Tackling the Corona Borealis Galaxy Cluster

by Steve Gottlieb

One by one, deep sky objects traditionally listed as severe challenges or even flatly unobservable have become standard fare at today's star parties. Nebulae such as the Veil, Eagle, Rosette, Pelican, California and even the Horsehead have succumbed to modern equipment such as the narrow bandpass Oxygen III and Hydrogen Beta filters and large transportable optics. Many Palomar globulars and Abell planetaries-very low surface brightness objects discovered photographically in the 1950's were once considered totally beyond visual detection. Yet 50% of these modern discoveries have proved to be within the grasp of large Dobsonians operating in dark skies. Want some challenging galaxies? Look for the dwarf galaxy Leo I very near Regulus, or the heavenly reddened Maffei I discovered in 1968. The latter is virtually invisible on the blue POSS print yet I've observed it with my 13" from the Fiddletown observing site.

Still, one well-known deep-sky object has resisted any large scale amateur observation-the Corona Borealis Galaxy Cluster (Abell 2065). I was first introduced to this remarkably rich cluster by an intriguing photograph in Burnham's Celestial Handbook which shows a field filled with small galaxies among two "bright" milky way stars. Their diminutive sizes and extreme faintness (the brightest members shine at mag 16.5!) a result of a truly staggering distance of nearly 1.5 billion light years. This distant cluster seemed way beyond the reach of my meager 8" and 13" scopes and this was confirmed in the Webb Society Handbook on Galaxy Clusters which claims it is "undetectable in 16" reflectors even under good sky conditions." A short report and sketch is given by Ron Buta who described Abell 2065 as "very difficult in the 36 inch" although he identified 16 members. So, this object was put on indefinite hold as far as my observing lists go, and over the years I've rarely heard this cluster mentioned in observing articles though both the Sky Atlas 2000.0 and the Uranometria 2000.0 both plot it as if it were standard observing fare. Nevertheless, as the more distant Abell clusters have passed through the eyepiece field of my 17.5" I've wondered about the observability of the Corona Borealis cluster.

Several years ago I had the opportunity to spend the better part of a week observing at the San Francisco Field Campus at the Sierra Buttes at an elevation of 7200' in conjunction with their Observational Astronomy class. After being thrilled at the deep sky sights in my 17.5" (the naked-eye threshold was close to 7.0), I knew that the following year I wanted to return and see if Abell 2065 could be conquered.

So, armed with the finder sketches in the Webb Society Handbook, a photocopy of the relevant POSS print and an article by Jeff Corder on the cluster which appeared in the Webb Society Quarterly Journal, I arrived at the class last year with high hopes. With careful starhopping I located the field and identified the two mag 11 and 12 stars which are displayed prominently on the Webb Society field sketch. Although no galaxies stood out with any certainty, I saw fleeting images of some extremely faint nebulous images very near the mag 12 star. Jim Shields was able to confirm that indeed several cluster members were visible.

I probably spent a good hour straining at my visual threshold to positively identify members which were barely non-stellar and visible with averted vision only. At the end I had logged 6 definite members in the central core and felt immensely satisfied just to have glimpsed a galaxy cluster at such a mind boggling distance. For those wanting to pursue Abell 2065, I discovered an interesting fact. The brightest cluster member is actually not plotted on the Webb Society chart at all, but is located about 20' SSW of the central core just northeast of a mag 9.0 star. None of the cluster members were discovered visually, so they are not found in the NGC or IC, although Morphological Catalogue of Galaxies (MCG) does include several members. In addition, I've included the Goodwin numbering system found in the Webb Society Handbook. Positions given are for epoch 2000.0 though keep in mind that the 5 Goodwin galaxies will easily fit within a 10' circle!

Observing Notes

Goodwin #2 15 22 29.3 +27 42 28 17.5: extremely faint, extremely small, round. Forms a close double with Anon #3 off the N side. The pair is cleanly resolved for moments. Located 2.5' E of a mag 12 star. Similar appearance as M+05-36-020 = Anon #1 in Abell 2065.

M+05-36-019 = Goodwin #6 = PGC 54875 15 22 22.8 +27 44 18 17.5: extremely faint, extremely small, round, only glimpsed for moments. Located between two mag 11/12 stars and 1.5' N of M+05-36-020 = Anon #1 in Abell 2065.

M+05-36-020 = Goodwin #1 = PGC 54876 15 22 24.0 +27 42 52 17.5: extremely faint, extremely small. Visible almost continuous with averted vision. This member of Abell 2065 is located 1.5' ENE of a mag 12 star. Similar in brightness to Anon #2.

M+05-36-022 = Goodwin #3 15 22 29.0 +27 42 46 17.5: extremely faint, extremely small, round. Forms a close double with Anon #2 off the S side. Located 2.5' E of a mag 12 star in Abell 2065.

M+05-36-023 = Goodwin #4 = PGC 54891 15 22 39.2 +27 40 38 17.5: extremely faint, extremely small, almost round. A mag 14 star is 1.2' NE. Last of six galaxies viewed in the core of Abell 2065. Forms the SE vertex of an equilateral triangle of sides 4.8' with a mag 12 star to NW and a mag 11 star to N.

NPM1G +27.0482 = PGC 54846 15 21 55.6 +27 24 53 17.5: very faint, extremely small, round. Located 2' NE of mag 9.0 SAO 083789. This galaxy is the brightest in Abell 2065 although not shown on the Webb Society finder chart!