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M71 from bortle 8?

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#26 chrysalis

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Posted 11 August 2020 - 03:29 AM

Additionally I find it strange the difference between seeming equal magnitude stars When I’m using my naked eye. I can see Albireo even though it’s faint but all but one star in is invisible Saggita and that star is extremely difficult. The coat hanger is invisible.

Albireo = ~ 3.3 mag

Sagitta: Alpha, ~ 4.4; Beta, ~ 4.4; Gamma, ~ 3.5; Delta, ~ 3.8 magnitude

Coat hanger (Collinder 399; "Brocchi's Cluster"): brightest star ~ 5.2 magnitude



#27 Tony Flanders

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Posted 11 August 2020 - 05:35 AM

(M71 is) easy to resolve because there are so few stars visible.


Yes, that is true. It is also one of the closest globulars, making its individual stars relatively bright.
 

I find M4 much easier to resolve under bright skies, dark skies too.


That may be a matter of latitude. Under dark skies, I find M4 by far the easiest globular to resolve in my 70-mm refractor, followed by M22 and M71 in that order. One of the reasons that M4 is easier to resolve than M22 is because it too is fairly sparse as globulars clusters go.

But in light-polluted skies at latitude 42N, both M4 and M22 are buried deep down in the bright skyglow along the horizon, so M71 might actually be easier.



#28 Tony Flanders

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Posted 11 August 2020 - 05:55 AM

Additionally I find it strange the difference between seeming equal magnitude stars When I’m using my naked eye. I can see Albireo even though it’s faint but all but one star in is invisible Saggita and that star is extremely difficult. The coat hanger is invisible.

This is all as it should be -- relatively speaking. Albireo is magnitude 3.0 (including both components), while Gamma, the brightest star in Sagitta, is mag 3.5 -- just 2/3 as bright as Albireo. And second-brightest Delta is even fainter, at mag 3.8.

 

The Coat Hanger is definitely invisible to the unaided eye in badly light-polluted skies.

 

However, a limiting magnitude of 3.5 or worse hints that your sky is likely Bortle 9, not Bortle 8. Granted, the ability to see faint stars varies greatly depending both on visual acuity and experience. Nonetheless, part of the definition of Bortle 8 is a naked-eye limiting magnitude of 4.5, and one full magnitude is a pretty big discrepancy. How do you do on the two other criteria for Bortle 8: the naked-eye visibility of M31 and M44? Those would be affected by experience but not by acuity, since they are fuzzy as well as faint.

 

For what it's worth, from Danehy Park in Cambridge, Massachusetts, on a night of good transparency, I can see M31 but not M44, and stars down to magnitude 4.7 with about the same effort that it takes to see M31. I therefore rate it as Bortle 8/9, since it meets two of the three criteria for Bortle 8.

 

It's possible that your sky might be Bortle 8 at other times of year, but is Bortle 9 in the summer due to the haze that often accompanies high humidity.

 

Anyway, in skies where I cannot see stars fainter than mag 3.5, I would expect M71 to be a fairly difficult target through a 130-mm scope. But it will certainly be fun to try! Sagitta is a very cool constellation in its own right, as seen through binoculars and finderscopes.


Edited by Tony Flanders, 11 August 2020 - 05:58 AM.


#29 Redbetter

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Posted 11 August 2020 - 06:12 AM

Albireo = ~ 3.3 mag

Sagitta: Alpha, ~ 4.4; Beta, ~ 4.4; Gamma, ~ 3.5; Delta, ~ 3.8 magnitude

Coat hanger (Collinder 399; "Brocchi's Cluster"): brightest star ~ 5.2 magnitude

For clarification, Albireo is not as dim as 3.3 mag.  It is considerably brighter and this is clear just from looking at it in bright sky.  The primary alone is ~ 3.18 or 3.19 mag, and depending on the source, the secondary component is ~4.7 to 5.5. mag.  Naked eye these are inseparable resulting in an effective magnitude of about 3.0 to 3.1.  On the brighter side, WDS gives the pair as 3.19 and 4.68 which would give a net of 2.94 mag.

 

On the dimmer side, even Jim Kaler's values of 3.3 and 5.5 (a real outlier) result in a net of 3.17.   Peterson's old field guide gives 3.2 and 5.4 for a net of 3.07.  


