15 replies to this topic

[kingsbishop]

Is there really any difference between a bortle 2 and a bortle 1 sky since bortle 1 is 22 and bortle 2 is 21.9 which is only a 0.1 magnitude difference which sounds like it’s barely anything yet bortle 5 to bortle 4 is around a whole magnitude difference which makes sense but a 0.1 magnitude difference doesn’t sound like anything so is there something I am missing?

[hyiger]

The whole Bortle scale is subjective anyway. I would say SQM is more accurate and certainly more quantative instead of speculative. 

 

Basically if I ask the question "How's the weather?" Bortle is "It's warm" or "it's cold" and SQM is "Temp is 21C, relative humidity is 30%, winds are 5 kph from the north east, chance of precipitation 10%"

Edited by hyiger, 02 April 2024 - 01:51 PM.

[deSitter]

In practice, none. I actually prefer a little skylight so I can see the edge of the field of view. You can see dark lanes in galaxies in Bortle 3 sky.

 

But, here is the generally accepted hand-waving description..

 

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

 

-drl

[kingsbishop]

In practice, none. I actually prefer a little skylight so I can see the edge of the field of view. You can see dark lanes in galaxies in Bortle 3 sky.

But, here is the generally accepted hand-waving description..

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

-drl

but would you even notice any difference in sky background from 21.9 to 22 because 0.1 mpsas difference sounds like nothing

[deSitter]

but would you even notice any difference in sky background from 21.9 to 22 because 0.1 mpsas difference sounds like nothing

It would have more to do with what you can't see - your hand in front of your face

 

-drl

[Tony Flanders]

Is there really any difference between a bortle 2 and a bortle 1 sky since bortle 1 is 22 and bortle 2 is 21.9 which is only a 0.1 magnitude difference which sounds like it’s barely anything yet bortle 5 to bortle 4 is around a whole magnitude difference which makes sense but a 0.1 magnitude difference doesn’t sound like anything so is there something I am missing?

There are many things you are missing.

 

First of all, the statement "Bortle 1 is 22 and Bortle 2 is 21.9" makes no sense. The Bortle Dark Sky scale and zenithal mpsas measurements are not interchangeable. The Bortle Dark Sky Scale is based entirely on subjective measures, and is defined by John Bortle's article in the February 2001 issue of Sky & Telescope.

 

Measurements of the zenithal skyglow in magnitude per square arcsecond can be obtained by various devices, all of which will likely give slightly different answers. The device most often used these days is the SQM-L, the lensed version of Unihedron's Sky Quality Meter. (The unlensed version doesn't have enough angular resolution to measure the zenith without picking up much lower parts of the sky.)

 

And finally, there are two dominant light-pollution atlases that give forecasts of what the zenithal skyglow will be at any given site. It's important to emphasize that these are forecasts, and like all forecasts, will inevitably differ from reality.

 

So, what question are you actually asking? Are you asking about the difference between skies that John Bortle would rate Class 1 versus Class 2? Or are you asking about the difference between skies where an SQM-L gives a reading of 22.00 mpsas versus skies where an SQM-L gives a reading of 21.90 mpsas? Or are you asking about the difference between skies where either lightpollutionmap.info or Light-Pollution Map 2022 forecasts a reading of 22.00 versus 21.90? The answers to each of those questions will be profoundly different; at the level of 21.9 versus 22.0, the differences between those methods are huge.

 

Second, you're completely ignoring transparency. Most experienced observers agree that John Bortle's descriptions of Class 1 and Class 2 don't make much sense to them, but insofar as they do make sense, they depend much more on transparency than on subtle variations of skyglow. However, neither the light-pollution atlases nor the SQM-L say anything whatsoever about transparency.

 

Third, you're glossing over the critical distinction between natural and artificial skyglow. Both of those vary a great deal from one part of the sky to another, from one night to another, and from one hour to another.

 

Just to take one example of angular variation, the SQM-L can never read as dark as 22.00 when the Milky Way is overhead. That doesn't mean the sky is worse when the Milky Way is overhead; it just means that the standard spot chosen for taking measurements -- the zenith -- happens to coincide with an area where the sky is brighter than average. If your objective is to observe the Milky Way, that's a good thing, not a bad thing.

 

As for the variation of natural skyglow in time, it happens that airglow, one of the major components of natural skyglow, has been much stronger than average for most of the past year. That again precludes an SQM-L reading of 22.00, but for completely different reasons. And unless your objective is to observe airglow itself, this is a bad thing for amateur astronomers, since it obscures other natural sources such as the Milky Way. The increase of airglow isn't subtle, by the way. Very few people have recently obtained zenithal readings as dark as 21.8, much less 21.9, much less 22.0.

