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.
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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.
How to Determine the Back Spacing Range on Your Petzval System
Start with a standard 55 mm back spacing configuration
Assemble your camera system so that the total back spacing is approximately 55 mm, a standard value in astrophotography.
Focus the telescope at infinity
Point the telescope at a distant astronomical target and carefully achieve accurate focus.
Read the focuser position
Once focus is reached, note the value shown in the Focus Ruler Scale Window on the focuser.
Identify the usable range
The available focuser travel before and after this position indicates how much the back spacing can be increased or reduced while still allowing the system to reach focus.
Important Notes
Leave focusing margin for safe operation
When determining the usable back spacing range, allow at least 2 mm of inward marginto ensure sufficient focuser travel and avoid reaching the mechanical stop.
Recommended focuser position for astrophotography
For astrophotography, keep the focuser reading below 10 ( or simply 55±3 ) to minimize drawtube extension and reduce the risk of light leakage. Higher scale values are intended for terrestrial photography.
For example:
If the focuser travel range is 0–30, and a 55mm back spacing configuration reaches focus at 5, the usable back spacing range is 31–58 mm.
( For astro imaging, a back spacing of approximately 55 ±3 mm is recommended. )
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.
