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Resolving globular clusters with refractors

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#1 Jeronimo Cruz

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Posted 17 October 2014 - 11:34 AM

Hello all,

 

Last night was the seeing was good and the air was nice and dry. To my surprise, I was able to resolve 14 stars, using direct vision, across M22 using a 101mm refractor working at 26x! Using averted vision, the star count doubled. Increasing the magnification to 154x, I saw a myriad of stars spanning the entire face of M22.

 

Next, for comparison, I swung over to M13. I could see no individual stars even with averted vision at 26x. Switching to 154x, M13 resolved into stars nicely, albeit not as spectacularly as M22. From memory M22 was about 30 degrees up whereas M13 was roughly 45 degrees up from the horizon.

 

I was never able to resolve stars in either glob using 20x100mm or 16x70mm binoculars. I can only imaging what one would look like through a large (8"+) refractor...

 

It sure would be nice to see Omega Centauri and 47 Tucane from Australia!

 

What is the smallest refractor/lowest mag combo you have used to resolve globular clusters (particularly M22 and M13)?

 

Thanks for your responses.


Edited by Jeronimo Cruz, 17 October 2014 - 11:35 AM.


#2 Love Cowboy

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Posted 17 October 2014 - 11:44 AM

Hello all,

 

Last night was the seeing was good and the air was nice and dry. To my surprise, I was able to resolve 14 stars, using direct vision, across M22 using a 101mm refractor working at 26x! Using averted vision, the star count doubled. Increasing the magnification to 154x, I saw a myriad of stars spanning the entire face of M22.

 

Next, for comparison, I swung over to M13. I could see no individual stars even with averted vision at 26x. Switching to 154x, M13 resolved into stars nicely, albeit not as spectacularly as M22. From memory M22 was about 30 degrees up whereas M13 was roughly 45 degrees up from the horizon.

 

I was never able to resolve stars in either glob using 20x100mm or 16x70mm binoculars. I can only imaging what one would look like through a large (8"+) refractor...

 

It sure would be nice to see Omega Centauri and 47 Tucane from Australia!

 

What is the smallest refractor/lowest mag combo you have used to resolve globular clusters (particularly M22 and M13)?

 

Thanks for your responses.

 

Granted, it would look better from Australia, but do you not realize Omega Centauri is in fact visible from your latitude?



#3 ggalilei

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Posted 17 October 2014 - 11:56 AM

I posted recently in the General Astronomy forum about my experience with M22 and M13 in small apertures: I was surprised too to realize that of the two it's easier to resolve stars in M22. I was using an 80mm refractor and the cluster was low on the horizon, yet I could make out a number of stars. I'm pretty sure it was at significantly less than 100x; the higher powers would wash out the cluster because of the small aperture and the low altitude.



#4 Jeronimo Cruz

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Posted 17 October 2014 - 12:10 PM

 

Hello all,

 

Last night was the seeing was good and the air was nice and dry. To my surprise, I was able to resolve 14 stars, using direct vision, across M22 using a 101mm refractor working at 26x! Using averted vision, the star count doubled. Increasing the magnification to 154x, I saw a myriad of stars spanning the entire face of M22.

 

Next, for comparison, I swung over to M13. I could see no individual stars even with averted vision at 26x. Switching to 154x, M13 resolved into stars nicely, albeit not as spectacularly as M22. From memory M22 was about 30 degrees up whereas M13 was roughly 45 degrees up from the horizon.

 

I was never able to resolve stars in either glob using 20x100mm or 16x70mm binoculars. I can only imaging what one would look like through a large (8"+) refractor...

 

It sure would be nice to see Omega Centauri and 47 Tucane from Australia!

 

What is the smallest refractor/lowest mag combo you have used to resolve globular clusters (particularly M22 and M13)?

 

Thanks for your responses.

 

Granted, it would look better from Australia, but do you not realize Omega Centauri is in fact visible from your latitude?

 

Yes, Omega Centauri is of course visible from my location. However, for me it never really high enough from the horizon to really see it in all of its splendor. At its peak it is only about 10-15 degrees up from the horizon.

