I have read about how the thermacell works. It gives off a small amount of smoke. I would be hesitant to use it in my observatory, but would not advise others either way. I think it might have the same effect as any volitale liguid that "out-gasses", but likely would not be a huge factor. Any that got onto a mirror over several observing sessions could likely be removed with ethanol, collodion, or acetone.