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#30 Jon Isaacs

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Posted 11 August 2020 - 06:19 AM

Scott - a word, or two, of advice. CN has a very broad cross section of observers. Some of us are simply novices, others have been observing the sky for decades, while still others do most of their observing from sites with skies of Bortle class 3 or better. One must always keep this in mind when reading advice from others here. To the highly experienced old-timers picking up  Messier 71 is a very simple, quick, easy process, even from a site of Bortle class 7 or 8. But for the notice it is likely to prove a major challenge, especially in skies so poor that the brighter stars of Sagitta aren't even visible to the unaided eye.

 

I see that you only joined CN in May of this year, so I would assume that you are likely very much a beginner at hunting down DSOs. If that is so, then contrary to what many of the more experienced folk have posted, I suspect that you are likely going to find it difficult to even recognize M71 if you do manage to come across it. From a Bortle class 8 site, a novice  may well " see" only a very faint diffuse patch of light devoid of any resolved stars. I assume from your posting the site you use is indeed Bortle class 8, so you must be deep within a densely packed, well lit, urban region. Such is just about the worst place to attempt serious DSO as a novice, the level of sky background brightness will dramatically limit the effectiveness of your 10" scope. If at all possible, see if you can at least occasionally get to darker skies if you wish to peruse DSO, else you are setting yourself up for a great deal of frustration.

 

BrooksObs.

 

:waytogo:

 

I am an experienced observer but try to calibrate my replies for the beginners forum.  As he says, it's relatively easy for an experienced observer to see M71 from a light polluted site but it's not easy for someone just starting out.  I spent some time last night looking at M71 from my light polluted backyard with my 10 inch Dob.  Generally the skies measure around 18.3-18.6 mpsas, pretty bright.  Last night it was compounded by the marine layer sneaking in. 

 

But it was as I remembered it, it's not an easy, can't miss object from light polluted skies. It's not M13 or M3 that are relatively easy to see in 10x50s from a urban backyard, it was a faint cloud at low magnifications and just a sprinkling of faint stars at 160x.

 

Jon


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#31 Jon Isaacs

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Posted 11 August 2020 - 06:34 AM

Yes, that is true. It is also one of the closest globulars, making its individual stars relatively bright.

 

 

According to my simulations with Sky Safari, there are 17 stars in M71, 14.0 magnitude or brighter, none brighter than magnitude 12.4.  Compare that to M13. I count more than 20 stars magnitude 12.4 and more than I can count magnitude 14.0 or brighter. 

 

M20 is small, the numbers i see are about 20,000 stars and young.  

 

Jon



#32 Redbetter

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Posted 11 August 2020 - 06:37 AM

In the northern hemisphere globular clusters don't get easier to resolve than M22 or M4.  These begin resolving at finder level magnification (18x) in 72 mm in Bortle 6 sky (as in last night.)  M22 has the advantage of being looser, making the resolution more obvious because the surface brightness is lower.  At 18x with the 72mm scope it has that very faint speckled sand look despite being low in bright sky.

 

The horizontal branch of globulars provided some indication of the ability to resolve the brighter stars (which are brighter than the HB.)  M71's HB is at ~14.4 mag.  This is quite bright, only slightly dimmer than M22 and equivalent to M55.

 

With the exception of the core collapse globular M30, I find globulars with a horizontal branch of 15.1 are partially resolvable in 60mm scopes under dark skies.   By that I mean that I detect enough stars and incipient resolution in averted vision in the primary glow that it is clear the object is a globular.  My criteria is being able to identify positions of 6 stars clearly within the confines of the object.  Some of these might prove to be field stars in a rich field, but there are enough that the concentration stands out.



#33 Redbetter

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Posted 11 August 2020 - 07:22 AM

According to my simulations with Sky Safari, there are 17 stars in M71, 14.0 magnitude or brighter, none brighter than magnitude 12.4.  Compare that to M13. I count more than 20 stars magnitude 12.4 and more than I can count magnitude 14.0 or brighter. 

 

M20 is small, the numbers i see are about 20,000 stars and young.  

Sky Safari is not accurate in this regard from what I have seen.  There are several stars brighter than 12.4 in M71, whether Sky Safari acknowledges them or not.  There are several others from ~10 to 11 mag which frame and define the position of the globular at low power in a small scope in the suburbs.    M71 is one of the easier globulars to partially resolve in dark sky, despite being sparse.  This is due to its more open nature.  That is why it is on my standard test target list for small apertures.    

