Why are you advised never to lower the HPO with coarse adjustment knob?

The coarse adjustment knob is designed for use with low-power objectives, allowing for significant changes in focus. When using high power objectives (HPO), lowering the stage too much can lead to the objective lens contacting the slide, risking damage to both the lens and the slide. Therefore, using the fine adjustment knob with HPO ensures precise focusing without the risk of collision. Always start with low power to safely locate the specimen before switching to higher magnifications.