 

I sure would like to see it high in the sky from somewhere in the southern hemisphere!



#5 Peter Besenbruch

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Posted 17 October 2014 - 12:20 PM

Last night was the seeing was good and the air was nice and dry. To my surprise, I was able to resolve 14 stars, using direct vision, across M22 using a 101mm refractor working at 26x! Using averted vision, the star count doubled. Increasing the magnification to 154x, I saw a myriad of stars spanning the entire face of M22.

 

Next, for comparison, I swung over to M13. I could see no individual stars even with averted vision at 26x. Switching to 154x, M13 resolved into stars nicely, albeit not as spectacularly as M22. From memory M22 was about 30 degrees up whereas M13 was roughly 45 degrees up from the horizon.

 

I was never able to resolve stars in either glob using 20x100mm or 16x70mm binoculars. I can only imaging what one would look like through a large (8"+) refractor...

 

It sure would be nice to see Omega Centauri and 47 Tucane from Australia!

 

What is the smallest refractor/lowest mag combo you have used to resolve globular clusters (particularly M22 and M13)?

 

80mm is the smallest I have gone with the two you mentioned. A 4" refractor should also got decent resolution on M5 and M3 (too late in the season, now). The real test for a 4" is M15. I could just get a bit of resolution around the edges from a darkish site.

 

I see Omega Centauri in the spring from where I live at 21.3 north. It's easy. I have seen 47 Tucanae on a trip to New Zealand through a 4" refractor. I found it prettier, if smaller than Omega. "Smallness" is relative, or course.



#6 Jeronimo Cruz

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Posted 17 October 2014 - 12:23 PM

I posted recently in the General Astronomy forum about my experience with M22 and M13 in small apertures: I was surprised too to realize that of the two it's easier to resolve stars in M22. I was using an 80mm refractor and the cluster was low on the horizon, yet I could make out a number of stars. I'm pretty sure it was at significantly less than 100x; the higher powers would wash out the cluster because of the small aperture and the low altitude.

80mm?! Wow!  Its nice to realize what 80mm can actually deliver on a good night...


Edited by Jeronimo Cruz, 17 October 2014 - 12:25 PM.


#7 Sasa

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Posted 17 October 2014 - 12:24 PM

Nice observations, Jeronimo. For, me it was a revelation when I first saw M13 in my ED100 refractor at 186x. I had no idea that 4" refractor was capable of showing such views.

 

Beside M3 and M5, I would also recommend M4. The brightest members are at the edge of visibility in 63mm refractor with direct vision at 84x.



#8 ensign

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Posted 17 October 2014 - 12:38 PM

Perhaps this is not a reply to the OP's question, but I think it's related and may be of interest. 

 

I find that globs take magnification very well and have had astounding views of M22 in my 9.25 Edge HD with a Delos 14 at 168X.  The nice thing about the Edge is that it delivers enough light at that magnification to make the views really exceptional.

 

Out of curiosity I've tried globs in my Equinox 120 (essentially the same scope as the Eon 120) with a Delos 4.5 (200x).  This view seems to resolve a similarly large number of stars, but with the exit pupil at just a hair more than .5mm, it's quite dim.



#9 Scott99

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Posted 17 October 2014 - 12:55 PM

I had a lot of fun doing the Messier objects with a Tak FS102.   It could get some degree of resolution on most of the globulars, and each one looked different enough to make it interesting.   For me, this would be the smallest refractor I'd want for regular viewing of DSO's.

 

A 6-inch refractor resolves all the Messier globulars except for the 1 or 2 most difficult ones, I'm forgetting which ones right now, maybe M12, was that it?  M10?  Not sure, one of the Ophiuchus globulars is always blurry.

 

>>>Yes, Omega Centauri is of course visible from my location. However, for me it never really high enough from the horizon to really see it in all of its splendor. At its peak it is only about 10-15 degrees up from the horizon.

 

You are lucky!  this pretty much describes viewing of M22 from up here, no chance of Omega Centauri.


Edited by Scott99, 17 October 2014 - 12:57 PM.


