Our vacuum aluminizer at the Chevy Chase Community Center's amateur telescope making workshop has given me quite an education over the past few months or so.
We began finding that the pressures would not go down to the required levels for a good coating, last year -- but intermittently; sometimes it worked.
(How can we tell? Discovered that unless the pressure on the Varian Cold Cathode gauge goes below about 8 or 9X 10 ^ -5 Torr, then the coating is somewhat transparent - you can see lights in back of the mirror. We were only getting it to about 1.5 x 10^-4 torr, which one would be forgiven for thin\king are not too far apart, but it was too far for success. Plus, in the Strong Procedures in Expewrimental Physics and elsewhere tab les are given for mean free path at various vacuum pressures; at 1x10^-4 torr the MFP for nitrogen at 0C is about 60 cm, or 2 feet, and that's about the distance between our filament and the mirror surface. So the gauge and the book and the mirror coatings roughly agree. By the way, the scotch-tape test shows no pull=off of aluminum in either case)
?Tightened up hose clamps on the rubber hoses, then found that sometimes cuts the hose! So, doubled up on clamps and don't tighten them so tight - lesson imparted, lesson learned. Also replaced hoses made by me, earlier, from auto parts store with proper thick vacuum rated hoses; David H cleaned out the mechanical vacuum pump, replaced the oil, fixed a number of unsafe electrical features. Sam C added a bleeder valve from mechanical pump and added a senson, discovered that the two of the sensor gauges were contaminated and thus giving false readings. The sensor leading to the bell jar ?was full of aluminum flakes and the one leading to the back
of the diffusion pump ?was full of DP oil. Was told NOT o clean them out with compressed air because it would destroy the filament.?
Discovered bleeder valve itself was leaking, so soldered in two new ones in series for reliability in the line from the mechanical Sargent pump.
David H
removed a mechanical passthrough and examined it, began fixing it, and decided that since there was no need for it in the first place, it would be better to plug it with a plumbing part.?
John P discovered that the 'monnkeyshit' (technical nickname, I'm told, for Apiezon Q vacuum putty) that was keeping the home-made high-voltage passthroughs not leak, had failed. Located and ordered and received and installed new commercially made ones for about $90-120 each at Lesker, learned about 1/2 national pipe thread angles and threading facts, ordered enough O-rings (in lots of 100 each, from McMC) to keep us supplied for many
lifetimes. David H tried warming up the hard wax used to make the homemade ones that Jerry S and Bob B had made many years ago that had sprung leaks; if David's method works, and the passthroughs from Lesker fail, then we have possible backups, as well as some of that monkeys##t....
Friday night, John helped clean and re-assemble and tighten and pump down the system to test it, as I was helping other folks make telescopes. It's working fairly well, but it would only go down to 1.1 x 10^-4 torr, and it needs to be into the 10^-5 torr range in my experience.
I suspect we still need to outgass the entire system for quite a few hours until it's back working properly , but also that cleaning out the diffusion pump is going to be needed as well. Then rebuild the high-voltage ion plasma geneator, using our copious photographs and our carefully kept containers of parts as guidance.
Without the assistance and example of Bob B, Jerry S, David H, John P, and Sam
S, Mike F, Alan T, Bill R, and Bill B, and Jean-Paul R, and others, I could not have gotten as much done as we did. I'm sure I've left out a lot of steps and gotten stuff wrong, but I would like to thank all of those kind souls.?
?
Guy Brandenburg, Washington, DC?
http://gfbrandenburg.wordpress.com/
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
============================
