E3
There have been few IIS posts about the faint globular E3 in Chameleon. I’m not sure how many of you have spotted it, or what kind of equipment you used, but the IIS references to this off-the-beaten track globular perked my curiosity. It is not shown on my 1989 version of Uranometria nor on the Torrés Tri-Atlas B set, but is accurately located on the set of charts I use in the field, Michael Vlasov’s
Deep Sky Atlas.
E3, properly known as ESO 37-01, is a relatively unknown and little-studied globular. It is listed in the
Harris Catalog at magnitude 11.35. It’s not an easy find, nor an easy look when you do. E3 is 26,400 light years from the Sun and 29,600 from the centre of the Galaxy. That’s not notably remote as globulars go. E3 is closer us than NGC 288 in Sculptor and NGC 362 next to the SMC. E3’s visual faintness implies that it is either very small and sparse or very reddened. In 1985 the reddening was measured at only 0.3 magnitudes, so that’s out. The Harris catalog lists E3’s horizontal branch stars—which are about the same intrinsic mass and brightness in all globulars—at magnitude 16.15. That’s actually a bit brighter than NGC 2808’s HB stars. Yet 2808 is a dazzling whopper while E3 is a fiddly glow. What gives?
If you have a go-to, E3 lies 43 arcmin south of the galaxy NGC 2915. For me, limited to a simple alt-az and no electronics besides a laser pointer, I am the ‘go’ and my eyes are the ‘to’. In an inspired case of beginner’s luck, E3 was a very faint fuzzy nearly centered in the eyepiece on the very first try. Credit goes to the accuracy if the Vlasov charts. I printed mine on A-3 paper for a large image scale out in the field. This was in my 180mm Mak at 100x using an ES 18mm 82° field. There being no other faint fuzzies nearby, the glow had to be E3. The patch quavered in and out of view at the beginning but settled down to a steady but dim and small glow. Averted vision revealed the soft round fuzzball we expect from globulars. An 11mm showed it larger but not better at 164x. It faded into sky background at higher power.
E3 looks brighter in the eyepiece than in
the picture below. I confirmed the observations over the next three nights using the 180mm Mak, an 152mm Intes MK-66 Mak-Cass and a 150mm Intes MN-61 Mak-Newt. The Mak-Newt’s view was as good as my 180mm scope’s, which shows that very sharp optics can squeeze every available photon into an Airy disc.
E3 appears smaller than its listed diameter of 3.35 arcmin—it’s more like its listed half-light radius of 2.1 arcmin. It also appeared w-a-a-y-y dimmer than the listed integrated luminosity 11.35. I’ve found the integrated luminosity numbers shown on the usual tables and lists to be misleading for fainter clusters. Before looking for it I had calculated a real-feel eyepiece view by adding the listed mag 11.35 plus the numerical value of the half-light radius (2.1 arcmin). I looked for a mag 13.6 cluster, and that’s what I saw. I don’t know why this odd back-of-the-envelope system works because the units are totally unrelated, yet I’ve found a lot of faint clusters using it as a guideline.
E3’s listed surface brightness (SB) of 23.10 mag/arcsec accords with my impression. The surface brightness limit for the eye is considered to be 25 mag/arcsec, which explains the phorescent look of faint objects and the long wait for superb skies before attempting them.
E3 is far more interesting than this visual report suggests. I've posted an analysis of its curious features in the Astronomy and Science Forum.
Cheers, Dana in SA