NGC 6540 is now considered a globular cluster . My copy of the Sky&Tel Pocket Atlas shows it as an open cluster. My Sky Atlas 2000 does not even show the cluster at all. Strange considering it is magnitude 9.3.
It was historically thought to be an open cluster. Djorgovski suspected it to be a globular (it now carries the honorary name Dgorg 3); and it has since been shown to have a blue horizontal branch and low metallicity.

I was looking at NGC 6540 this past weekend with my 11" scope. It was very faint from my NELM 5 back yard. A faint smudge with some periods of mottling now and then. The most striking thing to me about NGC 6540 is that it appears substantially elongated east/west. I thought my recent sketch had perhaps overemphasized this, but a look at the Digitized Sky Survey confirms its' shape. I can not think of any other globular that is so elongated. Can anyone else?