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8" Dob and percived Magnatude

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#1 NerfWrangler

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Posted 09 July 2020 - 06:30 AM

I recently went up form a 4 inch to 8 inch, and boy are there a lot of stars in the night sky. I particularly enamoured with globular clusters, and I have been spending lots of time with Omega Centauri/Caldwell 80 as it is nice and high form my vantage point.

 

Last night though, I tried my hand at M4 – it is the same Apparent magnitude and apparent size as C80 according to Stellarium. What I ended up seeing was something much smaller, but not necessarily fainter.

With C80, it sits comfortably in my 20mm eyepiece and if I swap in my 10mm it fills the entire field of view.
The object I observed list night was so small I ended up using my 10mm to observe it. Going back to my star chart I think it must have been NGC 6144, a 9.63 magnitude cluster….

I live in a city centre with a street light right next to me. Not ideal observing circumstances. 

My question then, would it be possible for me to have seen the NGC 6144 cluster at that magnitude with an 8inch dob?

Would it not have been to faint at a magnitude of 9.63?

 

 

I am very confused/

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#2 Tony Flanders

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Posted 09 July 2020 - 07:28 AM

I recently went up form a 4 inch to 8 inch, and boy are there a lot of stars in the night sky. I particularly enamoured with globular clusters, and I have been spending lots of time with Omega Centauri/Caldwell 80 as it is nice and high form my vantage point.

 

Last night though, I tried my hand at M4 – it is the same Apparent magnitude and apparent size as C80 according to Stellarium. What I ended up seeing was something much smaller, but not necessarily fainter.

With C80, it sits comfortably in my 20mm eyepiece and if I swap in my 10mm it fills the entire field of view.
The object I observed list night was so small I ended up using my 10mm to observe it. Going back to my star chart I think it must have been NGC 6144, a 9.63 magnitude cluster….

I live in a city centre with a street light right next to me. Not ideal observing circumstances. 

My question then, would it be possible for me to have seen the NGC 6144 cluster at that magnitude with an 8inch dob?

Would it not have been to faint at a magnitude of 9.63?

 

 

I am very confused/

First of all, nobody -- but nobody -- calls the sky's great globular cluster C80! It is usually called Omega Cen, Omega Centauri, or (fairly rarely) NGC 5139.

 

Second, if Stellarium says that M4 is the same size and brightness as Omega Cen, it is flat-out wrong. Omega Cen is at least four times brighter, and half again wider. In light-polluted skies, you would only see M4's core, so my guess is that's precisely what you saw. Certainly not NGC 6144.


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#3 NerfWrangler

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Posted 09 July 2020 - 07:56 AM

First of all, nobody -- but nobody -- calls the sky's great globular cluster C80! It is usually called Omega Cen, Omega Centauri, or (fairly rarely) NGC 5139.

 

Second, if Stellarium says that M4 is the same size and brightness as Omega Cen, it is flat-out wrong. Omega Cen is at least four times brighter, and half again wider. In light-polluted skies, you would only see M4's core, so my guess is that's precisely what you saw. Certainly not NGC 6144.

Thanks Mr Flanders, that makes a load of sense. I will look closer next time to see if I can spot the belt of stars in the centre of M4…. If I can even spot it again.

 

On the nomenclature of C80… better start calling me nobody as in my reference notes I have listed it as such. Omega Centauri is too much of a bugger to jot down in the dark.

I am also in the Southern Hemisphere, so I got used to looking for C numbers instead of M numbers. I don’t care much for the Parisian fop that is Messier.

 

NGC numbers can also jump off a building


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#4 Andrekp

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Posted 09 July 2020 - 10:01 AM

Don’t worry about Tony, he hates the Caldwell numbers!  Everybody does usually call it Omega Centauri, but call it what you want.

 

M4 is easy to find, being so close to Antares.  Also, it should have an obvious structure with a line of stars cutting across the middle.  It’s obvious.  If you don’t see that, it’s not M4.  But I usually find it by starting at Antares and then just scanning over a little bit.  It’s helpful if you use a chart that shows things in their actual sky orientation.  The one you posted here is rotated such that you don’t see the easy way to find it.  I don’t know if you were using that picture to find M4, but it would not be easy.  It IS easy in real life with a good chart.


Edited by Andrekp, 09 July 2020 - 10:01 AM.


#5 Tony Flanders

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Posted 09 July 2020 - 10:06 AM

On the nomenclature of C80… better start calling me nobody as in my reference notes I have listed it as such. Omega Centauri is too much of a bugger to jot down in the dark.
I am also in the Southern Hemisphere, so I got used to looking for C numbers instead of M numbers. I don’t care much for the Parisian fop that is Messier.
 
NGC numbers can also jump off a building


That's fine; I certainly understand the benefits of using simplified notation for your observing notes! Just be aware that few amateur astronomers and no professionals will recognize that name, whereas all amateurs and all professionals recognize Omega Centauri.

You're going to need to make your peace with the NGC sooner or later, since the Caldwell catalog is extremely limited.


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#6 NerfWrangler

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Posted 10 July 2020 - 03:55 AM

That's fine; I certainly understand the benefits of using simplified notation for your observing notes! Just be aware that few amateur astronomers and no professionals will recognize that name, whereas all amateurs and all professionals recognize Omega Centauri.

You're going to need to make your peace with the NGC sooner or later, since the Caldwell catalog is extremely limited.

Learning this lesson, Our national observatory publishes a list of DSO and it is always in NGC winch I then go and plot against my Caldwell poster.

 

I am also realising how big the jump form 4inch to 8inch really is. There is so much more to see and I will run out of C numbered gobs soon.



#7 Waddensky

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Posted 10 July 2020 - 04:17 AM

Although NGC 6144 is not as obvious as some other globular clusters, it is well within reach of a 8" scope. In this case however, like Tony and Andrekp, I think you most likely saw the core of Messier 4. It's quite large and bright but really can suffer from light pollution.

 

Keep in mind that the apparent magnitude listed for deep-sky objects is the total brightness of the object. For most objects, this brightness is spread out over a large area, making larger objects more difficult to see than smaller objects with the same brightness. If you want to get an idea of how bright an object will appear in your telescope, the surface brightness is a more useful value. You can find this in Stellarium, too.


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