The knob you turn to focus an image under low power on a compound light microscope is called the coarse adjustment knob.

Here's why:

* Low power means you're using the objective lens with the lowest magnification. This lens has a longer working distance, meaning there's more space between the lens and the slide.

* Coarse adjustment refers to the large, usually outer knob that allows for quick, significant changes in the stage's height.

The coarse adjustment knob is used to bring the specimen into approximate focus under low power. Once you have a rough image, you can then use the fine adjustment knob for precise focusing.