[ATM] glued aluminium mirror cell update

["Andreas Derwahl" <andreas@duda-derwahl.de>]

Hey everyone,

I recently introduced my ATM 10" travel dobson (URL: www.duda-derwahl.de), with one of the features being a glued mirror cell made from aluminium square tubing (www.duda-derwahl.de/mirror_cell.htm).

After a couple of observing nights with cool, but not too cold tempereatures (plus 3 - 5 degrees celsius) I had another failure of two of the glued connections and I came to the decision to not pursue the glueing any further. Though I cannot exclude mechanical stress as cause of the failure, my guess is that the glue I used (epoxy brandname "Uhu Endfest 300") is not flexible enough to acommodate for the thermal expansion of the aluminium. 

In my opinion it does not neccessarily follow, that glueing cannot work - as far as I know, glueing aluminium is sucessfully used in automobile and window making industry. But the way I did it, is obviously not suitable to form a longterm stable connection.

There's still room for improvement, though. Either using a more suitable glue (more flexible or with matching thermal expansion), increasing the overlap area of the aluminium parts, or reducing the gap between the glueing surfaces.

But since the mirror is relatively lightweight (2.4 Kg) and I have some scrap birch plywood at hand, I will make a new cell from wood that hopefully won't have these problems.

Just thought I'd let you know in case someone want's to go along this track...

Andreas Derwahl




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