Hunting for faint globulars at Fiddletown April 29th

by Steve Gottlieb


In a message dated 5/4/00, Matt Tarlach writes:

Sunday night, April 30, I enjoyed a fine observing session at my favorite site near Fiddletown, in the California Motherlode. Conditions were excellent, with fine transparency (limiting magnitude 6.4+) and seeing 4 out of 5.

Wonderful observing report -- I wish I was there to share the views! In fact, Jim Shields are I had a less successful evening on Saturday night -- the skies didn't completely clear up until after midnight, but were wonderful as the summer milky way wheeled into view. Observations were with my 17.5-inch f/4.4 (optics by Galaxy, mount by Ray Cash).

I always love revisiting the rich open cluster NGC 6520 in Sagittarius and perhaps the best dark nebulae in the sky on the west side - Barnard 86, which earns the moniker the "Inkspot Nebula".

Much less known, though, are two globulars within the same low power field. NGC 6540 is situated 40' east of NGC 6520 nearly midway along a short 1.5' arc of a half dozen or so mag 13-14 stars which are concave to the north. The globular, itself, is just a faint, round, 40" glow embedded just inside the center of this string. which. At 100x, the string extends beyond the globular to the west and east and creates the impression that the globular is quite elongated.

Although this globular was first discovered by William Herschel on May 24 1784, and catalogued as an open cluster, it wasn't officially listed as a globular until two centuries later (as Djorgovski 3 in 1994).

A more recent discovery is ESO 456-SC38, found photographically on the European Southern Observatory Sky Survey and initially catalogued as an open cluster. It was reclassified as a globular by Djorgovski in 1987 (color-magnitude plot) and is situated 3' north of a mag 9 star - just 21' WNW of NGC 6520/B86!

At 140x I picked up a faint, oval glow ~1.5'x1.1' with no hint of resolution other than one or two faint star glimpsed at the edges. The rich milky way background is quite prominent in the field except in the vicinity of the globular which appears to be nestled in a darker "hole". At 220-280x, ESO 456-SC38 appeared elongated SW-NE and stood out well with averted vision with a fairly sharp outline. Interestingly, it seemed more like a faint patch of nebulosity than a globular. A few mag 15+ field stars were visible at the edges - the easiest at the SW end and a second star at the NE edge. One of two additional threshold stars intermittently glimmered. This globular is sandwiched between a trapezium, consisting of a mag 9 star to the south, a close double star 2.5' SSW with two mag 10 stars bracketing the cluster on the west and east sides.

Terzan 3 is one a dozen faint globulars, first catalogued by Agop Terzan in a far red survey of the milky way. You'll find it plotted on the Uranometria 2000.0 atlas about 9 degrees south of Antares at 16 28 40 -35 21.2 (2000). It was picked up fairly readily at 140x as a fairly large, irregular low surface brightness glow with no central brightnening or resolution - certainly not your typical globular! It appeared perhaps 3'-4' in diameter although there really was no well-defined edge. The "glow" was situated east of two mag 13 stars and close west of a 20" pair of mag 14 stars. At 220x, a slightly brighter 20" knot was seen with averted vision near the center of the glow (possibly the core?), just south of a line through the close pair of mag 14 stars. For more info and data on the Terzans (something to add for your Lassen challenges!) see http://redshift.home.pipeline.com/terzan.htm