An expanded Bortle Scale for modern times

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An expanded Bortle Scale for modern times

#1

Post by Lokifish »


Due to my skies being nothing but clouds, I watched live streams of the Alpha Monocerotids. While the "storm" didn't happen, it was not what stuck with me. I noticed a reoccurring theme in chat on multiple live streams. It was an unrealistic level of expectation, and a lack of understanding of how much light pollution affects viewing such events. Most negative comments were along the lines of "I live in (insert city here) and haven't seen squat."

This got me thinking. The original Bortle Scale by John E. Bortle was written in 2001, and essentially gave up after Bortle 9. As light pollution has been an exponentially growing problem, 18 years is also long time for light pollution levels to increase. In that time astrophotography has also become easier, the cameras less costly and far more sensitive, and the U.S. Solar Eclipse rekindled interest in astronomy for many. So with that, and the observation of multiple chats, I thought it time to expand the Bortle scale. The expansion is to better reflect our current skies, and be a better reference point for those living under the ever brightening night sky.

I know some may disagree with my expanded chart, but I am open to feedback from the community as it uncharted territory. I would also be very keen on fine tuning the expanded SQM column at some point with help from the community. As we are seeing SQM readings pushing single digits in some areas, the original cross reference charts fall apart.



Clear skies, and revel in your uniqueness in this vast universe
Russell Hippert

Link to live Expanded Bortle scale
https://docs.google.com/spreadsheets/d/ ... sp=sharing

Attached PDF is current as of the date of this post.
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Re: An expanded Bortle Scale for modern times

#2

Post by Lady Fraktor »


Thank you for the article Russell :)
My local skies are between a Class 4-5 according to the scale with occasional nights going to Class 3.
Unfortunately I have never paid much attention to the scale and have rarely used a meter so cannot provide any extra information for you.
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Re: An expanded Bortle Scale for modern times

#3

Post by notFritzArgelander »


It's an interesting proposal. At my current location in Pocatello ID I get a measured NELM of 5.48. I'm fortunate that I get an extra magnitude over the average. When I was younger I got 1.5 but the pupils don't dilate as much.
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Re: An expanded Bortle Scale for modern times

#4

Post by JayTee »


The problem was all of these scales is that you are basing an evaluation on a subjective judgment. If everyone's Mark I eyeball performed exactly the same then this would make sense. Unfortunately, every eyeball is different. So your Bortle/Hippert observation may be a Hippert 4 to you, but my observation on the same night at the same time and location, may rate it at a 5 or even a 6 because I'm older and my pupils don't dilate as much.

The only way to get past using the human eyeball and level the playing field is to base all of your gradations on an SQM reading and I have pretty good knowledge on that because I own an SQM-L.

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Re: An expanded Bortle Scale for modern times

#5

Post by Lokifish »


JayTee wrote: Sun Nov 24, 2019 9:05 pm The problem was all of these scales is that you are basing an evaluation on a subjective judgment. If everyone's Mark I eyeball performed exactly the same then this would make sense. Unfortunately, every eyeball is different. So your Bortle/Hippert observation may be a Hippert 4 to you, but my observation on the same night at the same time and location, may rate it at a 5 or even a 6 because I'm older and my pupils don't dilate as much.

The only way to get past using the human eyeball and level the playing field is to base all of your gradations on an SQM reading and I have pretty good knowledge on that because I own an SQM-L.

JMHO,
JT

Thanks for pointing that out! I agree that NELM variance has always been an issue. Why it was included is many of the often referenced equivalency charts also include it. I'll add a notation on how age and other factors affect NELM. That shouldn't throw off the table in general as we are dealing with averages.
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Last edited by Lokifish on Sun Nov 24, 2019 9:44 pm, edited 1 time in total.
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Re: An expanded Bortle Scale for modern times

#6

Post by helicon »


My NELM is about 4 on bad nights and 5 on good nights, the light pollution tends to vary based on cloud cover in the area. Many times each year I am above the clouds and fog, which tends to help obscure city lights. Most of the time the fog level is 2000-3000 feet thick, so I am clouded out as well.
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Re: An expanded Bortle Scale for modern times

#7

Post by Arctic »


I think the statement that most folks cannot see past a NELM of 6 is highly speculative.

At my house a night of average/decent transparency will reveal mag 6.3 stars to my eye. A superb night will show reveal mag 6.7 or fainter.
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Re: An expanded Bortle Scale for modern times

#8

Post by Lokifish »


If this will help clarify things, here's the factors I considered before I put this together.

