2 added another solution
source | link

Ad 1):

Applying

Closing[EdgeDetect[image, 3], 5]

Produces a well defined image: detected disk

Afteredit: another option:

Binarize[ImageAdjust[TopHatTransform[image, DiskMatrix[20]]]]

detected disk 2 After this, it's the same procedure as in the related examples you've found.

Ad 1):

Applying

Closing[EdgeDetect[image, 3], 5]

Produces a well defined image: detected disk

After this, it's the same procedure as in the related examples you've found.

Ad 1):

Applying

Closing[EdgeDetect[image, 3], 5]

Produces a well defined image: detected disk

edit: another option:

Binarize[ImageAdjust[TopHatTransform[image, DiskMatrix[20]]]]

detected disk 2 After this, it's the same procedure as in the related examples you've found.

1
source | link

Ad 1):

Applying

Closing[EdgeDetect[image, 3], 5]

Produces a well defined image: detected disk

After this, it's the same procedure as in the related examples you've found.