In my old 10 inch (25 cm) Newtonian, five stars strung east-to-west gave this globular an initial appearance of being quite elongated. Additional time at the eyepiece revealed a faint nebulosity (light from unresolved cluster members) sharing the same area as the string of stars. Overall, this globular appeared 1'.5 by 1'.0 in size in my 10 inch: NGC 6540.

Luginbuhl and Skiff have published what may be the best modern visual description of this object, "In 25 cm this cluster is just visible as a tiny, faint patch involved with a partially resolved 1' string of stars that bends S at its W end. With 30 cm the cluster seems quite elongated at low power due to this string. At 225x the string is resolved into seven or eight faint stars in an arc concave to the N, running through the center of the cluster and out the E and W ends. The cluster is a 30" spot centered on the arc and is without resolution."

NGC 6540's appearance begs an interesting question: are the stars in the E-W string associated with the cluster? Bica, et al write in their 1994 paper, "NGC 6540 is very peculiar structurally with a dense elongated nucleus and two clumps of bright stars which extend the cluster in the east-west direction."

This is a fun deep-sky challenge and a good target for moderate aperture under dark skies.

Bill in Flag