Bortle scale and NELM are purely perceptual scales. One could argue that neither are valid for numerous reasons. You could take the dimmest documented unaided observation and counter that using stellar limiting magnitude based on pupil diameter. Another could then counter with photon count and rod response. That can be countered by fluid and lens clarity, and presence of floaters. This could then be countered by variances pupillary response and contrast ratio required for recognition. One could even go into spectral response, which would then be counter by individual response variances. As cameras and optics are also affected by similar issues, same applies there.

SQM, has issues as well. There are environmental factors like weather, particulate density, the characteristics of the particulates, and so on. This is without adding user error, and variances between readings and meters. Even if SQM readings were 100% accurate, the very second one asks "what can I see" it reintroduces all the issues of Bortle/NELM. This still doesn't get into one's experience observing dim objects, or anomalies (environmental or physical).

The table is not best case or worse case. The table is an extension based on what has been cited and used by the astronomy community as a whole. Even Bortle contradicts himself on this matter by saying NELM can't be trusted then giving examples of what can be seen using magnitude. So is this or any chart perfect? No, for the very same reasons I mention above. About the closest one could get to an accurate table is to base it solely on sky glow photons received vs stellar photons received, and leave the rest to a "must be you" reason that everyone would fall victim to. Well except for maybe Julius Schmidt.
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Re: An expanded Bortle Scale for modern times

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Post by notFritzArgelander »


Lokifish wrote: Mon Nov 25, 2019 5:40 pm If this will help clarify things, here's the factors I considered before I put this together.

Bortle scale and NELM are purely perceptual scales. One could argue that neither are valid for numerous reasons. You could take the dimmest documented unaided observation and counter that using stellar limiting magnitude based on pupil diameter. Another could then counter with photon count and rod response. That can be countered by fluid and lens clarity, and presence of floaters. .......
Exactly. One can measure ones own "personal correction" to the scale in order to account for individual physiological differences in sensitivity. I occasionally do this using the Bortle scale and, for instance, counting Pleiades. Or I use a telescope and use a standard magnitude sequence in one of the open clusters compared with the usual predictions for the average 6.2 magnitude limited eye. That is how I know that in my youth I did 1.5 magnitudes better than average and now only about 1.0.

It's just scientific method to apply individual sensitivity corrections. (Any astrophotographer knows this. Thats what darks and flats are for.) It's not the fault of the scale if I don't measure my personal sensitivity!
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Re: An expanded Bortle Scale for modern times

#10

Post by Steveinit »


Do you mind if I make a copy to take some note on? I'd like to compare it against the sheet I validate a site's (and) clear outside bortle rating.
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Re: An expanded Bortle Scale for modern times

#11

Post by Bigzmey »


Hi Russ
Your extension looks good to me. However, I am not sure what it could be used for. Once you get to white / Bortle 8 skies you stop observing and go to the pub to drown your sorrows. Not sure if exact shade of white matters. :)
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Re: An expanded Bortle Scale for modern times

#12

Post by mikemarotta »


Lokifish wrote: Sun Nov 24, 2019 6:50 pm ... The original Bortle Scale by John E. Bortle was written in 2001, and essentially gave up after Bortle 9. As light pollution has been an exponentially growing problem, 18 years is also long time ...
Thanks! I printed both versions for my astronomy viewing notebook and saved them both on my computer, of course.
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Re: An expanded Bortle Scale for modern times

#13

Post by Lokifish »


Bigzmey wrote: Tue Nov 26, 2019 1:41 am Hi Russ
Your extension looks good to me. However, I am not sure what it could be used for. Once you get to white / Bortle 8 skies you stop observing and go to the pub to drown your sorrows. Not sure if exact shade of white matters. :)
Thanks.

The expanded list is more for outreach and helping novices have a simple reference point. I know I wish I had done this before the last club outreach I volunteered for. The view between that event's B9/white skies, and another popular event my club hosts were vastly different. The worse one was more like a B13 on my expansion.

Nah. No pub, just drunk astronomy. Makes you forget how bad the skies are, and you discover things you drunk mind forgot were there.
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Re: An expanded Bortle Scale for modern times

#14

Post by Harmonious »


Things I do not do include measuring the sky brightness above my little roll off roof shed, attempt to assign a Bortle number to the visible sky or any part of it or look online for charts and maps that purport to show the numbers for my sky. Why? Because it is a pointless activity. The conditions are what they are and will become worse barring some sort of crisis. I do use software to assess sky conditions for setting camera controls on occasion but the electrons per numbers are not in themselves interesting.

Every year the number of people living in the state increases as does the number of commercial airline flights leaving visible trails, the number of satellites captured in images, the number of cars turning highways into rivers of lights (moving and fixed). Lighting is converted to LED but at higher illumination levels and buildings and bridges are lighted as art. The false premise of lights = security spreads from cities to suburbs, THE F'ING MARCH OF PROGRESS.

