Flipping image (horizontally)

Hi everyone,

Any idea to how to flip an image while still in fits format? Trying to merge multiple integrations from different scopes, but one camera saved images flipped horizontally and Registration is failing. I can save it as tiff, flip it, reload it, but then it loses the exposure information (useful for merging with weights according to exposure).

Any ideas? Am I missing an option in one of the tools?

Thanks!