 

Finally -- and striking truly to the heart of your question -- if you were to assume that the natural skyglow was a uniform 22.00 over the entire sky (which can never be) and the transparency was perfect, then an SQM-L reading of 22.00 could only be obtained at a site where the artificial component of skyglow was zero. Under those same assumptions, the artificial component of skyglow would be quite substantial at a site where an SQM-L read 22.90. So whereas the variation in total skyglow is proportionally small, the variation in the artificial component is proportionally infinite. And since nobody except students of light pollution is interested in observing the artificial component, that's a very bad thing.

 

Moreover, artificial component may still be fairly modest at the zenith, but it's bound to be much greater down toward the horizon, especially if you're looking in the direction of the major light source. That's particularly bad news if you're observing from the northern hemisphere and the brightest light source is due south. In that case, while light pollution will have very small effects on your views of objects that pass directly overhead, it will have major effects on your views of objects at far-southerly declinations. Which, for us northerners, includes the heart of the Milky Way, alas.

 

For those of you south of the equator, the effects of having a bright light source directly to the north have less effect on the Milky Way, but will have a large effect on objects with far-northerly declinations, like the Andromeda Galaxy.

Edited by Tony Flanders, 03 April 2024 - 05:38 AM.

[GeorgeLiv]

The exact same question was asked here. You may also review these threads..1, 2, 3, & 4.

 

One can never find adequate resolution pertaining to sky-glow if you keep obsessing over numbers. For my dark site here, I have gotten SQMs from 21.57 through 22.24 m/arcsec² for clear nights. My sky-glow is unique to my location (100 km away is Ottawa @ my NE), and although one night can look quite similar to another, the position of the summer Milky Way will skew your results by 0.35 m/arcsec². Good Luck!

[Redbetter]

Tony covered it pretty well and in depth. 

 

A more succinct answer would be:  yes.  Anyone who has observed in both can appreciate this even without appreciating the numerical "why?"  Of course the OP would have no idea why, because the question reveals that the OP doesn't understand what the Bortle light pollution scale means.

 

I'll add here that even if the difference of light pollution's effect on the sky was as little as 21.9 vs. 22.0, the relative differences would substantially underestimate the actual visual impact.  Why?  Because what really matters is not just the whole sky darkness but the contrast produced by the sky between stars whether naked eye or in the eyepiece.  Unfortunately, that results in a floating scale that is dependent on aperture (to know what to subtract out.)  

 

However, we can skip the floating reference by subtracting all of the stars down to 18 mag or so.  The background sky without them will be perhaps 23 mpsas or darker.  But the background sky will have a higher delta due to light pollution since it is still producing the same flux as before.  So it becomes a much larger component of the contrast loss (over twice as much.)

[Toddeo]

Why was this question even asked???????????????????

[GeorgeLiv]

For one location near the earth's surface, without MANY readings through the year, a difference of 0.1 on the SQM scale near the apparently magical 22.0 value, is statistically no difference. However, from one night to another, over many nights, an experienced observer can determine if such a small difference is transient or permanent. For example, a few nights in September of 2023, although it seemed quite dark, I was getting 22.35 m/arcsec2. But the stars were dimmed and had a haze around them when viewed through binocs, yes, high-level smoke. Skies in 2023 were atrocious. Removing the effects of the stars/MilkyWay and the somewhat fixed sources of LP you STILL have A BEAST of an atmosphere to consider, which is highly contaminated today PLUS solar activity is far too high for dark skies in the evenings. Before dawn is better, but even in 2022 the skies have been awful, mainly because of the Hunga Tonga–Hunga Haʻapai eruption on January 15 2022.

[KD5NRH]

Is there really any difference between a bortle 2 and a bortle 1 sky since bortle 1 is 22 and bortle 2 is 21.9 which is only a 0.1 magnitude difference which sounds like it’s barely anything yet bortle 5 to bortle 4 is around a whole magnitude difference which makes sense but a 0.1 magnitude difference doesn’t sound like anything so is there something I am missing?

 

This is 15 seconds at ISO800 in a Bortle 1, and I still bumped exposure quite a bit in post to make it look like a Bortle 2.  The actual eyeball view is just the stars, with a Casa Grande shaped chunk of no stars.  If you stare long enough, you start to sort of imagine the outline of the mountain, but it's still hard to be sure if you're really seeing it or just filling it in subconsciously.

 

The High Chisos is an amazing place, day or night.  One of these days maybe I'll feel strong enough to lug a decent tripod up to the South Rim and try some night shots over the Quemadas.

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Edited by KD5NRH, 21 May 2024 - 12:42 AM.

[Sebastian_Sajaroff]

B2 is to B1 like a Porsche 911 is to a Lamborghini.
Yes, B1 is better, but we’re comparing two excellent scenarios.
I’d kill to have my B6 skies back…

[csrlice12]

It's dark out, the stars are out, you're out, a scope is out....the rest don't matter near as much.

