Good day Ron!

Is this energy at frequencies above nyquist?

Yes!

For discrete Fourier transformations the planes must be imagined on a torous or periodically repeated.

Your image data isnâ€™t correctly sampled (Shannon theorem).

This happens if the objects youâ€™ve imaged contain high frequencies and your imaging device doesnâ€™t provide the necessary lowpass.

It may also happen in case of synthetic images that are most often not correctly bandlimited. Think of an image consisting of a single pixel different from zero. Such an image has an infinitely extending Fourier-spectrum.

What you see â€śspectrally folded inâ€ť are so-called alias-components that generally canâ€™t be removed *post hoc*.

HTH

Herbie

**PS:**

I must admit that I canâ€™t reproduce the Fourier spectrum of the provided image (looks like a photograph of a rotating grating)!

What I get as the log Power Spectrum is: