开云体育

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


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