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Naked eye limiting magnitude

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#26 Keith Rivich

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Posted 14 February 2024 - 08:48 AM

In the little area to the Southeast around NRG.

 

I tried again tonight, glimpsed it for a moment several times, much more difficult than it was last time and nearly impossible, any observing at all was more difficult, better than normal but still bad. I need another last night sometime soon .

 

This time I was able to see HD37744 (Mag 6.1) without a ridiculous amount of difficulty, completely confident in this observation as it was actually my younger sister who pointed it out and told me to look there. I am very confident that I did indeed see the 6.4 star yesterday as I didn't actually know it existed until I saw it and checked its magnitude through stellarium. 

 

Speaking of observations, how did you go about proving that you saw the m87 jet?

We all did a sketch at the eyepiece then compared the sketches to an image. 



#27 KBHornblower

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Posted 14 February 2024 - 11:02 AM

...snip...

Remember LP is a backscattering function and has NOTHING to do with a Bortle rating.
And EVERYTHING to do with the amount of junk between you and the target.


Clearest Skies

My bold.  That is an odd choice of words.  Mr. Bortle's ratings were very much driven by light pollution in what otherwise would have been a dark sky.  


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#28 Sebastian_Sajaroff

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Posted 14 February 2024 - 02:17 PM

Supposedly, some people can see them just by looking. Not me.

In my young years, I could see Ganymedes by blocking Jupiter with a brick wall.

Try it close to Jupiter opposition and at a moment when Ganymedes is at its farthest from Jupiter.

It's not difficult (and I have moderate astigmatism and myopia)



#29 BFaucett

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Posted 14 February 2024 - 05:13 PM

...<snip>...

Remember LP is a backscattering function and has NOTHING to do with a Bortle rating.
And EVERYTHING to do with the amount of junk between you and the target.


Clearest Skies

 

 

What ??

 

 

GAUGING LIGHT POLLUTION: THE BORTLE DARK-SKY SCALE
BY: JOHN E. BORTLE JULY 18, 2006
Sky & Telescope
Editors' note: This article on light pollution and astronomy appeared in the February 2001 issue of Sky & Telescope.
https://skyandtelesc...dark-sky-scale/
 

Bob F.



#30 Keith Rivich

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Posted 14 February 2024 - 06:25 PM

In my young years, I could see Ganymedes by blocking Jupiter with a brick wall.

Try it close to Jupiter opposition and at a moment when Ganymedes is at its farthest from Jupiter.

It's not difficult (and I have moderate astigmatism and myopia)

What kind of sky was needed to see this? From the OP's location the SQM-L is probably around 15 to 17 on the best of nights. 



#31 SrAstro

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Posted 15 February 2024 - 09:14 AM

What kind of sky was needed to see this? From the OP's location the SQM-L is probably around 15 to 17 on the best of nights.

At opposition it's around mag 4.6, which is direct-vision visible from here. Doesn't sound especially difficult.

 

I could probably see it when Jupiter is at opposition again in December provided I block it out with something. Right now, it's mag 5.4 which is too difficult verify when jupiter is right next to it.


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#32 SrAstro

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Posted 15 February 2024 - 10:47 AM

If the OP is seeing down to mag 6.4 in Orion, then the Milky Way should be easily visible from overhead down to about 45 degrees above the horizon

Can you see that, @SrAstro?

How long did it take you to see the star? What percentage of the time did you see it? Were there multiple attempts where you did not set it, and only a few that you did? How many times did you glimpse it before you confirmed?

The milky way is not what I would call easily visible in detail, at zenith late last year I could sort of make out a hint of brighter sky patch but that's about it. I have never tried to follow it down the sky, I've always considered that hopeless from my location. Besides, the bright parts of the milky way aren't high over the horizon at this time. Though even in dark skies I have always had trouble seeing the millky way specifically, it's obvious from bortle 4 but not to the extent you'd expect it to be. Seems that I do better with point sources than diffuse objects.

 

That star took about 2-5 minutes to see the first time after looking in that area. Every time I checked back on it in that night I could see it again after looking for 30 seconds or so. The next night it was much more difficult but I did see it again.



