A fastening device for releasably fastening an external apparatus (3) to a holding structure (2) and having an adjustable clamp (10) configured to engage with the holding structure (2) with a clamping force (F). An indicator (11； 11') is provided, configured to assume a first discrete state indicating that the clamp (10) is fastened to the holding structure (2) with a clamping force (F) below a predefined force (Fpre) and a second discrete state indicating that the clamp (10) is fastened to the holding structure (2) with a clamping force (F) equal or above the predefined force (Fpre). In this way a fastening device is provided, by means of which an external apparatus may be securely fastened to a holding structure with sufficient force.
