Re: [ATM] New home built 12" ultralight dob

[Mark Holm <mdholm@telerama.com>]

You really can't keep dust out while observing.. OK, maybe you can, but 
that is only a fraction of an amateur telescopes typical life.  The 
larger fraction is spent in storage, waiting for the owner to get free 
of job, family, sleep, etc. in order to do the things that really matter ;)

You can drill your structure full of holes, and still keep dust off of 
your mirror, during storage, with a close fitting mirror cover.  I mean 
something that sits right on the rim of the mirror, leaving little if 
any gap for dust to get through.  That close to the mirror, you have to 
worry a bit about chemicals outgassing from the cover material and 
depositing on the mirror. 
The only easily availble, easily worked, fairly light, non brittle 
material I can think of to fit the bill is aluminum.  To keep it light 
and relatively cheap, you could get the required stiffness by laminating 
the aluminum to some thin plywood with epoxy.  That way, you could use 
very thin aluminum, perhaps even aluminum foil!

For lightness, you could use foam core board instead of plywood.  
Laminate both sides to make it waterproof and more warp resistant.  For 
laminating aluminum foil, you may find that thinned white glue works as 
well as epoxy and is a lot cheaper and cleaner to handle.  Um, I have to 
add a caveat.  Water based glue probably won't work for aluminum foil on 
foam core.  The path to the atmosphere is too long for the glue away 
from the edge.  It won't dry.  3M Spray 77 would probably be just fine 
for this job too.  It has the advantage that you let the solvent 
evaporate before you put the layers together.  Just use a light coat on 
the foam core, because if the solvent penetrates the paper, it may well 
melt the foam.  If one layer of foam core isn't stiff enough, laminate 
two together to get 8 times the stiffness.

Reminds me, years ago, I was to play a Roman centurian in a Christmas 
pagent.  My Dad laminated aluminum foil to cardboard, then cut that into 
shapes that he fastened together to make the armor.  A lot of work for 
15 minutes worth of Christmas pagent, but I sure did look better than 
those shepherds wearing bath robes!

-- 
Mark Holm
mdholm@telerama.com


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