If you have a large reflector telescope, you probably leave it outside for 20 to 30 minutes before using it, so the mirror can reach ambient temperature. Not doing so will lead to distortion in your view through the eyepiece.
Another option is a fan. You can spend 30 dollars and up for a fan. But I found a way to set myself up with one for 17 dollars. It’s a small fan, but cools down the mirror quickly.
Grab this quiet 5-volt fan on Amazon for 11 dollars.
And get this 6 dollar battery case. It has a built-in on/off switch, and take 4 AA batteries.
I attached the fan and the battery pack to the scope with Velcro-type connector fabric tape (which is a great thing to have anyway. Many uses!) The screw size is exactly right for my scope, but I want to be able to remove the fan and battery pack., and didn’t want to remove the mirror to add the fan, so I used Velcro.
Don’t overcharge the batteries. I charge them less than full, and it works fine. I use these rechargeable batteries and this charger.
I had to add a little bit of aluminum foil to get the last battery to fit, for some reason, the clamps on mine were backwards on that one.
Clip the wires on the end of the charger and of the fan, strip them, and twist them together. Solder them, or wrap electrical tape around them.
Then apply the Velcro, with the paper still on the top side:
Then peel that paper and put in place:
When working with electricity, be careful, and you are under your own responsibility. I recommend removing the fan and battery for long term storage, or even for short term if you want.