals; e.g. mirrors used near oceans will tarnish more quickly compared to those used in deserts. There are also purely mechanical tarnishing processes from particulate matter hitting the mirror. | |
| Jul 15, 2016 at 21:40 | comment | added | uhoh | My theory is that the actual thing that needs to be done is simply cleaning, and the easiest way to clean such a delicate coating is to strip it and redeposit it. As for the gold, I am not sure it chemically reacts with the air. However if it builds up a thin layer of organics from the atmosphere, those can be strong absorbers at some IR wavelength, and if an organic layer is able to absorb moisture from the atmosphere, that might be an even stronger absorber. But I don't think the gold is actually chemically reacting with the air an actually tarnishing. | |
| Jul 15, 2016 at 21:37 | comment | added | uhoh | Thanks! We should call the amorphous $\text{Al}_2 \text{O}_3$ layer alumina. Sapphire is a crystal. As a rough analogy, we usually call amorphous $\text{SiO}_2$ silica, not quartz Why do you think aluminum actually continues to oxidize significantly after the native oxide layer forms - even years later? Did you read that somewhere - is it possible to track that down? Is there any data anywhere that actually shows that good quality aluminum coatings degrade by chemical reaction of the aluminum with something in the air? Are you sure the gold "tarnishes"? | |
| Jul 15, 2016 at 19:58 | history | answered | Florin Andrei | CC BY-SA 3.0 |