Back Focus Defined
In astrophotography, back focus refers to the optical distance between the last optical element of your telescope (or corrector, such as a flattener or reducer) and the surface of the camera’s sensor.
This distance is critical because it determines where the corrected focal plane falls relative to the imaging sensor. If the sensor does not sit at the designed back focus position, stars—especially near the edges of the frame—will appear distorted, elongated, or out of focus.
Why manufacturers don’t measure back focus from the lens surface?
Because these elements are placed inside the housing, measuring directly from the glass is impractical. Instead, manufacturers set the reference surface at the rear mechanical thread, which users can measure easily. This practice led to the industry-standard 55 mm back focus, making filters, adapters, and camera mounts modular and cross-compatible.
▲ All William Optics back focus measurements are referenced from the rear thread surface.
Back Focus vs. Focusing
Although they sound similar, back focus and focusing are not the same.
Focusing:
Move the camera assembly with a focuser, or shift the entire lens group (e.g., WIFD), to align the focal plane with the sensor and bring stars into sharp focus.
Back Focus Adjustment:
Place the sensor at the designed distance from the field-flattening optics (flattener or reducer) to align with the corrected image plane and ensure sharp stars across the entire field.
Why some systems do not require back focus?
Not all telescopes require the user to set a fixed back focus distance. For example, Petzval designs are built with internal elements that naturally flatten the image plane. The corrected focal plane falls directly at the focus point, so users do not need to set or calculate a specific back focus distance.
Summary
Back focus is the fixed optical spacing that ensures your camera sensor is positioned at the precise point where the telescope’s optics deliver a corrected image.
It is not the same as focusing, which you adjust dynamically every session.
Understanding whether your system requires a back focus setting—and why—is the foundation for capturing sharp, flat astrophotography images.
For practical methods on how to evaluate and set back focus, see our article: Image Plane Calibration > Back Focus Adjustment.