ATM Aluminum Mirror

[Jack Schmidling <arf@mc.net>]

Well, they said it couldn't be done.  And.... they were right!

My 4.25" cast aluminum mirror project has been put aside for greater
minds to work on.  All of the predicted problems and then some,
did indeed rear their ugly heads.

I started with an aluminum blank and the pre-curved tool from the glass
one just completed.  Many hours of rough grinding failed to produce the
desired curve in the mirror and removed glass from the tool at a faster
rate than from the aluminum mirror.

Not wanting to see the pyrex blank vanish, I cast another aluminum 
blank and used this for the tool.  I used a belt sander, lathe and
grinder to bring it to shape but the free abrasive grinding was
pitifully slow.  I finally got something close to overall contact at
the right sagitta and went on to fine grinding.

What surprised me the most is the way the ground surface reacted to the
tumbling operation.  My intent was to tumble the mirror between each
grit to peen the surface into submission and hopefully reduce the
air pits.  We tumble all aluminum parts for our MALTMILL with stainless
sheet metal screws, soap and vinegar and they come out sparkling bright
as did the rough cast surfaces of the mirror but the ground surface
just stayed a muddy grey and showed no signs of the tumbling action.

Next problem I ran into was when I got to 5 micron prepolish.  As soon
as the surplus was worked out, it locked up like glue.  I could not take
a single stroke of any length.  It was hopeless and I quit.

After pressing on the old pitch lap left from the glass version,
polishing felt like rubbing it on the driveway.  The only way it 
felt right was if I left the faceting netting between the pitch and
mirror after pressing.  After several hours, all I achieved was a dull
grey sheen and nothing resembling a polish.

I quit!  So much for fun at Astrofest.  Guess I will just annoy 
folks by bringing along an EASYTESTER.

js

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