#33 careysub

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Posted 17 February 2024 - 09:31 AM

Some observations about the Borlte Scale and a proposal for implementing a modernized version.

 

With the rise of light pollution there was a need for a convenient scale to rate the degree of pollution at a given spot just as we have hurricane scales, tornado scales, earthquake scales, and so forth.

 

Bortle's proposal addressed an urgent need in the amateur astronomy community. Bortle's zones are not going anywhere, but the observational yardsticks Bortle laid down do not actually define distinct zones. The zones and their quality descriptions ("inner city", etc.) should be preserved, which just need a better way (i.e. consistent, repeatable, objective) of defining the boundaries.

 

The sticking point is who is going to do it? How would it be instituted or promoted?

 

This question has been answered, the answer is here:

 

https://en.wikipedia...ki/Bortle_scale

 

And here (from 2016):

 

https://web.archive....rg/nomogram.php

 

It appears that a system was devised for mapping one method of characterizing light pollution to another for the International Astronomy Year which was 2009 -- IYA2009. Mapping SQM values to Bortle zones is not measuring the SQM values for Bortle zones, but redefining them in terms of an objective measure of light pollution - SQM. I don't know who should be credited with doing it (it would take a bit of Google archaeology to investigate), but someone in the astronomy community put some effort into defining a meaningful 9 zone SQM scale.

 

The problem was this was really part of a one-time, one-year event that failed to effectively promote the system, and then vanished from the Internet  (darkskiesawareness.org is long gone).

 

The new development is that this new SQM based scale has been placed in Wikipedia making it an easily accessed reference on a reliably maintained, universally known information resource.

 

Since we now have a central reference point to use for an SQM based zone scale I say we start using it -- calling it the Bortle-SQM Scale (or Zones).

 

I think I will start promoting this idea myself - writing letters to astronomy magazines, making presentations to my club, etc.

 

I'll have to do a write-up and  start a thread with this title here presently - the focus will be on getting people with access to SQM meters (every club has got to have people who have them) to field test the Bortle-SQM scale and give feedback. That should probably be done on the light pollution forum, but a notice on this forum would be a good idea.

 

Other thoughts:

Although any competently done SQM reading should be good to characterize a site by zone I would favor using narrow SQM zenith readings. Reported SQM readings should specify the meter type.

Has anyone done an evaluation of the effectiveness of phone sky brightness apps and the SQM meter?


Edited by careysub, 17 February 2024 - 09:40 AM.


#34 PKDfan

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Posted 17 February 2024 - 06:16 PM

My bold. That is an odd choice of words. Mr. Bortle's ratings were very much driven by light pollution in what otherwise would have been a dark sky.



Yes your right of course KB !

--yet with a B9 sky like Vancouver (or Edmonton) my very best DSO observation of Alnitaks grouping were achieved right after torrential rain fell scrubbing the atmosphere clear BUT the Big Picture naked eye view was very very poor and the 'occulted' image of the 2degree B33 ngc 2023/4 were contained within the black fieldstop were so VIVID @40X and a pupil much smaller than you'd imagine using but 70mm Pronto 12mm Nagler Type2 view-window was quite frankly Just Shock heart stopping IMPOSSIBLE to believe.

Veritable clouds of light up there.

While at a proper B2 site with the milky way gloriously overhead ngc 2024 was a vivid flame

astrophoto like with my 100mm and 20mm Type2 and the HH No Where to be found.

In my opinion using this sky rating metric induces a mindset that is wholly unrealistic or inappropriate.

Reducing the liklihood that someone in downtown Van would EVER try to make this observation of Barnard 33.


I call this ultra-transparency and even now after 20years of growth Vancouver with some of its EPIC rainfall i'd 'kill' to try again with my 62mm apo and the same Type2 12mm Nagler after one such deluge.

I wish i had more time and other power options with the Magnificent semi-achromat Pronto back then.



CS

#35 CHASLX200

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Posted 17 February 2024 - 07:39 PM

Mag 5 on my best nites at best. Avg is about 3.



