keegreil

Hi all, thanks for the ideas! Sorry for the late reply, I've been in the middle of moving... Enthalpy, I do not doubt at all that you needed a seriously good vacuum for semiconductor work. For a real scientific grade telescope mirror a super clean vacuum like 10-9 torr may be desired. I was going for something much less impressive, I'm really just interested in testing the boundaries of what is possible for an amateur. Likewise, electron beam evaporation of the target Al is definitely the ideal method to eliminate contamination, but for an amateur attempt I think a simple tungsten filament or other crucible is easier. Interesting point about the substrate being heated, I haven't heard of that being done for telescope mirrors but I may look into it. I agree oil diffusion pumps are simple in principle, I had read however (can't find my source now) that the oil for them is pretty specialized and can be rather expensive (i.e., not motor oil or olive oil, haha). That's why I was hoping to avoid those pumps. Lastly, gold is very reflective, but not as much as Al in the visible range. I understand it is the ideal material for Infrared mirrors (such as those used in CO2 lasers). Using a Mg getter is a great idea, thanks! I have a question that kind of goes back to my first post though. If the aluminum will not react with Nitrogen and remove it from the chamber, why exactly does the Nitrogen need to be removed at all? Is it that the aluminum would cool off when bumping into N2 before it reached the glass? I also really like the idea of a roll of Aluminum foil as a getter, I'm all for using household materials! Can you help me flesh out how exactly that process would work? I'm picturing a roll of foil lying next to a inductive heater coil, being stretched across the heater and rolled up by a motor on the other side. Is it alright to just be heating a small portion of the foil at a time? Or to work properly does the whole thing need to be heated at once? My guess is if done right, the foil would be rolled up and protected before cooling off, preventing the recently released molecules from reattaching right away. After the whole roll has been heated and re-rolled and the mechanical pump did it's work, slowly unroll it again while cold and wait for the vacuum to drop.

Thanks for the ideas everyone, I may look into actually trying to make a vacuum. Might be a fun project, if difficult. Does anyone have experience with creating vacuums? Specifically with evap getters? My latest idea is this. Get a $100 Harbor Freight pump to take the chamber down to about 10^-3 Torr. Place the mirror in the top of the chamber facing down, covered by a remote controlled shutter. Place the aluminum in a small crucible, and gradually evaporate it. At first, the Al atoms will react with everything in the chamber, hopefully turning them into solid products and condensing in ugly black powder all over the walls. Basically, we are using it as a "getter" to take the vacuum down to about 10^-6 Torr (ish?). After a while, the Al will have "gotten" all of the gas in the chamber and will start depositing as a clean shiny layer. When that happens, remotely open the shutter, uncovering the mirror and coating it perfectly (in theory ). My theory is that by making an Al evap getter of sorts, we could get around having to use other more exotic pumps like oil diffusion or turbo-molecular pumps. Another advantage is that we don't need to figure out a way to measure when the vacuum is good enough to deposit Al (though that admittedly should be easier than making a pump). When the Al starts making shiny layers, then the vacuum is good enough. No fancy vacuum gauges required? I know this idea is probably ridiculous too, but thanks for entertaining my curiosity!

Wow, I see. I never realized aluminum would react with water. Guess that answers my question then, you really do need a vacuum. Just out of curiosity, would doing this with tin work? Or is tin to reactive as well? Thanks!

Hi all, I'm new here. My professional background is in computers and airplanes, but about a year ago I started getting really curious about how telescope mirrors were made. I know that typically they are coated with aluminum in a very good vacuum. What follows is a dreamed up alternative process that is almost certainly less effective (or totally useless?), but since this is far out of my profession I need some help coming up with some reasonable hypotheses for what might happen if this were tried (not just what I wish would happen). Right now I live in a hotel room for work, but perhaps later this year I will have a garage again and may try it myself. Most hobbyists, when finishing a telescope mirror, send the glass out to get professionally coated. Most hobbyists (not all!) cannot build a good enough vacuum chamber to successfully vapor deposit aluminum themselves. My understanding is that the reasons a high vacuum is required are twofold. 1) In a low vacuum, even if successfully vaporized, the aluminum will bump into other air molecules and cool off to a solid along its transit, and so would deposit as a (black?) powder. 2) In the presence of oxygen, aluminum quickly forms aluminum oxide, which deposits as a white powder. A vacuum solves both of these problems. Are these assumptions correct and are there other reasons a vacuum is required? Here is my hypothetical experiment. 1. Place a small piece of aluminum inside an empty jar, preferably made of Pyrex 2. Add a small amount of water to the jar (enough to cover the aluminum?) 3. Cover the jar with a flat piece of glass to be coated 4. Place a second jar full of water on top of the flat glass 5. Put the whole stack on a heat source, VERY slowly bringing it up enough to melt the aluminum Then wait (hours? days?). The water in the first jar will have all boiled, hopefully forcing all of the air (especially O2) out the top and filling the space with superheated steam, at the melting point of aluminum (660 C I think). The water in the second, top jar, will be boiling and, by evaporative cooling, self regulating its own temperature to 100 C. The glass plate on which it rests should be slightly above 100 C. In my ideal imaginary world, the aluminum would (albeit slowly) vaporize and be carried by the superheated steam to the cool plate, where it would instantly deposit itself in a nice shiny coating. By filling the chamber with 700 C steam, in theory we have displaced all of the oxygen, and prevented the aluminum from cooling to a solid prematurely. This would allow simple hobbyists like myself to perform aluminum vapor deposition without the need for a high vacuum system. So here are my questions. While I feel like this shouldn't work (it would be too easy), why exactly wouldn't it? 1. Would the aluminum still cool off because of a boundary layer of cooler steam on the plate? Is it ok if the aluminum atoms become supercooled before freezing onto the glass plate? 2. Is the vapor pressure of liquid aluminum just too low to get any useful deposition rates? Perhaps trying this with tin first would be a better idea, less extreme temperatures required Thanks in advance for any input or other wild ideas, cheers! -- Keegan PS: Attached is an embarrassingly crude drawing of the idea, if that helps picture it.