Can I use another target for calibration?

Reikan FoCal is optimised to work with a specific target design and that's definitely the best and recommended way to use FoCal. Technically you can override this recommendation and use a different target but results may be inconsistent or inaccurate.

We very much recommend using the FoCal designed target, it provides extra features such as target validation and accounts for target/camera movement during calibration and testing. It is possible to self-print the FoCal target on any normal home printer, see FAQ entry How do I print the FoCal Target?

Using a different target would remove the ability for FoCal to do any sort of target validation, distance and other target checking. A user chosen target must have well defined, sharp high contrast edges in both horizontal and vertical axis. Caution is advised, since user chosen targets can vary widely, calibration and other results may also vary depending on target chosen.

To use a different target change Help > Settings > Target Validation to "No Target Validation".