#10 schang

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Posted 17 October 2014 - 01:40 PM

 

What is the smallest refractor/lowest mag combo you have used to resolve globular clusters (particularly M22 and M13)?

 

Thanks for your responses.

I did that with a 4" refractor and a 25x100 bino on M22, but M13 is a little tough to resolve, maybe a few stars...M13 is a lot farther away than M22, though.

 

The question is a little odd to me and beg for "question"...with these small scopes, I only acknowledge them that they are there in the scope then move on, but not admire them for what they entail in a larger scope. These are the objects that deserve big scopes.  



#11 KJL

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Posted 17 October 2014 - 01:55 PM

 find that globs take magnification very well and have had astounding views of M22 in my 9.25 Edge HD with a Delos 14 at 168X.  The nice thing about the Edge is that it delivers enough light at that magnification to make the views really exceptional.

 

Out of curiosity I've tried globs in my Equinox 120 (essentially the same scope as the Eon 120) with a Delos 4.5 (200x).  This view seems to resolve a similarly large number of stars, but with the exit pupil at just a hair more than .5mm, it's quite dim.

I have an EdgeHD 9.25 as well as a 123mm f/6 APO and completely agree with this assessment. From Boston I can only regularly resolve M13 with concentration through the 123mm triplet. It's not much fun because I know, deep down, that M13 is the only globular that scope can resolve. Scott99 said something to the effect of how a 6" refractor is the minimum to comfortably start breaking apart most Messier globulars, and based on my experience I'm sure that is right.

 

On one, incredibly clear, Boston summer night I was able to clearly resolve M13 using a 90mm MCT. From that single experience I am certain -- in totally clear, dark skies -- that an 80mm APO would be able to do the same thing. So I would probably find myself in agreement with Peter's experience above as well.

 

After that amazing evening I resolved that my life was far too short to have to wait for that one night a year in Boston to be able to resolve a globular with a small refractor. That's why I have a big SCT now. :)



#12 Astrojensen

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Posted 17 October 2014 - 02:54 PM

On clear, good nights, I regularly resolve many stars in the halo of M13 with my 63mm Zeiss and 93x magnification, using a 9mm ES100 eyepiece. On one particularly superb evening, I had a fantastic view of it in my 72mm f/6 ED, using similar magnifications. My 85mm Zeiss, using an 8mm Ethos, giving 200x, has given me an absolutely fabulous view, with hundreds of stars resolved all across its face. The Danish TeleVue dealer was with me at the time and he declared it to be by far the finest view of M13 he had ever seen in a scope smaller than 4". 

 

M22, sadly, is deep in the soup here from 55° north. One superb night, almost 20 years ago, I resolved it stunningly well in an old 4.5" newtonian at around 100x, using a 9mm Zeiss ortho. 

 

 

Clear skies!
Thomas, Denmark



#13 KJL

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Posted 17 October 2014 - 03:13 PM

On clear, good nights, I regularly resolve many stars in the halo of M13 with my 63mm Zeiss and 93x magnification, using a 9mm ES100 eyepiece. On one particularly superb evening, I had a fantastic view of it in my 72mm f/6 ED, using similar magnifications. My 85mm Zeiss, using an 8mm Ethos, giving 200x, has given me an absolutely fabulous view, with hundreds of stars resolved all across its face. The Danish TeleVue dealer was with me at the time and he declared it to be by far the finest view of M13 he had ever seen in a scope smaller than 4". 

 

M22, sadly, is deep in the soup here from 55° north. One superb night, almost 20 years ago, I resolved it stunningly well in an old 4.5" newtonian at around 100x, using a 9mm Zeiss ortho.

I've read about your success with small refractors and globulars before: oh what a tease you are! I'm so glad you can't see M22 from your location. Serves you right.

 

;)

 

I had totally forgotten about one fairly clear and dark evening in the Bay Area when I had my SV50A with me (50mm doublet APO). I thought I might have resolved M13, until I realized I was looking at the granularity of my retina in such a low-photon environment. Yet I also felt I was really, really close to breaking it apart, so I think maybe 50mm is close to the lower bound of aperture needed for resolve ol' Hercules from anywhere excepting maybe a mountaintop view from Hawaii.

