The newest globular cluster – seen!

Globular cluster AL 3

Globular clusters are roughly spherical collections of old stars that circle around the centre of the Galaxy. Some like Omega Centauri are massive objects with hundreds of thousands of stars. Others like the just announced AL 3 are faint and have relatively few stars. Amateur astronomer extraordinaire Les Dalyrmple in his latest observing report claims to have made the first visual detection ever of AL 3. Here is his story:

Those who know me, know I have a soft-spot for globular clusters. As of 29 June 2006, the “official tally” of G.Cs attached to our galaxy stood at 153. The next day it ticked up to 154 with the announcement that a previously identified open cluster candidate — AL-3 had now been positively identified as a globular from it’s colour-magnitude diagram which clearly shows the hallmarks of a G.C — a short main sequence, strong sub-giant branch and most importantly a distinct, blue horizontal-branch. The “discovery” paper is here.

A POSS II image is above. The image looks like a huge maze of teeny dots with a marginal overabundance of dots in small zone at centre.

Actually, when printed in negative, it instantly reminded me of a visit I made to a vocational guidance counsellor when I was in high school. It looks like one of those sheets of dots and blotches that supposedly contain a hidden picture or pattern of some sort (no, not the more recent 3-D art), that they’d show you after the IQ test while asking what thing you could see amid the dots. Depending on our response (or lack of), they could then determine with great accuracy whether we were headed for a career in brain surgery, advertising or ditch digging etc.

Come to think of it I can’t remember what fate the vocational guidance cousellor pronounced on me all those years ago back in high school. All I can remember upon being shown the dot mazes in succession was answering “… a star”. “… a telescope”, “… Saturn”, “… a galaxy” etc etc. Despite that I don’t think he said “… Astronomer” at the end. But, I digress …

So, armed with the approppriate maps and a POSS II image of the newcomer, off I went in search of it with the thought that I just might be the first person on the planet to make a _visual_ detection of it.

This was my proceedure — my 46cm dob is equiped with Argonavis DSC with 8192 tic encoders, and stellarcat dual axis drives ( upon which I had performed a “local sync” on NGC 6528 which is only 2.5 degrees away) — and then went to the RA and Dec for AL – 3. The pointing should have been very accurate (in my experience somewhat better than 5 arc-mins error in local sync mode with this ‘scope). I then looked for the East – West running “droopy bow-tie” type asterism made up by GSC 6855:396 and GSC 6855:1804 at the eastern end, SAO 186501 at the “knot” and GSC 6855:864 and GSC 6855:790 at the W end (visible in the pic not far below centre). This asterism is only 3.5 arc-mins long and the brightest star at “the knot” is magnitude 8.9. After the slew, that asterism happened to land esentially dead centre in a x185 27 arc-min true field of a 12mm T II Nagler. It took quite a while to actually recognise that asterism because, couter-intuitively, dead-centre of the field was the last place I looked!

I then located the two stars to the east of the position for the cluster that don’t quite point at it : GSC 6855:724 and GSC 6855:336 at mags 11 & 12 respectively and identified those two. The latter is somewhat less than 2 arc-mins from the position for the cluster. At the time, the field was at about 65 degrees in elevation above the ESE horizon. At that altitude the atmosphere is a negligible issue, but it is far enough from Zenith that “Dobsons hole” ought not be an issue either (and I’m only 1 step up the ladder!).

There was nothing visible after a couple of minutes of looking at x185 so I switched to the 9mm Nagler at x247. Again nothing. Then the 7mm Nagler at x317. At this magnification I spent about 5 minutes looking and was nearing the point of giving up on it, when I thought I saw a tiny patch near a mag 15 – 16 star a couple of times while I was shifting my gaze within the field of view. It would sort of turn on and then turn off almost instantly. I couldn’t quite determine at that point whether it was real or “averted imagination”.

As the seeing was very good — at a rough estimate about 0.7 arc secs or better (I didn’t attempt to verfy that with some double star splitting — it is a “guesstimate” based on experence and the appearance of the star images), I went for the doctor and slipped in the Ultima barlow with the 7mm Nagler giving x634 and a tiny 7.5 arc min field.

At that magnification, AL – 3 was intermittently visible near the mag 15 star with averted vision about 20% of the time as no more than a small, slightly less than 1 arc min diameter “field enhancement” — a tiny patch which seemed to have marginally higher surface brightness than the surrounding sky and there seemed to be a couple of 16th magnitude (or worse) sparks that were very occasionally visible at the centre. Very, very nasty indeed. Gary confirmed it’s existance and then Col & Jeff took their turn at the eyepiece. Elation!


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