 

In town, it is tough for small apertures to make up for increased background sky brightness for resolving faint stars.  The lower surface brightness of the globular might result in it being nearly entirely washed out.  That is why the 10" would be the instrument of choice, because it can resolve over a handful of stars with enough incipient resolution to provide confirmation that something is there..  This is analogous to what I have done with the 20" looking for Palomars and other globulars at the limit of detection.  In the case of Palomar 4 in low elevation Bortle 3 conditions, the slight over-brightening of the globular was confirmed by the presence of a few exceedingly faint stars (17.5 to 18.0 mag) scintillating by periodically popping into view over the face of the globular when seeing improved. 



#34 Tony Flanders

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Posted 11 August 2020 - 08:52 AM

waytogo.gif
 
I am an experienced observer but try to calibrate my replies for the beginners forum.  As (John Bortle) says, it's relatively easy for an experienced observer to see M71 from a light polluted site but it's not easy for someone just starting out.


I agree with Jon and John (a.k.a BrooksObs). I'm not in Cambridge now, so I can't verify the observation. But in my Urban/Suburban Messier Guide I listed it as a 3, meaning that an experienced observer might need to look around a little for the object, even once it's known to be in the field of view of a wide-field eyepiece. And if an experienced observer can't pick it up at a glance, a novice might not be able to see it at all, unless there's an experienced observer right there to point it out and verify that it's real, not an illusion.

 

Curiously, I rated it the same regardless of the size of the telescope. That's characteristic of a largish object with low surface brightness.



#35 JustAnotherScott

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Posted 11 August 2020 - 09:40 AM

I haven’t tried to observe M31 yet but I do remember the Beehive cluster is difficult to see from my house. I don’t remember if I saw it or not with the naked eye. I never noticed it before I looked with binoculars though.

Edited by JustAnotherScott, 11 August 2020 - 09:41 AM.


#36 chrysalis

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Posted 12 August 2020 - 03:44 AM

I haven’t tried to observe M31 yet but I do remember the Beehive cluster is difficult to see from my house. I don’t remember if I saw it or not with the naked eye. I never noticed it before I looked with binoculars though.

Beehive (M44) very likely impossible (or at best quite difficult) naked-eye in Bortle 8. Same goes for M31 - although the integrated magnitude is quite high, it is spread out over a large area.



#37 Tony Flanders

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Posted 12 August 2020 - 05:26 AM



Beehive (M44) very likely impossible (or at best quite difficult) naked-eye in Bortle 8. Same goes for M31 - although the integrated magnitude is quite high, it is spread out over a large area.


The definition of Bortle Class 8 says: "M31 and M44 may be barely glimpsed by an experienced observer on good nights."

 

Mind you, the Bortle Dark-Sky Scale includes multiple criteria for each class -- that's the whole point of it. Different people find that these different criteria line up differently. But John Bortle has spent relatively little time in his long life observing in urban settings, so I think he's a little off in his criteria for classes 8 and 9. The next clause in the definition states: "only the bright Messier objects are detectable with a modest-size telescope."

 

I'm not sure what "modest-size telescope" is supposed to mean. In my youth, an 80-mm refractor was a modest-sized telescope, but today people are more likely to think of an 8-inch Dob, which is another thing entirely. Regardless, on a night good enough to see M44 with my unaided eyes, I also expect to be able to see all of the Messier objects through an 8-inch scope.

 

On a night with decent transparency, when M44 is reasonably high in the sky, I expect to be able to see it without optical aid when my SQM reads 18.5 but not when my SQM reads 17.5. I find M31 considerably easier to spot naked-eye than M44, and the Double Cluster easier still.


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#38 Voyager 3

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Posted 12 August 2020 - 06:17 AM

Albireo = ~ 3.3 mag
Sagitta: Alpha, ~ 4.4; Beta, ~ 4.4; Gamma, ~ 3.5; Delta, ~ 3.8 magnitude
Coat hanger (Collinder 399; "Brocchi's Cluster"): brightest star ~ 5.2 magnitude

I can also feel this at times . I saw the key stone fully but I couldn't see the 3rd star of Ursa minor , sulafat etc 😟 and these observations were made at the same day

#39 JustAnotherScott

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Posted 16 August 2020 - 10:06 PM

Currently out in bortle 4 skies and found it with ease. First of all it’s cake when you can see all the stars. Second of all this thing is super faint compared to everything else. No way I could ever see this from my house
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