Resolving member stars M54

[Weltevreden SA (Dana)]

I’m curious how many IIS observers have experienced clear resolution of M54 (NGC 6715) member stars. We’ve had several nights of better-than-average seeing over here in SA, and I’ve logged three observations on different nights of ~20 faint stars in M54 using a 200 mm Mak at up to 358x, and ~10 in a 150mm Mak mounted alongside at 269x. The cluster stars are evenly spread across the 8 arcmin face and all have nearly the same visual mag of about 15.5. The cluster stars possess a uniformity clearly distinct from seven adjacent field stars, which are irregularly half a mag to 2 mag brighter. A colleague with me on one night confirmed the above apparent resolutions.

M54 has a foreground reddening of only 0.15, so in our eyepieces its stars are little diminished from their actual magnitude. The brightest 10 red giants start at vm 15.5 and the effective limit for visual observers is the cluster’s considerable population of horizontal branch stars at vm 17.5. While red giants can differ visually over a couple of magnitudes, HB stars in a cluster are nearly uniform in apparent visual mag. While M54 is important to the professionals because it is likely the core of the tidally stripped Sagg Dwarf Gx, I am more interested in what we see from our backyards. Can anybody confirm this kind of resolution with these apertures? Thanks, Dana in SA

[AG Hybrid (Adrian)]

I was looking at this object last month from Katoomba and I could not resolve it into even a slight granular field in my 12". Astrojason was looking with me. We even went to Alex's 17.5" and could not resolve it into a granular object.

It was a very windy night though. Perhaps the seeing was bad. I didn't really get past 182x that night.

According to sky safari m54 may not even be part of the milky way and is actually part of the Sagittarius dwarf galaxy. Current estimates is that it's 87000 light years away. Calculations indicate it is 300 light years across. That makes it second largest and brightest glob we can see, second only to omega centuri when looking at absolute magnitude. With the distances involved with this object we need much larger instruments to resolve m54 well.

[mental4astro (Alexander)]

The GC I brought up for Adrian I wasn't aware of it being M54 - at least I don't remember being told it was. Heck, I just drive the scope...

M54 would require very good seeing conditions to resolve. That night at Katoomba, seeing conditions were terrible, so resolution even in my 17.5" was no chance. I'll have another go at it this weekend.

[AG Hybrid (Adrian)]

Originally Posted by mental4astro

The GC I brought up for Adrian I wasn't aware of it being M54 - at least I don't remember being told it was. Heck, I just drive the scope...

Actually I think I did. Jason is my witness . I don't think you took a decent look in to the eyepiece though. You said it was "easily resolved".

[mental4astro (Alexander)]

Ah, that did the trick, Adrian. Yes, now I recall the instance. I had written off the whole night's observing in my mind. But yes, I do now remember seeing that GC resolved. It was faint (the member stars), and required a patient eye to allow the pin points of light to 'pop'. I recall it was a dense little GC, but actually quite bright for such a small diameter GC. Reading the links provided, I now see why as it is a type III GC, which is one of the densest types. It is surprising how bright. That was one of the few highlights of the evening.

Later that night, after you and Jason left, seeing deteriorated even further. I couldn't even use 120X without seeing thermal disturbance. Dreadful conditions, .

Resolving GCs takes a certain knack. A fleeting averted vision look won't do it. I've found that our eyes take well to a little bit of 'ramping up' with a fixed gaze (using averted vision). It's like our eyes require a bit of pumping or flooding by the photons to get the component stars to finally 'pop'. I do this with all GCs. GCs fainter than mag 12 begin to resist resolution in my big dob. Some GCs in the LMC are mag 12 and brighter, and I have managed to resolve these, but good seeing is a must. The sketch below is of a small area of the LMC I did early this year. I'll put M54 on the list to pen this weekend and add it to this thread.

[Weltevreden SA (Dana)]

Hi fellas, I'm back in civilization after nearly 2 weeks at the farm. Great skies after a slog of rain, but oh my heaven how good that burger tastes at the local teenager hangout!

I looked over my logs, and there are very few NGC globulars I haven't resolved from the overhead menu this time of year. In Norma, 5946; Lupus 5927 & 5986; Sco 6541, 6496, 6453 adj to M7; Ara 6352; Cor Austr 6723. N6526 in Sco is unusual in that it resolves into ~20 shimmering specks without any sign of subliminal glow. The toughie of the less-reported GCs in the area is 6388 on Sco-Ara border. It's very bright & pops out against the field, but is a tight Class III with so much crowding effect nothing stands out. It laughs at 370x in my 200mm.

Skies may have something to do with the general resolvability, but it also may be that I look at anything for a half hour at a time, switching eyes frequently. When the object is really dim, I hyperventilate the way sharp-shooters do. Three or four rapid deep breaths followed by half-exhaling and holding it. I can't quantify the improvement, but I'd say offhand I can pull another half mag out of a field for the 10 or so seconds the effect lasts. The decline in acuity is unmistakable as the oxygenation wears off.

There's a long, interesting thread in CN re. the GC N6749 in Aquila. Said to be the toughest NGC globular around, I'm going to try it when I get back to the farm. Anyone else want to give it a look so we can compare notes? I've been researching it in the literature and the current theory is that 6749 is a remote halo GC plunging through the Galactic bulge and being stripped naked as it goes. It is now the loosest GC in the sky and may emerge from the present tidal ordeal a shell of its self on arrival. (The same thing happens when you go to the casino.) It's pretty much agreed that our Galaxy's outer halo comprises a lot of stripped globular stars. Very low metallicities and in many cases retrograde motion are the tip-offs. Keep your eyes on N6749 for a gigayear or so and we'll know for sure.