All Members

Hi everyone,

I have a frustrating issue which I would like to get thoughts on how to resolve.

I have an 8" f/7 parabolic glass mirror for my reflector telescope. The mirror is 36 years old. I recently had it stripped and re-coated with a fresh aluminium coating. Unfortunately there is a patchy aluminium oxide layer which wasn't able to be removed. The layer is about 60-100 nm thick which is enough to cause issues in the views I am getting through the eyepiece.

I am looking for a safe method to be able to remove the aluminium oxide layer so that a fresh uniform aluminium coating can be applied. I have been advised by the person who did the re-coat (a professional in the optical coating field) that he tried sodium hydroxide (have yet to ascertain what concentration), hydrochloric acid and nitric acid (separately) but was unsuccessful in removing the oxide layer.

An internet search has revealed suggestions such as citric acid or an aromatic solvent although I can't see how something like toluene or xylene would work. I myself had thought of aqua regia, a mixture of hydrochloric and nitric acid. These suggestions haven't been tried yet. Ferric chloride is used normally to strip the aluminium as is a mixture of hydrochloric acid and copper chloride or sulphate, can't remember which.

Before attempting to try to remove the oxide layer, I am trying to find a method which may have a reasonably high chance of success without damage to the optical surface. Hence my plea to the RACI community to see if there are any experts which may be able to help.

Please post any suggestions. I will be happy to discuss any solutions.

regards,

Werner