Small Telescopes Aluminizing

Small Telescopes Aluminizing Procedures


(chamber in the 1.5m dome)

This aluminizing chamber accepts mirror up to 1.5m in diameter and is used for all CTIO mirrors but the 4m M1.

Mirror washing: see 4.0-m procedure.

Picture of the 0.9m tel M1 mirror after sitting 2 years in the tube without any cleaning and a picture of the washing (aluminium removal with acid).

Note: units used are : 1.3x10-6 atm = 1 micron (Hg) = 1 millitorr (=0.13 Pa)

Vacuum sensors and controllers: on the main panel, there is an analog controller NRC725 (Norton) for the Varian 524-2 cold cathode (1 input, reading from 10-3 to 10-7 torrs), and for Varian 531 thermocouples (2 inputs: not used not working anymore!). On top of the panel, there is an extra analog controller NRC721 fot 531 thermocouples (2 inputs: one for the diffusion pump and one for the tank, both reading from atm to 5 microns). There is also a thermocouple in the roughing line, which can be used to check good funcionning of the pump (turn off the meter, unplug the diffuser sensor and plug into the roughing line). Calibration of that dual thermocouple meter must be checked: when rough vacuum is reached in the roughing line (close diffuser and tank valves and plug the sensor in the roughing line), press the 'fil MA' button and turn the 'fil adj' knob until the needle reads 200 on the top red scale.

We indicate the elapsed time for the aluminizing (rather slow in this example, the entire process could last as little as 3h00).



A.1. Put a roll of paper on the bottom of the chamber to step inside without leaving dirt.

A.2. Replace all the damaged filaments (there are 24 of them on a single diameter): they must be straight when loose (i.e. unstressed), not covered with aluminium (drops). See picture of this operation inside the chamber. Press gently the filaments with the fingers : if they are stressed, they break immediately. Filaments can usually be used for 2-3 aluminizings. Handling of the filaments must be done with gloves to avoid their contamination with grease.

A.3. Prepare 96 aluminium clips and wash them in acetone. Hang 4 clips per filament : one on each loop. Again use gloves to hung the clips.

A.4. Check thoroughly cleanliness of chamber, wash walls with acetone and all the surfaces touched by hands or tools, check the chamber Orings (clean and lubricate)

A.5. Use mouth mask from the moment the mirror is dried out until the chamber is closed. Wash with acetone the inside of the mirror holding ring and attach to the mirror. Bring the mirror into the aluminizing tank (picture). Wash with acetone the outside of the mirror holding ring and any spots touched by tools.

A.6. Install the thickness monitor: it must face the center of the filament array and be at the same distance from the filaments as the mirror (otherwise you need to enter a 'Tooling' coefficient in the thickness monitor meter. Check that it is working (see instructions)

A.7. Blow off laterally dust with dry nitrogen. This is a critical step to ensure that the mirror is absolutely free of dust. Close tank and seal it by tightening the 4 bolts. Add at least 2 C-clamps near the top (original bolts are missing).

B. ALUMINIZING (see picture of control panel for location of all knobs)

B.1. 0 min: Connect black and orange power cables (filaments and glow discharge) on the side of the main control panel. Connect 2nd black cable from glow discharge controler to valve at the rear of the tank (on the air inlet tube). Connect main thick black power cable to wall plug.

B.2. Take the extremity of the black flexible exhaust water pipe outside the building.

B.3. Open the water circuit valve (green pipe), the valve to the roughing pump and to the thickness monitor.

B.4. Close the copper tube valve (water to diffuser) on the right side of the control panel

B.5. See all following elements described on picture 1. Connect the tank vacuum gauge. Close both valves (diffuser and tank): the handle of the tank (top valve) should be on the left side and the handle of the diffuser (bottom valve) should be on the right side.

B.6. Picture 2. Turn on the air compressor: it must show 100 lbs to be able to open/close the diffuser valve. Make sure the voltage wheels for glow discharge and filaments are at 0 and their respective controllers off.

B.7 Turn on the roughing pump, plug a thermocouple into the line and check the meter calibration (as described above) once good vacuum is reached (5 um). Reconnect the sensor to the diffuser and open the diffuser valve.

B.8. When diffuser gets to 5 microns, close diffuser valve and open tank valve. Some smoke is generated by the compressor for a few minutes: close the doors and turn on the room air extractor.

B.9. 25 min: When tank vacuum reads 200 microns, turn on the power knob on the control panel to start the glow discharge. Turn control knob to raise current progressively up to 500mA (about 700V) (picture) and check the ionization color in the tank (watch out for arcs). The vacuum should drop around 30 microns. Maintain glow discharge for 20min. Decrease slowly to zero the voltage, switch off the controller and turn off the glow discharge power knob on the main control panel.

B.10. 1h35min: When tank is at 5 microns, close the 2 valves (diffuser and tank) and open the poppet valve (diffuser-to-tank valve) by switching on the appropriate knob (this makes a loud noise, don't panic!). Turn on the diffuser, the refrigeration and open the copper tube water inlet. Turn off the vacuum gauge and turn on the cold cathode on the first scale (10-3 to 10-5 torr).

B.11. Open the diffuser valve (so that the roughing pump starts pumping the tank through the diffuser).

B.12. 3h50min: the tank reaches 4x10-5. Check the thickness monitor is ready to use. Turn on the filaments power on the main panel (left 'low voltage' knob) and on the filament controller.

B.13. Always reading the values of the current on the wheel -not on the small meter- (picture), raise slowly to 35A and check for 2-3 min until the filaments color is uniform.

B.14. Raise to 45A for 1 min, then to 55A for 2 min: you should see the aluminium melt on the filaments (the filaments darken, then redden again). The aluminizing process is initiated and you should see some activity on the thickness monitor display.

B.15. Raise to 65A for 30sec: all clips should be melted and the filaments of a uniform color with a deposition rate around 15A/sec. Raise to 80A for 30 sec then 90A until thickness reaches 890A (maximum rate should be around 17A/sec). Decrease the power down to 0 in 10 sec. Switch off the power on the controller and on the main panel. Real aluminizing time should be around 1min30sec and final thickness should be around 950A.

B.16. Close the poppet valve, and turn off the compressor. Switch off the diffusion pump. Turn off the cold cathode and disconnect it.

B.17. Open a little the air inlet valve to the tank (with a piece of cloth on the pipe entrance). When diffuser cooling water exhaust is cool again, close the diffuser valve, turn off the roughing pump, the refrigeration and close the main water valve on the wall (leave the copper tube opened).

B.18. It will take about 20 minutes for the tank to be at atmospheric pressure so that you can open it (listen when air leaking in stops).

B.19. Inspect carefully the coating and look for water marks with intense light shining on the coating. Check visually that there is no transmnission through the coating (ie. thickness is adequate) nor tiny 'dust' holes in it by shining a bright light from behind the mirror. Do the adhesive tape test on the edge of the mirror to check the quality(adhesion) of the coating. Measure the reflected and scattered light in 3 different places to compare with data before aluminizing.

Total: 4h30min.


Max Boccas, last revision on February 1st, 2001