#36 Sebastian_Sajaroff

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Posted 18 February 2024 - 07:38 AM

What kind of sky was needed to see this? From the OP's location the SQM-L is probably around 15 to 17 on the best of nights. 

A suburban decent sky (which I had twelve years ago), one where you could reach mag 5.0-5.2. 



#37 NinePlanets

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Posted 18 February 2024 - 08:13 AM

A close friend of mine could see the Galilean moons naked eye without an occulting bar of any kind. I tested him on it. It can be done! But certainly not by me. Everyone's eyes are different.


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#38 KBHornblower

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Posted 18 February 2024 - 08:43 AM

I saw a sweetheart of a story about an observer and his 7-year-old daughter.  He was aiming his telescope at Jupiter and telling her that the moons would be above the planet in the field of view.  She replied with something like, "No, they are below Jupiter".  She was staring at the planet with no prior knowledge of the inverted view in the scope.  This little girl was blessed with sharp vision and no confirmation bias.


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#39 NinePlanets

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Posted 18 February 2024 - 09:55 AM

Yes, it does happen. I was continually amazed at what my friend could see, day and night and have often wondered what the world looked like through his eyes.



#40 Keith Rivich

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Posted 18 February 2024 - 06:33 PM

Yes, it does happen. I was continually amazed at what my friend could see, day and night and have often wondered what the world looked like through his eyes.

A long time ago we would do a "how many stars in xxx". Sketch the stars we could see and compare this to a chart of the area. I always lost. Usually a full half magnitude worse then my friends. However, at the eyepiece, I could go much deeper then they could.

 

The magic of the optics!


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#41 SrAstro

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Posted 18 February 2024 - 07:52 PM

A long time ago we would do a "how many stars in xxx". Sketch the stars we could see and compare this to a chart of the area. I always lost. Usually a full half magnitude worse then my friends. However, at the eyepiece, I could go much deeper then they could.

 

The magic of the optics!

Yeah, I get the feeling that I would suck through an eyepiece. Dunno how that works, though.



#42 CarolinaBanker

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Posted 18 February 2024 - 08:01 PM

It is Bortle, not "bottle".  And no, 6 mag as a limit is nowhere near correct.  If you can only see down to 6 mag in near pristine skies your uncorrected vision is not good.  I can see past 7.0  in Bortle 4 conditions without correcting for my mild myopia or astigmatism and I am no youngster.  When I was in my late 20's I reached 8 mag in pristine skies, even though I had some myopia and astigmatism even at that time.

 

Having lived in Houston.  I don't know what the OP saw, but I do know that cloud in Houston would make the sky brighter, not darker.  That light pollution all around gets reflected right back at you when there is cloud above, which is why it is so bright even without cloud. 

 

Excellent transparency could help even in otherwise bright sky, but assuming the OP is correct on the ID's, this is likely more about young eyes and visual acuity..  Some folks can pick out dim objects next to very bright ones far better than other people do, and this could be another example of that (dim star on bright background.)  Just because I can't go this dim in such bright sky doesn't mean that nobody else can.  

I mean if you’re seeing an NELM of 7 it’s a Bortle 3 sky. Here’s the link to the original article: https://skyandtelesc...dark-sky-scale/



#43 Sebastian_Sajaroff

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Posted 18 February 2024 - 08:05 PM

I saw a sweetheart of a story about an observer and his 7-year-old daughter. He was aiming his telescope at Jupiter and telling her that the moons would be above the planet in the field of view. She replied with something like, "No, they are below Jupiter". She was staring at the planet with no prior knowledge of the inverted view in the scope. This little girl was blessed with sharp vision and no confirmation bias.


Young children tend to have better visual acuity than us.
Ask them to look at Venus when it’s close to inferior conjunction, in a group of 40 there’s always 2 or 3 that can discern the crescent naked eye and draw it correctly.
I did that test several times during outreach activities for kids between 5 and 9 years old.
By the way, I could never see it myself.

