ICM images frequently demand the use of a Neutral Density (ND) filter to allow the longer exposures needed in bright (typically lower than 1/15sec).
In the absence of an ND filter the aperture can be stopped right down (eg f22) to allow some options on speed, but that comes with a caveat.
Small apertures tend to magnify the impact of dust spots on the camera sensor, this tends to lead to circular 'smudges' throughout the image, most noticeable in areas lacking detail (such as the sky).
Checking for dust spots should be part of any workflow and performed prior to publication of printing, this I prefer to do so in Lightroom using the spot removal tool (with image size set to 100%), but other tools can certainly be used.