I live in a town of under 17,000. It was a town of under 15,000... and once it was home to a few dozen year around residents and a lot of pear trees. As a Boy Scouts we camped nearby under a sky so dark it looked like the special effect sky in movies and TV and yet is was only about 12 miles from a major urban area. Now the town wants more and brighter LED street lighting, the college adds lights every year to make the campus safe for women and proposes to operate lighted sports facilities 24/7.

The cities and their suburbs merge and then these complexes spread along freeways and connect and before you know it you have a chain of light that runs from the ocean to the mountains and down the great valley for 200 miles. Progress always trumps astronomy and that's why a scale that differentiates degrees of badness for people under a bad sky is irrelevant.
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Re: An expanded Bortle Scale for modern times

#15

Post by kt4hx »


Hi Russ and thanks for your article. I have been using the Bortle scale for many years as one tool for assessment of the night sky, be it at home in our typical suburban sky or at our dark site house in the western part of the state. The Bortle has been praised and criticized, but for the most part I have found John’s original criteria reasonably accurate for my situations. I know in reading John’s posts on astronomy sites and in my own communications with him he has staunchly defended his scale as a very accurate tool for real time assessment of one’s sky in any given moment.

He specifically is not a fan at all of some trying to correlate his scale to the colored maps/scales that are sometimes employed,. That is simply because they are static in nature, whereas his scale is a variable one, with real time changes possible over the course of an evening. The maps are of course based on nighttime satellite imagery with an algorithm applied to estimate the spread of sky glow over terrain. While they can be a useful tool for identifying locations that might possibly be darker, they don’t give any assurances. I see them as a broad brush approach to locating sites that still need to be vetted to ascertain their true usefulness. John will also argue against any correlations between his scale and the SQM methodology. He truly believes his scale stands alone without embellishment. :)

On a personal note, our dark site varies from as good as a Bortle 2, with 3 being typical, to increasingly poorer levels as conditions deteriorate. That is the beauty of the scale, it slides with changes in conditions. However, as JT points out, which you echoed, individual optical variation and experience can noticeably impact where one falls on the Bortle scale. So as with NELM measurements, which I never use personally, the Bortle scale is not equal for all people all the time.

Mention was made of the SQM, which I also own, having an SQM-L. As you rightly pointed out, there can be variation there as well because of the unit’s battery status and its calibration accuracy. I suppose there could be some user error, but I don’t see that as a significant factor as they are quiet easy to use. Again, though there are variables associated with the meter, I still use it to get a sense of the actual sky glow levels at our dark site. I do find it a useful tool, bearing in mind, the potential flaws.

As to your expanded scale, I tend to agree with Andrey that there doesn’t seem to be any significant utility for levels beyond a Bortle 9. While I understand your thoughts that perhaps for beginners it might be useful for them to gain a perspective, I don’t believe it takes a lot of experience to understand that if you assess your conditions at a Bortle 9, you are in a seriously compromised area. It is obvious you have given a lot of thought and study to the subject matter to come up with your expanded scale. I personally appreciate and applaud your interest in the subject matter and the obvious desire to take a serious look at the Bortle scale. However, I am just not sure how much true benefit will come from going beyond Bortle 9. As I said, if you know you are in Bortle 9, then you are well aware that you are in deep kimchi and anything worse would just be more depressing.

On a side note, I recall an argument I had with John about observing in general. He stated to someone that if he had to observe from anywhere brighter than a Bortle 3, he simply would give up the hobby. While I understood his sentiment, I countered that his way of thinking was very defeatist in nature and discouraging to a lot of folks. I told him I simply could not agree with his thinking because even from our Bortle 5 backyard, I could still observe a vast number of galaxies and other DSOs with my dobsonian reflectors and would never exhaust the potential targets from that location. Anyway, my stance didn’t persuade him, nor did I expect it to. However, I wanted other readers to not be discouraged because they have to deal with conditions worse than Bortle 3. So we basically agreed to disagree! :lol:

Again, thanks for your hard work and thought on this expansion. I hope some find it useful for their purposes in the field, and just for those folks who may not yet know the full criteria of the original Bortle scale, it can be found at the below link.

https://www.skyandtelescope.com/astrono ... sky-scale/
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Re: An expanded Bortle Scale for modern times

#16

Post by Lokifish »


Alan

If what you say is correct, then John really did give up when he reached the 8/9 threshold. The scale being considered variable is also an odd concept as the scale is mostly static, it's the Bortle at the time of measurement. So just like SQM and NELM, it's only valid for the time when it was measured and will change as the night progresses. As a result, it suffers the same failings which I accept. For it being standalone, all the scales can be standalone. However, there is a general equivalency between them that can be used for cross referencing.