 

That lower bound is consistent with Thomas' amazing experiences. Yet again, I find myself in agreement with the experts here!



#14 rmollise

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Posted 17 October 2014 - 04:43 PM

Hello all,
 
Last night was the seeing was good and the air was nice and dry. To my surprise, I was able to resolve 14 stars, using direct vision, across M22 using a 101mm refractor working at 26x! Using averted vision, the star count doubled. Increasing the magnification to 154x, I saw a myriad of stars spanning the entire face of M22.
 
Next, for comparison, I swung over to M13. I could see no individual stars even with averted vision at 26x. Switching to 154x, M13 resolved into stars nicely, albeit not as spectacularly as M22. From memory M22 was about 30 degrees up whereas M13 was roughly 45 degrees up from the horizon.
 
I was never able to resolve stars in either glob using 20x100mm or 16x70mm binoculars. I can only imaging what one would look like through a large (8"+) refractor...
 
It sure would be nice to see Omega Centauri and 47 Tucane from Australia!
 
What is the smallest refractor/lowest mag combo you have used to resolve globular clusters (particularly M22 and M13)?
 
Thanks for your responses.

 
My 80mm APO resolves quite a few stars in M13 from a dark site--but it takes 100x or, better, 150x to do it. 25x100s? The magnification is just a bit on the low side. Under good conditions they show some up in M22 and a couple of others...but generally just not enough power. ;)


Edited by rmollise, 17 October 2014 - 04:43 PM.


#15 mogur

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Posted 17 October 2014 - 05:02 PM

I posted recently in the General Astronomy forum about my experience with M22 and M13 in small apertures: I was surprised too to realize that of the two it's easier to resolve stars in M22. I was using an 80mm refractor and the cluster was low on the horizon, yet I could make out a number of stars. I'm pretty sure it was at significantly less than 100x; the higher powers would wash out the cluster because of the small aperture and the low altitude.

 

I think it may be because M22 is a bit "sparser" with the stars having more separation. I've never tried to resolve with any scope smaller than my 102ED. I don't really own anything smaller. :smirk:



#16 Bomber Bob

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Posted 17 October 2014 - 06:13 PM

I would also recommend M4.

 

Yep!  It's my favorite GC:  Easy to find, and easier to resolve for me and my small achros.



#17 Bill Barlow

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Posted 17 October 2014 - 07:58 PM

Isn't M22 quite a bit closer to Earth than M13, which would make resolving stars in M22 a little easier with smaller apertures?

 

Bill



#18 Peter Natscher

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Posted 17 October 2014 - 09:07 PM

Isn't M22 quite a bit closer to Earth than M13, which would make resolving stars in M22 a little easier with smaller apertures?

 

Bill

M13 is a class V GC with a slightly more concentrated core vs. M22 class VII and less core concentration.   If you catch M22 at dusk before it gets too dark, you can see the numerous red super giants pop in view as reddish stars on top of the remaining cluster's bluish-white stars.  3D and beautiful!



#19 ggalilei

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Posted 17 October 2014 - 11:27 PM

I have found a logbook entry for my observation of individual stars in M22 in the SV80T: it was at 120x after all (4mm setting in 2-4mm Nagler zoom). The target was about 25 degrees above horizon, and the transparency was eyeballed at 6 out of 10. At the same time, M13 was 55 degrees high and showed "some granularity" at 120x.



#20 george tatsis

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Posted 18 October 2014 - 05:51 AM

Isn't M22 quite a bit closer to Earth than M13, which would make resolving stars in M22 a little easier with smaller apertures?

 

Bill

 

M13 is 25,000 light years away , as opposed to 10,000 for the M22. Big difference!

 

George



#21 mogur

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Posted 18 October 2014 - 03:43 PM

Isn't M22 quite a bit closer to Earth than M13, which would make resolving stars in M22 a little easier with smaller apertures?

 

Bill

 

Any star more than a couple light years away is going to be resolved as a point source at infinity focus. I doubt the difference in distance matters much.



