Free planning tools · Check equipment compatibility

Backfocus spacing calculator

Build your imaging train, then see the spacer gap between your optical reference plane and camera sensor.

No account needed. Results update here as you change the inputs. Measurements stay in this tab unless you choose to share a link; shared links contain the inputs, including any location coordinates.

Your inputs

The starting values are examples. Replace them with your own measurements, prices or documented requirements.

What to have ready
  • The required distance and reference plane in the optical manual.
  • The camera’s sensor recess, not its overall body length.
  • The installed optical length of each wheel, drawer, adapter and ring.
Measurements for Backfocus spacing calculator
Installed optical distances

Calculated from your inputs

Your plan

The example will calculate here when JavaScript is ready.

What to do next

    Calculation details

    Inputs used in this result

    Compare a change before committing

    Keep this plan, change the inputs and see the difference. The comparison stays in this tab.

    How much spacing does your camera train still need?

    Use this backfocus calculator before ordering another adapter or spacer. Start from the distance specified for the exact reducer or corrector, then subtract everything between its reference plane and the sensor. The result tells you which dimensional gap to investigate.

    A club member or supplier can open a shared plan and see the same component list. You can also keep a plan in the tab, change an adapter thickness and compare the effect without losing the first calculation.

    How to use this planner

    1. Enter the optical manufacturer’s required backfocus. Keep filter correction at zero unless separately justified.
    2. Name each installed component and enter its contribution once. The camera distance has its own field.
    3. Read the gap or excess, then check whether real spacers and threads can produce that distance.

    The calculation

    Remaining spacing = specified backfocus + explicit correction − camera distance − installed component thicknesses.

    A worked example

    55 − (17.5 + 20 + 2) = 15.5 mm of remaining spacing.

    Example inputs are demonstrations. Replace them with your own measurements, prices or documented requirements.

    Assumptions and limits

    Measure from the reference plane specified by your reducer or corrector manufacturer. Count each ring or adapter once. An exact numerical match does not establish thread compatibility or optical performance. Filter correction starts at zero: only enter a correction justified by your optical manufacturer, and do not add it if their specification already includes it.

    ZWO manufacturer guides and connection diagrams

    Questions about this calculation

    Is every camera setup supposed to have 55 mm backfocus?

    No. The example uses 55 mm to demonstrate the arithmetic. Your reducer or corrector sets the target and the plane it is measured from; substitute its actual specification.

    Does matching the calculated distance guarantee good stars?

    No. Thread fit, tilt, the corrected field, measurement accuracy and the optical system still matter. Use a real star-field test after assembly and change one variable at a time.

    Put the result in context