Old 15-02-2014, 09:52 AM
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Weltevreden SA (Dana)
Dana in SA

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E3 globular in Chamaeleon observation


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
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Old 15-02-2014, 10:31 AM
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Thanks for the interesting report, Dana - sounds like its worth the hunt
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Old 15-02-2014, 10:42 AM
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Excellent report Dana! I've had a few goes at it with a small scope but no luck. Here's a link to the thread I put up - if you scroll down to reply #9 you can read Les Dalrymple's description of E3.

Mind you, I'm still waiting for that happy conjunction of perfect dark skies and perfect dark-adaptation...

Cheers -
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Old 15-02-2014, 07:18 PM
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Weltevreden SA (Dana)
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E3 and Ru106

Rob, thanks for the link to Les's E3 observation. That was the one which piqued my interest in E3. I notice he also mentioned Ru106. I've observed 106 several times and it's a lot easier than E3. Ru106 is visibly granular while E3 is evanescent & opaque. Ru106 is a curiosity as well, but for different reasons than E3. I've read a few papers on it & will go back and summarize them for a report for the IIS glob fans. =Dana
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Old 17-02-2014, 10:46 PM
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Very nice write up Dana. Thanks for taking the time to write up the posts in the astro and science forum as well very interesting reading. It's nice to understand exactly what your looking at through the eyepiece and reflect upon that during your observation. Makes it more worthwhile I reckon.
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Old 21-02-2014, 06:26 PM
SteveG (Steve)
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Excellent post, Dana. I took a look at E3 six years back on one of my visits to Australia. I'm curious about the few "resolved" stars I noted as Les didn't mention seeing any. Do you know the magnitudes of the brightest members? This observation was through a 24-inch f/3.7.

At 200x, three faint stars were resolved over a 1' low surface brightness hazy glow with no concentration. In addition two brighter field stars are situated at the outside edge of the glow. At 260x, a fourth superimposed star was glimpsed, though these are possibly line of sight stars and perhaps the cluster, itself, was unresolved. Located 43' SSW of NGC 2915, though the precise position was pinned down moving 13' W of a mag 7.7 star using a 16" pair of mag 12/14 stars as a reference that is situated at the midpoint of this line.
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Old 23-02-2014, 05:15 AM
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Weltevreden SA (Dana)
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Hi Steve, and thanks for the question. Harris 2000 quotes an HB mag of 16.15. Mochesjka et al 2000 http://adsabs.harvard.edu/abs/2000AcA....50..105M measured an MSTO of 19.5, 3.35 mag fainter than the HB. CMD data from p.110 the Mochesjka paper plot 6 stars in the range of mag 15.5 to mag 16. Three of them are >0.5 (B - V) blueward of the RGB which indicates field star contamination. This paper has quite a lot of magnitude derivation detail, though it was compiled from a 2048 px imager at 0.41” arcsec/pixel—coarse by today’s standards. The 1985 McClure http://adsabs.harvard.edu/abs/1985PASP...97..665M CMD plots only two red giants definitively on the RGB (Table III p.673); the other four between mag 15.5 and 16.0 are >0.5 mag (B - V) blueward and thus suspect. Les Dalrymple may well have been seeing some of these six, so he had a 50/50 assurance of resolving actual cluster stars. Les likely gets the gold for at least three as definite members in his report. Dadgum better than the averted-only glow I can see. I wish I could put a ‘1’ in front of the ‘8’ in my largest aperture.

I don’t know why Harris 2000 listed the extinction of E(B - V) as 0.81 when the 1984-85 papers he likely drew upon said 0.3. E3 data has a lot of blank spots in Harris, esp all the luminosities rightward to the absol. mag -4.12 in Sect 2 and every item leftward of the core concentration in Sect. 3. The core surf brightness is mag 23.1, which makes it an elusive visual object using any aperture.

I sure appreciate the detail and accuracy of your myriad visual reports. I think you’ve single-handedly invented a whole scientific / literary cross-breed. Certainly a lot more credible than most of the fiction my wife tries to cajole me into reading.
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Old 27-02-2014, 03:47 PM
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Paddy (Patrick)
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Thanks for another fantastic thread Dana.

Here are my fairly lame notes on it from May 2011 using 16" dob:

175X A barely perceptible 2’ round glow that may require better conditions for proper observing.

It was a night of pretty poor transparency according to my notes so probably worth a revist.

Looks like I was only using the Night Sky Observers Guide for navigation that night.
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