Understanding “No Back Focus” in Petzval Systems
Petzval systems use an internally corrected optical design, with a rear lens group integrated into the telescope. Unlike systems that rely on external flatteners or reducers, Petzval telescopes do not require precise millimeter-level back focus tuning.
Instead of a single fixed distance, Petzval systems are designed to work comfortably within a back spacing range — giving users more flexibility when attaching cameras and accessories while maintaining reliable image quality.
What Is Back Spacing
Back spacing is simply the distance from the telescope’s rear mechanical reference surface to the camera sensor.
In a Petzval system, back spacing works together with the focuser travel. As long as the total camera setup falls within the supported range, the focuser can easily bring the system to focus.
Back Spacing vs. Back Focus
Although back focus and back spacing both describe the distance from the final optical element to the camera sensor, they represent fundamentally different behaviors due to different optical design.
ㅤ | Back Focus | Back Spacing |
Nature | A single, fixed distance | A usable distance range |
Tolerance | Very narrow | Relatively wide |
Typical system | Refractors with external flatteners or reducers | Petzval optical systems |
If incorrect | Edge distortion (radial or tangential star deformation) | Unable to reach focus |
This distinction explains why Petzval telescopes are generally more forgiving in real-world setups.
Rather than fine-tuning to an exact millimeter value, users simply need to ensure their camera system remains within the supported back spacing range to achieve focus reliably.
Practical Considerations
In normal use, most common imaging setups will naturally fall within the recommended back spacing range, allowing the system to reach focus without special adjustment.
Small deviations may appear only in less typical situations, such as:
- Extreme temperature changes Large temperature variations can cause slight mechanical contraction or expansion, subtly shifting the effective focus position.
- Thick filters or stacked filters Filters with significant optical thickness increase the optical path length and may introduce minor spacing offsets.
- Additional optical elements Adding extra optical groups to the light path can change where focus is reached.