It can get cold at night. And if it gets cold enough to reach the dew point, your lens is likely to fog up. I’ve handled this in the past by paying attention to the front element of the lens (looking at it with my red-headlamp) and keeping a lens cloth in my jacket pocket.

Another solution is to heat the lens so it never fogs up! There are do-it-yourself remedies such as using hand or toe warmers in a sock and wrapping the sock around the lens, and you can find tutorials online for making an electric heater.

You can also use hand warmers along with rubber bands.

The easiest and least bulky solution I’ve seen so far is the LensMuff, produced and sold by Kevin Adams. At $29.95, it’s not the cheapest option, but it is less expensive than a heater and a battery. You can get one here.

The LensMuff holds standard chemical hand warmers and wraps around your lens. It doesn’t take much room at all in your bag, and you don’t have to worry about plugging things in or recharging a power pack. You do need to make sure you have hand warmers though.

Another solution is to purchase a commercially available heater for the purpose. You’ll need the heater and a power pack. The heaters can widely range in price but it seems that there are a number of them available for around $20. The power pack will likely be more than that, but not all that much.

Here’s just one example. This heater will fit just about any lens. It’s longer than I need for my night lenses, but it wraps around a couple of times and I like to think it makes the lens feel nice and cozy! I found this one at Amazon.

You can just search on “Lens Dew Heater.”

To power the heater, you need a power pack. I suggest getting one that has plenty of power. Not only to last all night, but to recharge your phone if needed! I also found this one on Amazon.

The capacity of this usb power pack is 20,000 Amp-Hours. That should be enough to get through the night and then some.

The advantage of this system is that you don’t have to stockpile hand warmers. But you do end up with the bulk of the power pack and the tangle of a cable and you need to come up with a way of attaching it to your tripos so you won’t be tethered to your camera while the heater is being used.

(these are not affiliate links; I’m not profiting from posting them. These are just examples of what’s available)