My favorite observing accessory

White duct tape. 

Okay, maybe not my favorite, but up there in the Top 10.

Why? Because it helps you find stuff at night. Not up there, down here. We worry about finding stuff in the sky, but when you drop something on the ground or walk into something in the dark, you’ll appreciate that being able to see stuff on Earth is almost as important as seeing stuff in the sky.

Few sites are so dark you won't see a piece of white duct tape more easily than something darker colored. I always keep a roll in the car.

Where to use it:

• On any equipment you don't want people bumping into in the dark, especially at public events with people unfamiliar with the size and shape of astro gear.
• Lens caps. Many lens caps are black. Drop one in the grass and you will need to turn a light on, not something you want to do unless you have to when you are observing. All my eyepiece caps and telescope covers have little squares of white duct tape on both sides so I can find them easily in the dark. Kudos to those companies that make the clear caps, but even they can benefit from a piece of white duct tape.
• Marking indicators. I have this thing about always forgetting to turn off red-dot finders. Always. So I put a couple of small pieces of white duct tape on the knob that turns it off. When they line up, it's off. Where to put the scope in the dovetail saddle? Mark it with white duct tape. Where does the telescope cap line up? Where do I grab something at night? White duct tape.
• At one site, we have a wooden fence gate that can be in various stages of open. I will invariably walk into it in the dark. Slap a piece of white duct tape on it.
• At another site, there was a big gopher hole. I stepped in it. You can be seriously injured by doing something like that. A stick with a piece of white duct tape on it kept me out of the hospital.
• Black telescope? Manufacturers love black telescopes, black binoculars, black cameras, black cases, black everything. A few strategically placed pieces of white duct tape will make it less likely to be stepped on, bumped, or run into at night, by you or someone else.
• Red LED flashlight. If it's dark colored, it's not going to do you much good if you can't find it in the dark. White duct tape.

And there's not much in the world you can't fix temporarily with duct tape, so it's always good to have it around.

I'm sure I've forgotten many other uses for it, and I'm sure you can come up with others.