#22 Thomas A Davis

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Posted 18 October 2014 - 03:56 PM

 

Isn't M22 quite a bit closer to Earth than M13, which would make resolving stars in M22 a little easier with smaller apertures?

 

Bill

 

Any star more than a couple light years away is going to be resolved as a point source at infinity focus. I doubt the difference in distance matters much.

 

Where the difference will come in is in brightness of the cluster stars.  At the closer distance, the brightest stars will be visible in the closer cluster, which in this case is M22.  As the distance increases, the combined light of the cluster members will be seen, but fewer individual stars will cross the threshold of visibility.  The same holds true for galaxies.  They are too far away for individual stars to be seen visually, but the combined light is seen of all the stars.  As aperture increases, individual stars in M13 become visible.  If you take CCD images with a smaller scope, you find that M13 resolves into many stars.  Resolution is not the issue, but the magnitude reach of the scope visually for individual stars, which are fainter at the greater distance.

 

Tom



#23 Astrojensen

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Posted 18 October 2014 - 04:00 PM

 

Isn't M22 quite a bit closer to Earth than M13, which would make resolving stars in M22 a little easier with smaller apertures?

 

Bill

 

Any star more than a couple light years away is going to be resolved as a point source at infinity focus. I doubt the difference in distance matters much.

 

We're not discussing resolving the disk of a star, but resolving stars next to one another. At less than half the distance, the stars of M22 will on average appear twice as far from each other as the stars in M13. 

 

The fact that M22 is not much, much brigther than M13, is because it's physically smaller, around half the size. If M13 was as close, it would be much bigger than the full Moon and shine at around mag 5.

 

 

Clear skies!

Thomas, Denmark 



#24 Thomas A Davis

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Posted 18 October 2014 - 05:47 PM

 

 

Isn't M22 quite a bit closer to Earth than M13, which would make resolving stars in M22 a little easier with smaller apertures?

 

Bill

 

Any star more than a couple light years away is going to be resolved as a point source at infinity focus. I doubt the difference in distance matters much.

 

We're not discussing resolving the disk of a star, but resolving stars next to one another. At less than half the distance, the stars of M22 will on average appear twice as far from each other as the stars in M13. 

 

The fact that M22 is not much, much brigther than M13, is because it's physically smaller, around half the size. If M13 was as close, it would be much bigger than the full Moon and shine at around mag 5.

 

 

Clear skies!

Thomas, Denmark 

 

But again, images taken with DSLRs and CCD cameras will resolve M13 with a small refractor.  Image brightness of the actual stars in the clusters is the issue.  A 60mm scope will show many individual stars in M13 with a CCD that the eye can't see.  But yes, resolving star chains in the cluster will improve with larger apertures.  Still if the brightness of the stars is below the visual threshold, you won't see them in the smaller scope.  The closer cluster will allow fainter cluster members to be seen in the smaller scope.

 

Tom



#25 17.5Dob

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Posted 18 October 2014 - 07:16 PM

I find that globs take magnification very well and have had astounding views of M22 in my 9.25 Edge HD with a Delos 14 at 168X.  The nice thing about the Edge is that it delivers enough light at that magnification to make the views really exceptional.

 

And in my 17.5",  most of the M clusters resolve to the core @ 222 -333x, many even showing individual star colors, and it's a very rare NGC cluster that I can't crack, but that's not the OP's point. I've never been able to visually resolve any cluster with my 80mm.

Somebody mentioned the difference between photo results and visual in an 80mm.

These photos were all taken with just a cheap kit 70-300 f5.6 zoom lens @ 200mm fl /f7, giving a "telescope" aperture of 28mm. And they weren't even taken using some big GEM. These were all taken using the cheapest "camera tracker" out there, the iOptron SkyTracker, and 30" exposures, using my dSLR.

M92

14175780737_621af05687_c.jpg

 

M5

14277245766_58f9efb799_c.jpg

 

M13 (and NGC 6027)

 

14094812647_b93120cfdc_c.jpg


Edited by 17.5Dob, 18 October 2014 - 07:19 PM.



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