You'll notice on this page - http://www.primalucelab.com/astronomy/d5500a-cooled-camera.html http://www.primalucelab.com/astronomy/d5500a-cooled-camera.h... - t

You'll notice on this page - http://www.primalucelab.com/astronomy/d5500a-cooled-camera.html http://www.primalucelab.com/astronomy/d5500a-cooled-camera.h... - that the before/after photo is taken at ISO 6400.I consider that to be reasonably high ISO. But yes, at some point, heat is not the primary issue anymore.