Setting up a field flattener without stars?

[buzz]

Hi there - is there a way to set up a field flattener on a refractor, trying different sensor spacings - to optimise the image quality across the field, without waiting for a clear night?

I thought I read somewhere of a torch with pin-holes that could be used for testing purposes.... I seem to recall that a laser is a collimated polarised beam - so passing that through a pin hole might make it a plane wavefront beam, appearing to come from infinity....hmmm.

regards

Chris