#44 Redbetter

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Posted 19 February 2024 - 02:33 AM



I mean if you’re seeing an NELM of 7 it’s a Bortle 3 sky. Here’s the link to the original article: https://skyandtelesc...dark-sky-scale/

That would be an unfortunate/inaccurate simplification and doesn't correspond with my eyes or my skies.  The key to arriving at a rating is in aspects of the scale other than NELM.  One has to weigh these to determine what one is seeing in terms of the scale, otherwise, one might rate a site in four Bortle classes at the same time.  

 

The way the scale is presented with respect to NELM isn't quite how it works, certainly not how the average observer's eye works either.  NELM was not meant to be the arbiter of Bortle rating.  It is ironic, because in the article Bortle made that very clear...yet he went on to put in some generally aggressive NELM (and TLM) values in for his scale, which undermined the message. 

 

Here is what John Bortle wrote in the actual article on the subject:

 

 However, naked-eye limiting magnitude (NELM) is a poor criterion. It depends too much on a person's visual acuity (sharpness of eyesight), as well as on the time and effort expended to see the faintest possible stars. One person's "5.5-magnitude sky" is another's "6.3-magnitude sky."

 

 

It has been fairly common recently for my sites that normally run Bortle 3  to run a little worse than that, into the good side of Bortle 4 with typical SQM readings hitting only ~21.2 mpsas.  Bortle 4 is very wide.  Yet I am still seeing stars fainter than 7.0 mag (Bortle 2 per the table you linked).  However, NELM is not the key to rating the darkness of the sky.

 

For perspective:  Bradley Schaefer requested NELM and TLM readings from Sky & Telescope observers in 1989.  The data (along with historical study data) was incorporated into a TLM study he published.  Despite dark sky skies still being substantially more accessible ~35 years ago, only two of the observers who used Schaefer's sequences (meaning they were part of the new survey vs .legacy data) reached 7.0 or beyond for NELM.  Skiff was "only" reaching 7.0, despite observing in very dark skies from what I can tell. Seal reported 7.2.    It is doubtful that Skiff was only observing in Bortle 3 sky, and from what I have seen of his period telescopic reports he went deep--most closely matching my own results three decades later.


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#45 David Knisely

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Posted 19 February 2024 - 04:56 AM

I mean if you’re seeing an NELM of 7 it’s a Bortle 3 sky. Here’s the link to the original article: https://skyandtelesc...dark-sky-scale/

Well, maybe yes and maybe no.  There are problems with the Bortle scale (which may be why some amateurs shy away from using it).  One problem with it is: some people have eyes that simply can't see any fainter than magnitude 6.0 to 6.5 or so, which has historically been a commonly cited level for what was considered to be a reasonably dark sky.  Thus, even if that person was observing at a "Bortle 1" site, they might not see stars fainter than that.  Weather and transparency variations also affect how faint a person can go at a given site.  The faintest I have gone from a true "pristine" dark sky site (Merritt Reservoir Dark Sky Park where the Nebraska Star Party is held) is magnitude 7.8, but a friend with better eyes managed to reach a verified magnitude of 8.15.  Last year at that very same site, I only was able to hit about 7.1 or so, partly due to my aging eyes and partly due to the reduced transparency from the Canadian forest fires.   At one of my regular dark sky sites much closer to home, I have occasionally seen stars nearly overhead as faint as magnitude 7.1 which would put it just barely in the "Bortle 2" range.  However, it takes a very good clear night to do this, as the site is more typically in the range of magnitude 6.4 to 6.8 (straddling the Bortle 3 and 4 levels).  I generally just use my own eyes and look up to determine what my Zenith Limiting Magnitude for a given night is to put down in my logbook during my observing sessions and leave it at that.  There is an extensive discussion about coming up with potentially better methods for judging how dark a sky is on the Deep Sky Forum, but suffice it to say, the Bortle "scale" has some difficulties and is probably best not taken as some sort of absolute standard.  Clear skies to you.


Edited by David Knisely, 19 February 2024 - 06:09 PM.

