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Observing Palomar Globular Clusters

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#76 Redbetter

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Posted 15 February 2020 - 01:55 AM

Pal 2 is as one might expect of a Palomar, fairly low surface brightness for a globular, about 22.7 MPSAS by my estimate.  That is easier than some of the other Palomars, but tough for a globular in general.  At 13 magnitude per Uranometria DSFG it is visible in dark sky if one can find it.  The brightest stars in it are about 18.8 mag, so this is not one that can be identified by incipient resolution in a scope less than 32" or so.  Horizontal branch is out there at 21.7.  I didn't see any members with the 20" some years ago.


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#77 Jeff Morgan

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Posted 15 February 2020 - 02:25 AM

Because of my m.21.4 skies and a 1 magnitude deeper reach, I found Palomar 2 to be constantly visible with a few stars visible.
M14 has a m.17.1 horizontal branch and I see stars completely across the face of M14.
I only saw a few stars in Palomar 2 so its horizontal branch is much lower. I don't have that data here on my vacation computer but I'll look when I return home.

 

 

The Uranometria Deep Sky Guide addresses this, but not very well. It was not clear if one needs to reach the magnitude of the horizontal branch to "resolve" a cluster, or just the magnitude of the brightest cluster members (which will generally be brighter). DSG usually gives both values.

 

I recently posted about the Galactic Wanderer (NGC 2419) where I was able to resolve only about 4 stars, which corresponds nicely to the brightest cluster members - 17th magnitude. A HR diagram I found for NGC 2419 only had a very few points around mag 17.2.

 

OTOH, I would have needed to reach down to 20th magnitude to get the horizontal branch. Not likely with SQM 20.5 skies and a 16" scope!

 

It would seem that just reaching the brightest individual members is enough to "resolve".

 

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#78 Redbetter

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Posted 15 February 2020 - 03:13 AM

Jeff,

 

Unless you noted them separately, there are a number of 14-16 mag stars on the periphery/edges of NGC 2419.   Some of these could quite easily be mistaken for cluster members as some are within the glow of the globular.

 

With the 20" in dark skies 3+ years ago I show it as unresolved.  I consider a globular cluster resolved if I can identify positions of 6 or more probable members.  At that point I can tell I am looking at a star cluster rather than a galaxy or nebula.  Of course that means only very marginally resolved, but it does imply seeing actual members (even if a few prove to be field stars.)  



#79 Starman1

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Posted 15 February 2020 - 09:59 AM

Pal 2 is as one might expect of a Palomar, fairly low surface brightness for a globular, about 22.7 MPSAS by my estimate.  That is easier than some of the other Palomars, but tough for a globular in general.  At 13 magnitude per Uranometria DSFG it is visible in dark sky if one can find it.  The brightest stars in it are about 18.8 mag, so this is not one that can be identified by incipient resolution in a scope less than 32" or so.  Horizontal branch is out there at 21.7.  I didn't see any members with the 20" some years ago.

Given that data, what I saw was a few superimposed 'field' stars and a glow where the cluster is.



#80 Asbytec

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Posted 15 February 2020 - 10:12 AM

Given that data, what I saw was a few superimposed 'field' stars and a glow where the cluster is.


Is it possible, given the data, for it to appear mottled?

#81 Jeff Morgan

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Posted 15 February 2020 - 12:11 PM

Jeff,

 

Unless you noted them separately, there are a number of 14-16 mag stars on the periphery/edges of NGC 2419.   Some of these could quite easily be mistaken for cluster members as some are within the glow of the globular.

 

With the 20" in dark skies 3+ years ago I show it as unresolved.  I consider a globular cluster resolved if I can identify positions of 6 or more probable members.  At that point I can tell I am looking at a star cluster rather than a galaxy or nebula.  Of course that means only very marginally resolved, but it does imply seeing actual members (even if a few prove to be field stars.)  

 

Possible they were foreground stars, chance alignment. They were over the central core, not edges.

 

NGC 2419 was definitely my toughest globular observation to date! I'll have it on my list for the next dark site trip. 

 

Looking forward to trying Palomars and Terzans.



#82 Starman1

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Posted 15 February 2020 - 12:16 PM

Is it possible, given the data, for it to appear mottled?

Not if the brightest stars are mag.18.8.  That would require a 24" scope in pristine skies, or absolutely perfect high altitude skies in a 20" (with sub 0.5" seeing).  No way I could see that in a 12.5" in m.21.4 skies.

Mottling is a term I used to hear a lot many years ago to indicate 'incipient resolution', i.e. the condition where a lot of stars were just a hair beyond the reach of the scope, making some points of the cluster appear brighter than others, leading to a mottled appearance.

I see that when looking at the Sagittarius region just above the Teapot spout--where the incipient resolution on thousands of points of light just beyond the reach of the scope leads to a 'grainy' appearance to the entire background sky.  On the best nights, I've seen the whole background become a carpet of tiny little faint points, but more nights the sky is not dark enough or transparent enough to see that.



#83 Asbytec

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Posted 15 February 2020 - 12:36 PM

Don, yes, that's what I meant. Not resolved in the real sense, but verging on it with the impression of seeing some stars that cause the mottled appearance. If I recall (I don't have my notes handy) I saw some mottling in the Inter Galactic Wondered mentioned above. Not sure if that's a significant thing or not, but it did feel like the verge of resolving it. It's certainly easier and certain to distinguish between resolution and mottling when some stars are clearly resolved in the image. But, small dim things without clearly resolved members? Maybe? Just curious.

Edited by Asbytec, 15 February 2020 - 12:41 PM.



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