Q. Build-An-Adapter says the adapter "is not possible" or "cannot be built".
A. Certain custom adapters cannot be created as a single piece. Typically custom adapters requiring a bolted flange at both ends are not possible. Also if an adapter is bolted at one end, the feature at the other end cannot have a diameter value too close to the diameter of the bolt circle since this would prevent drilling the holes or installing the screws.
There are also other constraints when building a SLR/DSLR lens adapter for an astronomical camera. Please see this answer if you get an error trying to create such an adapter.

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