James, Sounds like an exciting day! Don ** ----- Original Message -----
From: "arcstarter" <arcstarter@...>
Thanks Don, for your reply regarding the oxygen plasma.
This list sure has been quiet lately... ON that note, let me recount my latest "WTF?" moment :)
I've just sealed the chamber on an 8" optic I'm coating, pump down went as
usual. Did the glow discharge thing for 10 minutes or so (as usual). Opened
the diff pump valve, and the system dropped to 2e-5 torr on schedule (by the
time I manually spin the valve open, chamber is ready).
I do a quick cross check, everything is in order and its time to evaporate some
aluminum. I start slowly applying current to the filaments, I hear the step
down transformer starting to hum, I see the filaments start to glow dull red, I
wait for the magic moment when the aluminum goes from solid to liquid. In quick
succession each of the 6 filaments "blink to dark" for a moment as the aluminum
hits the transition temperature between solid and liquid. I now bring the
filament current up so each filament is glowing a bright orange/yellow color,
and watch as the "magic" happens.
Now the first off-nominal event rears its ugly head, as I'm watching the chamber
I hear two distinct "Clicks" from electrical relays changing position. My eyes
instinctively track to the position of the two clicks, and I see that the
Penning Ion Gauge is reading "Off Scale High", and the Thermocouple gauge is
indicating about a 1/2 torr chamber pressure! "UH OHHHH".
Now, in what feels like slow motion, I kill the filament power with one hand,
while the other starts spinning the diffusion pump to chamber valve handle as
fast as possible. I'm also thinking that perhaps this is why I see newer
diffusion pump valves are the 1/4 turn butterfly style, instead of the screw
type gate valve I have that takes "a gazillion" turns to close in an emergency!
:0
In what feels like an eternity, I have the diffusion pump valve about half
closed, and WOW look at that, the chamber pressure is back to normal
(2e-5torr)... I think to myself that's rather weird, and keep on spinning the
diffusion pump valve towards to the closed position...
Finally I've got the valve closed, and the chamber vented, lets have a look an
see what we see inside the chamber.
#1 Everything in "line of sight" of the filaments is the blackest black I've
ever seen!
#2 My first thought is "wow, this would be a great flat black for spider vanes /
focuser tubes"
#3 I drag my finger across some to see if it will wipe off, nope, seems pretty
well stuck in place.
#4 finally I think "what happened, what was different this time?"
Now, follow me on a mental flash back of early that day while I was clearing a
path to the alluminizer :)
I was moving some sheet metal out of the way when I slipped and cut my thumb.
Now, like most cuts, you stop what your doing and have a look, wondering "How
bad is it?" OK, its bad enough to need a bandage... (grumble)
So, as I scrounge around in the medicine cabinet for a band-aid, the Wifely
person comes along to see what I'm doing, and so it begins..
(Note WP = Wifely person)
WP: "What are you doing"
Me: "lookin for a band-aid"
WP: "Are you OK?"
ME: "Yea"
WP: "Let me see"
ME: "I'm OK, don't worry about it"
WP: "LET ME SEE IT!"
ME: Sigh, "OK"
WP: "You NEED stitches"
ME: Sigh, "No, I'm OK I need a band-aid"
WP: "You NEED a Tetanus shot"
ME: Sigh, "Nah, I had one a 'while' ago"
WP: "Well, at least let me put some Neosporin on it"
ME: "OK, sure, but I got to get back to work.."
After having my thumb properly treated and bandaged, I go back to work, where it
appears I promptly cover the tungsten filaments in an ever so thin coating of
Neosporin Ointment (Evil Stuff!). This petroleum product seems immune to plasma
cleaning, and is rather content to hang around in a rather good vacuum until
slightly heated.
Once this stuff is inside a vacuum chamber, forget about normal cleaning
methods! To get rid of the last traces (after six or eight failed coating
attempts!) I had to resort to Sandpaper, two cans of Automotive Brake Cleaner, A
heat gun, and evacuating the chamber empty while running naked tungsten
filaments at their highest current setting and repeating several tries.
Finally, at about 3am, I finally get that 8 inch mirror coated!
Have any of the amateur telescope metallizers ever attempted to
aluminize any mylar or similar plastics? I coated some plastic flash light reflectors once, does that count? :)
Take Care,
James
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