Too long: 55 mm required, 60 mm present
55 − 60 = −5 mm: reduce the assembled length by 5 mm. The result must not say that no further spacing is needed.
Load this example in the calculatorBack-focus spacing calculator
Calculate whether your imaging train is too short, matched numerically, or too long for the specified sensor distance.
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What this calculation tells you
Start with the reference surface specified for your corrector or telescope, then measure to the sensor plane. This calculator keeps the sign of the spacing difference: adding a spacer cannot fix a train that is already too long.
Inputs and workflow
- Enter the required distance in millimetres for the exact optical configuration.
- For a quick difference, enter the assembled reference-surface-to-sensor length, including the camera sensor setback once. Do not count overlapping threads twice.
- For an inventory search, open “Plan with my owned parts”. Select the camera with its stated front ring or tilt plate, then enter installed parts in telescope-to-camera order and available spares with their actual quantities.
- Check each seated added length and both thread endpoints, including pitch and gender. Search can insert spares but preserves installed order. Apply any manufacturer-required filter correction to the target first.
Formula and units
Adjustment (mm) = required distance (mm) − current distance (mm). Positive means add length; negative means reduce length.
Worked examples
Short: 55 mm required, 48 mm present
55 − 48 = +7 mm. A 5 mm plus 2 mm numerical example fills this gap, but those parts are not an inventory or a verified thread connection.
Load this example in the calculatorLimits before using the result
55 mm is an illustrative requirement, not a universal telescope specification. A numerical match is not a guarantee of corner sharpness.
The quick calculator uses repeatable nominal spacer examples, not owned parts. The separate inventory search uses only entered quantities: at most eight accessory pieces and eight rows, with up to four of each. A reached search limit is reported, never presented as proof of an optimal route.
Unknown interfaces stay unknown. A no-purchase result requires an exact numerical route, matching nominal thread specifications and confirmation of ownership and reference surfaces; it does not certify engagement, clear aperture, strength, tilt or optical performance. The quick-calculation share link does not save the inventory; copy the parts summary before reloading.
Sources and method notes
- ZWO: back-focus configuration examples
Manufacturer examples illustrate why the camera and accessory configuration must be checked instead of assuming a universal distance.
Calculation notes maintained by AstroGearFit. Updated September 7, 2026. Examples are numerical demonstrations, not hands-on equipment tests. The method notes are currently in English; the existing interface language selector remains available.
Choose a calculation
Five focused checks with reproducible examples and explicit limits. These tools do not certify a complete equipment combination.
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