Gerhard,
Additional concerns.
In the UK (and I imagine elsewhere too) museums generally require photographers to receive permission before they may take photographs and there may be a small fee for that permission. You may be able to get away with hasty hand-held snapshots, but anything deliberate is likely to attract official attention fast.
If you wish to publish your photos, then there are additional copyright considerations. Remember that museums usually have a small income stream from selling photos of their prize exhibits and it you want to compete with them, they may require more from you than a small fixed fee for permission to shoot

.
You may also find that they have rules that permit amateur photography by available light only. 'No flash!' rules are fairly common - and I whole heartedly support that; the use of flash can be a blight on the enjoyment of other visitors.
Just ask for the museum rules at reception and all should follow from that. There is no single set of rules.