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#46 CarolinaBanker

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Posted 19 February 2024 - 08:54 AM

That would be an unfortunate/inaccurate simplification and doesn't correspond with my eyes or my skies. The key to arriving at a rating is in aspects of the scale other than NELM. One has to weigh these to determine what one is seeing in terms of the scale, otherwise, one might rate a site in four Bortle classes at the same time.

The way the scale is presented with respect to NELM isn't quite how it works, certainly not how the average observer's eye works either. NELM was not meant to be the arbiter of Bortle rating. It is ironic, because in the article Bortle made that very clear...yet he went on to put in some generally aggressive NELM (and TLM) values in for his scale, which undermined the message.

Here is what John Bortle wrote in the actual article on the subject:



It has been fairly common recently for my sites that normally run Bortle 3 to run a little worse than that, into the good side of Bortle 4 with typical SQM readings hitting only ~21.2 mpsas. Bortle 4 is very wide. Yet I am still seeing stars fainter than 7.0 mag (Bortle 2 per the table you linked). However, NELM is not the key to rating the darkness of the sky.

For perspective: Bradley Schaefer requested NELM and TLM readings from Sky & Telescope observers in 1989. The data (along with historical study data) was incorporated into a TLM study he published. Despite dark sky skies still being substantially more accessible ~35 years ago, only two of the observers who used Schaefer's sequences (meaning they were part of the new survey vs .legacy data) reached 7.0 or beyond for NELM. Skiff was "only" reaching 7.0, despite observing in very dark skies from what I can tell. Seal reported 7.2. It is doubtful that Skiff was only observing in Bortle 3 sky, and from what I have seen of his period telescopic reports he went deep--most closely matching my own results three decades later.


Your earlier comment only stated the Bortle rating and NELM, with no mention of the DSOs that correspond to the rating. The big issue with the scale is interrater reliability. A beginning observer and an experienced one will differ. A young person with excellent eyesight and an older one with middling or poor eyesight will differ, SQM is objective but until they make it a $1.99 app I can run on my iPhone it will suffer from adoption issues, I’m not terribly interested in spending more than $100 to determine how bright the skies are.

#47 Redbetter

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Posted 19 February 2024 - 05:53 PM

Your earlier comment only stated the Bortle rating and NELM, with no mention of the DSOs that correspond to the rating. The big issue with the scale is interrater reliability. A beginning observer and an experienced one will differ. A young person with excellent eyesight and an older one with middling or poor eyesight will differ, SQM is objective but until they make it a $1.99 app I can run on my iPhone it will suffer from adoption issues, I’m not terribly interested in spending more than $100 to determine how bright the skies are.

You are missing the point of my posts in this thread, especially the initial one:  NELM is not a particularly good indicator of Bortle rating, nor is NELM that useful for rating the darkness of the sky (as Bortle himself explained.)  So saying someone's sky on a given night is Bortle X based on NELM is a fundamental error.  NELM can and does provide some weight toward defining the sky at the time, but it is nowhere near absolute.

 

This is another variation of the trap people fall into in declaring that 6th magnitude is the naked eye observing--which is what I responded to.  Any experienced observer who has observed with a mix of others will recognize the assertion of mag 6 as a limit is inaccurate.  

 

Unfortunately, you seem to be veering the other way now, from a specific mapping of NELM to Bortle scale, to a specific mapping of SQM (mpsas) values to Bortle scale.  While the SQM provides the best measure we have of actual sky darkness, it can't map directly to the Bortle scale because the Bortle criteria simply are not defined that way.  The Bortle criteria are more mixed and subjective.

 

I am not trying to sell meters, but it is somewhat amusing to point to an app as a logical replacement for a meter.  That looks like an example of false economy to me considering what folks spend on their phones every month and consider normal.  With a meter you pay for the device once, and have it for perhaps a decade or more (I bought my SQM-L eight years ago and it is still going strong, so I'll wager a decade.)  Never mind that the apps can be wildly off (there was a recent thread that after some discussion revealed that the person's phone app was running about 0.7 mpsas darker than the various skies they had surveyed with it.)




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