As far as the utility beyond 9, I make this case. Using the exact same setup, image/view under a sky that's just barely a 9, as well as the brightest inner city skies one can find. Under the current scale there is no distinction between the two, even though the number of observable objects varies significantly. So currently saying one is under Bortle 9 skies or white zone is meaningless as even our Sun going 1a nova would be Bortle 9. This leaves NELM, SQM, and "your skies are too bright" as a response to folks wondering why they can't see something. The first two are problematic for various reasons. The third lacks any real reference or educational points for the individual not well versed in these things. The expanded scale attempts to resolve that in a manner that even the "what's a galaxy" crowd can understand. Especially considering that the bulk of outreach is done with folks under those ambiguous Bortle 9 skies.
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Re: An expanded Bortle Scale for modern times

#17

Post by kt4hx »


Lokifish wrote: Wed Jan 08, 2020 3:02 am Alan

If what you say is correct, then John really did give up when he reached the 8/9 threshold. The scale being considered variable is also an odd concept as the scale is mostly static, it's the Bortle at the time of measurement. So just like SQM and NELM, it's only valid for the time when it was measured and will change as the night progresses. As a result, it suffers the same failings which I accept. For it being standalone, all the scales can be standalone. However, there is a general equivalency between them that can be used for cross referencing.

As far as the utility beyond 9, I make this case. Using the exact same setup, image/view under a sky that's just barely a 9, as well as the brightest inner city skies one can find. Under the current scale there is no distinction between the two, even though the number of observable objects varies significantly. So currently saying one is under Bortle 9 skies or white zone is meaningless as even our Sun going 1a nova would be Bortle 9. This leaves NELM, SQM, and "your skies are too bright" as a response to folks wondering why they can't see something. The first two are problematic for various reasons. The third lacks any real reference or educational points for the individual not well versed in these things. The expanded scale attempts to resolve that in a manner that even the "what's a galaxy" crowd can understand. Especially considering that the bulk of outreach is done with folks under those ambiguous Bortle 9 skies.
Given how John feels personally about observing under conditions any worse than B3, then one could look at it in a sense of he did give up at B9. Likely figuring what's the use in going any worse. Obviously I cannot speak for him on that point, but I at least suspect that might be his rationale.

Agreed, all the scales mentioned - Bortle, NELM and SQM, are variable based on the changes in localized conditions, often times rapidly over the course of an evening. If I seemed to indicate otherwise that was not my intention. The only LP tool mentioned that is static are the maps using the colored zones. That is why John can get a bit hot and bothered when someone tries to correlate his scale to the maps/colored zones. To that point I was pleased to see you did not follow that trend. Though they are both indicators of sky quality, they are derived in very different manners. However, with a properly calibrated SQM, I believe there can be some correlation between the meter and the Bortle scale, though I am quite sure John would argue otherwise. :)

Pertaining to your extension of the Bortle, I admit that I do not typically observe under any conditions worse that Bortle 5 to 6. Since my own backyard is B5 (on average) and the dark site B3 (on average) and those are the only two places I observe from on a regular basis, then I cannot comment on the true utility of going beyond B9. That is other than to say for myself, I don't see any significant benefit. If someone at a outreach function were to ask why they cannot see such and such from there, I believe that explaining to them, in a simple and concise manner, about the impact of sky glow due to the abundance of ground lighting emitted outward and upwards, that would (or should) carry more weight than using a scale which they would in all likelihood not grasp easily. Again, that is my personal viewpoint, and I concede I am no expert in the subject matter, but merely someone who has utilized the Bortle and SQM methods for several years. This may ultimately turn out to be that we simply agree to disagree upon the utility of the expanded Bortle scale. And of course that does not make it right or wrong, just differing viewpoints.
Alan

Scopes: Astro Sky 17.5 f/4.5 Dob || Apertura AD12 f/5 Dob || Zhumell Z10 f/4.9 Dob ||
ES AR127 f/6.5 || ES ED80 f/6 || Apertura 6" f/5 Newtonian
Mounts: ES Twilight-II and Twilight-I
EPs: AT 82° 28mm UWA || TV Ethos 100° 21mm and 13mm || Vixen LVW 65° 22mm ||
ES 82° 18mm || Pentax XW 70° 10mm, 7mm and 5mm || barlows
Filters (2 inch): DGM NPB || Orion Ultra Block, O-III and Sky Glow || Baader HaB
Primary Field Atlases: Uranometria All-Sky Edition and Interstellarum Deep Sky Atlas
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"Astronomers, we look into the past to see our future." (me)
"Seeing is in some respect an art, which must be learnt." (William Herschel)
"What we know is a drop, what we don't know is an ocean." (Sir Isaac Newton)
"No good deed goes unpunished." (various)
Some people without brains do an awful lot of talking, don't you think?” (Scarecrow, The Wonderful Wizard of Oz)
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