Because this required spacing can vary depending on optical design and operating conditions, William Optics has designed most of our flatteners with an adjustable back focus mechanism to compensate for optical path shifts caused by temperature variations during imaging. This adjustability also allows certain flatteners to be compatible with multiple telescope models. However, each telescope–flattener combination results in a different back focus distance, and in some cases, the required spacing may be long enough to require the use of extenders.