[Redbetter]

This is 15 seconds at ISO800 in a Bortle 1, and I still bumped exposure quite a bit in post to make it look like a Bortle 2.  The actual eyeball view is just the stars, with a Casa Grande shaped chunk of no stars.  If you stare long enough, you start to sort of imagine the outline of the mountain, but it's still hard to be sure if you're really seeing it or just filling it in subconsciously.

 

The High Chisos is an amazing place, day or night.  One of these days maybe I'll feel strong enough to lug a decent tripod up to the South Rim and try some night shots over the Quemadas.

At Bortle 1 and 2 sites on good nights the sky is much brighter in visual appearance than what you suggest.   It is pretty obvious in the image once one dims the lights and has a little dark adaptation. I don't have any trouble seeing the outline of objects on the horizon from Bortle 1 and 2 sky.  It is actually a pretty stark difference.  Conifer trees that are closer don't reflect much star light, so they create an even more marked contrast with the sky.

 

I get the impression that some folks never get dark adapted when they claim the natural night sky is so dark.  Once dark adapted one of the things a person can do to understand how the adaptation works is look at a fixed point around the horizon for 30 seconds or so at a dark site, then glance upward around 20 degrees higher.   The former dark outline of the horizon in the visual field will become a brighter patch of sky with the same shape, offset the same ~20 degrees higher.    It takes some time (seconds) for this to fade--the false image is the result of the rods that were formerly seeing darker sky having residual greater sensitivity for a time.

 

Like I said in an earlier post in the thread, in pristine sky the background between the stars is going to be something like 23+ mpsas.  It still looks rather bright visually, whether naked eye, or in the eyepiece.  It takes a very small exit pupil to make the sky background completely indistinguishable from the field stop in an eyepiece when one is fully dark adapted to the new conditions.  For me this exit pupil works out to around 28 mpsas apparent surface brightness.  That is ~100x dimmer than the 23+ mpsas sky between stars in pristine conditions.

[George N]

Why was this question even asked???????????????????

A> Some people like to wear their supposed "Bortle Number" as a chest medal -- "I'm Bortle 1, but you are only Bortle 2 - that's why I see more than you do and my Milky Way wide-field images have more 'likes' on FaceBook!" 

 

B> If you are looking for an extremely dark area for your observing (including maybe wide-field imaging) - and you have a spot 100 miles away that sits on a 'dark sky map' in a claimed Bortle 2 region -- is it worth it to drive another x miles to get to Bortle 1 - with all the baggage of being even more remote and needing to find another good safe legal spot to set up for a few nights. What would you get for all that effort and $$?

 

C> Go read Bortle's original article (https://skyandtelesc...dark-sky-scale/) - the first paragraph - on 'why' << partial quote: ....(an) answer to this question is useful for comparing observing sites and, more important, for determining whether a site is dark enough to let you push your eyes, telescope, or camera to their theoretical limits..... more..... >>

 

But it's probably "A" above......  

 

Or maybe we "Bortle Scale purists" just need to recognize that whatever John said in his S&T article - to most of the world today - including the world of younger amateur astronomers and the popular media - 'Bortle Number' has just become the title for regions on on-line dark sky maps? It is never going back to what John intended - mostly a visual observation of sky and ground conditions and a little bit about limiting magnitude - naked eye and visually in a mid size telescope - and nothing to do with quantitative measures of sky darkness with a device like an SQM.

[kingsbishop]

A> Some people like to wear their supposed "Bortle Number" as a chest medal -- "I'm Bortle 1, but you are only Bortle 2 - that's why I see more than you do and my Milky Way wide-field images have more 'likes' on FaceBook!"

B> If you are looking for an extremely dark area for your observing (including maybe wide-field imaging) - and you have a spot 100 miles away that sits on a 'dark sky map' in a claimed Bortle 2 region -- is it worth it to drive another x miles to get to Bortle 1 - with all the baggage of being even more remote and needing to find another good safe legal spot to set up for a few nights. What would you get for all that effort and $$?

C> Go read Bortle's original article (https://skyandtelesc...dark-sky-scale/) - the first paragraph - on 'why' << partial quote: ....(an) answer to this question is useful for comparing observing sites and, more important, for determining whether a site is dark enough to let you push your eyes, telescope, or camera to their theoretical limits..... more..... >>

But it's probably "A" above......

Or maybe we "Bortle Scale purists" just need to recognize that whatever John said in his S&T article - to most of the world today - including the world of younger amateur astronomers and the popular media - 'Bortle Number' has just become the title for regions on on-line dark sky maps? It is never going back to what John intended - mostly a visual observation of sky and ground conditions and a little bit about limiting magnitude - naked eye and visually in a mid size telescope - and nothing to do with quantitative measures of sky darkness with a device like an SQM.

lol i like to wear my earliest astronomical twilight ending time in winter in the evening as a chest medal but i can only do that to people in my country as i am in Australia and most people in the Northern hemisphere have an earlier astronomical twilight ending time in Winter evenings