I've used the Thermacell several times. Outdoors, and in friends' observatories. The question was always asked about the vapors harming mirrors. It's effective for about a 15' radius-or was it diameter? Think about how small the amount must be in each "pad" of repellent, and how long it takes to be used up. The amount given off in any time period cannot be very great. In any case, just put it a few feet away from your scope. Whatever may be harmful would be quite diluted before reaching your optics, but still effective in the average observatory-only 10-12' in diameter.