Yup... it's a sunshade. - Sightmark Sun Shade for Rifle Scopes

I've got a Sightmark 8.5-25X50 mounted on a rifle length AR15, and while it works, I did have a small problem with mirage. I have a vented free-float tube on the rifle, with slots on top that allow the gas tube and barrel to quickly shed their heat. Problem was that the gas tube gets so hot so fast that it would throw up a target obscuring mirage if several shots were fired in quick succession. The addition of this sunshade moved the objective opening out past the gas tube and pretty much eliminated the mirage problem.

As far as the price is concerned, it wound up costing me about $26 with shipping. Considering it's just a threaded aluminum tube with an anodized finish, that may seem excessive to some folks. But compared to other sunshades the price is reasonable, and it was worth every penny to me to get rid of that dang mirage.

And there's also the tactical benefit, if you use your optic in tactical situations. It reduces the angle at which the sun can reflect off the objective lens, and makes it less likely that your position will be compromised by a glint from the lens.

It's just a stupid aluminum tube... but it sure earns its keep.