[ATM] Aluminizing procedures

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

I spent a couple of amazing weeks in mid-August at the
Mount Wilson Observatory in the CUREA program. We also
visited Mt. Palomar, JPL (Jet Propulsion Laboratories)
and CalTech (California Institute of Technology). I
also talked to people from Kitt Peak; UCLA; University
of Arizona; Dartmouth College; Tokyo Institute of
Technology; Kenyon College; Edinboro University of Pa;
and Cambridge University in the UK. If you ever have
the opportunity to take the 2 weeks to join this
program, DO IT! The housing is rather spartan -- the
lodging is called 'The Monastery' -- but it does have
hot + cold running water, laundry facilities, showers,
toilets, and the occasional bear or rattlesnake to
make things lively. (In fact, one of the bears kept
raiding the kitchen and stealing our spices, mangoes,
and peanut butter.) It's not every day you get to see
and use both ancient (100-year-old) and brand-new
astronomical technologies (CCD's, infrared
interferometry) within a 24-hour period.

Among other things, I spent time talking to some of
the technicians at one or more of the above facilities
concerning how they go about re-aluminizing their
mirrors. These people, while extremely helpful and
informative, were most adamant that the information I
convey not be linked to any one particular observatory
and that I emphasize that this information is for
educational purposes only, and is not a practical
guide to be used by amateur telescope makers in their
own procedures. Also, they cautioned that some of
these procedures are potentially quite hazardous, and
require a complete knowledge and familiarization of
the process to be done safely.They also asked that
none of the nice photographs that I took of the
process be used on my website or anywhere else  : ( 

Those disclaimers being out of the way, here is the
way that at least **one** observatory in the Northern
Hemisphere performs the re-aluminization process:

 1. Take the mirror and rinse it with tap water to get
off the biggest dirt particles.
 
  2. Dissolve some sodium lauryl sulfate (Orvus paste
- available as sort of a horse shampoo at equine
supply places, or in small quantities for quiltmakers
and needleworkers to wash the things they work on) and
clean water; immerse mirror in the solution; use BARE
fingertips to go over the entire surface, using
fingernails to pick off anything that won't come off
with finger pressure and that sticks.The bare fingers
allow you to feel every little tiny piece of dirt.
 
  3. Rinse with more clean filtered tap water, or
perhaps distilled water if desired or needed.
 
  4. Use hydrochloric acid-cupric sulfate solution to
remove the old aluminium coating (4 liters of
distilled water; 4 liters of reagent grade [1 Normal]
hydrochloric acid; 100 grams of cupric sulfate
crystal, reagent grade). The cupric sulfate speeds up
the reaction. The solution should be left on for
double the amount of time that it takes to remove all
of the visible aluminium, which may take anywhere from
5 minutes to half an hour, depending on the condition
of the old coating and whether it was overcoated or
not.
  
  5. Rinse off with lots of distilled water.
 
  6. Mix up a slurry of Calcium carbonate powder and
water. Use cotton pads of the purest type to scrub the
mirror very carefully + thoroughly.
 
  7. Rinse off with lots of distilled water, using
clean new pads to make sure all of the residual
calcium carbonate is removed.
 
 8. Dry off with a spray of dry nitrogen gas from a
compressed-nitrogen tank.
 
  9. Put mirror in chamber; close chamber.
 
  10. Previously, small wires of 99.999% pure aluminum
have been crimped to the tungsten filaments in the
chamber; some of the filaments should not be able to
'see' the mirror and will be used as 'getters'. The
rest of the filaments should have a straight line of
sight between them and the mirror.
 
  11. While pumping down the chamber, re-insert some
nitrogen gas; perform high-voltage ion bombardment at
about 50 microns of pressure.
 
  12. Repeat step 11 with oxygen gas. 
 
  13. Do the nitrogen step (#11) one more time.
 
  15. Quickly but smoothly, bring up the low-voltage
current on the getter filaments until the aluminum is
vaporized, combining with what little free oxygen
(etc) remains.
 
  16. Make sure that the pressure is below 1 x 10^-5
Torr. (I mean, somewhere in the 10^-6 Torr range. If
you can get into the 10^-7 Torr range, then
wonderful!)
 
  17. Quickly and smoothly bring up the low-voltage
current on the rest of the tungsten filaments until
the aluminum is vaporized. The faster that you can
deposit the aluminum, the more reflective the coating
will be.
 
  18. Make sure everything went properly.
 
  19. Return pressure in the chamber to ambient
atmospheric pressure. Remove the mirror, but resist
the urge to blow on it or to touch it with your
fingers. You are done.
 
Guy Brandenburg 


=====
Guy  Brandenburg
Washington, DC
My home page:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
_______________________________________________
ATM mailing list http://www.atmlist.net/