Bortle scales and Millicandela measurements

[vlaiv]

23 minutes ago, jetstream said:

I really dislike the Bortle scale... all this, all that, can you see this can you see that. On top of this transparency changes the Bortle rating IMHO.

If the MW is bright and jagged, sharply defined with the Opi split showing down to the horizon things are about as good as they get. 21.7-21.8 skies will reflect the MW on a nice shiny car hood...or a calm lake.

The MW will go soft in less than transparent skies and when the readings go below 21.3 or so IME.

Vlaive, I wish you could come up with a simple sky rating for people.

I'm quite happy with mag rating - single verifiable / measurable number. What is not to like about it?

It suits me perfectly for imaging, and although it does not tell the whole story about observing site - unlike something like this:

(btw anyone knows software that produces such image? Guide cam with all sky lens is enough to record needed data - it's just calibration and nice display after that)

it is enough to do some rough calculations on what to expect from a night of imaging of particular target.

On the other hand - I have no idea how to relate that to what to expect from observing. I think we have all the data needed to build decent model of what can be seen or rather what would average person expect an object to look like given:

- scope, eyepiece

- SQM reading

- extinction (elevation, altitude, AOD)

couple that with some facts about human vision - like ability to detect only several photons high signal (like 5-7 photons) and that there is 1000:1 contrast at any one time, I still don't have a clue how to put all of that together.

I don't know the answer to a simple question - if SQM reading is mag20 and nebula brightness after extinction is mag23 - will contrast be enough to be detectable by human eye (I can tell you in percentage how brighter nebula will be compared to background but that tells me nothing if it will be visible by human eye)?

[pete_l]

I would put a large degree of uncertainty into Bortle / SQM data. My personal experience is that Mr. Bortle's descriptions do not match well with my reality. I am sure they work for him, in his country, but for me it is a different story. There are many reasons I can think of, not the least is the eyesight of the individual performing the assessment .

With SQM readings, there is no single value. My site varies from 21.3 to 22.0 according to my meter and the quality of the views changes with that. As others have said: transparency is a huge factor. The amount of water vapour in the air makes a significant difference. As does the amount of turbulence (seeing). The only real solution is to go to a place and check it out. Apart from anything else, that is just about the only way to assess non-astronomical questions, such as the placement of trees, paths, access and having a flat place to set up.

[vlaiv]

10 minutes ago, pete_l said:

I would put a large degree of uncertainty into Bortle / SQM data. My personal experience is that Mr. Bortle's descriptions do not match well with my reality. I am sure they work for him, in his country, but for me it is a different story. There are many reasons I can think of, not the least is the eyesight of the individual performing the assessment .

With SQM readings, there is no single value. My site varies from 21.3 to 22.0 according to my meter and the quality of the views changes with that. As others have said: transparency is a huge factor. The amount of water vapour in the air makes a significant difference. As does the amount of turbulence (seeing). The only real solution is to go to a place and check it out. Apart from anything else, that is just about the only way to assess non-astronomical questions, such as the placement of trees, paths, access and having a flat place to set up.

I'm sure that quality varies between nights but on a single night with two or more repeated measurements in same direction - there probably is no much difference in obtained value?

[pete_l]

16 minutes ago, vlaiv said:

I'm sure that quality varies between nights but on a single night with two or more repeated measurements in same direction - there probably is no much difference in obtained value?

Yes, that is my experience. It is helpful as a way to perform relative assessments over a period of time near my house. For that my SQM is a useful tool.
But just as you can't plot a graph from just a single point, a single SQM measurement on a single night (or even a series of measurements over a few minutes) with many unknown variables doesn't give much of a basis for comparing with other places on other nights under different sky conditions.
I find that even on a single night with no Moon, the darkness I measure at (say) 22h in the winter can be different from another measurement 4 hours later. I suspect that much of the difference is due to increased sky transparency, so less light backscattered from the nearest town/

Edited December 31, 2019 by pete_l

[Second Time Around]

I too wonder how accurate the scales are, especially Bortle.

According to Clear Outside, where I live is Bortle 4, mag 20.83.  However, it doesn't seem as good as that.

I'm guessing that the magnitude scale is based on the 2015 survey (anyone know?) Certainly light pollution has got worse here in recent years.

Also, these last few months have seen very few clear nights, and even then transparency has been poor.

Finally, my eyes are not nearly so young as they were first time around stargazing.  Indeed, my pupil opens to a maximum of only 4.5mm now.  

What effect would the latter make, and would there be a difference between naked eye and telescope limiting magnitude?

[estwing]

On 30/12/2019 at 13:53, bomberbaz said:

Just viewing Viaiv, I have been to Galloway once and it sticks in my memory how beautiful inky black the skies were, if they are anything like that I know they will be perfect. 

Yeah I can confirm that @mapstarraves how good it is at the star party there.

[Ships and Stars]

I'm a bit obsessed with Bortle and SQM readings, nearly ordered a SQM-L meter online a few days ago but hesitated after reading a few articles. If you take SQM-L readings, one caveat apparently is that the sensor doesn't deal well with LED light pollution, my understanding is that is doesn't register it well and will give a darker reading that what it actually is. I don't have enough experience to really rely upon visual limiting magnitude with the naked eye, but those really good nights are readily apparently as soon as I step out of the van at my dark sky spots and glance at the Milky Way - you know it's going to be great when everything just pops out.

I can flip-flop between trying to view from home (not very good) and 21.5-21.9 sites, the 21.5 site is 20 minutes away but half of the sky is restricted by trees. Still, can be quite a good site, that's where I got my first proper views of the Veil and Dumbbell, etc. my true dark sky site is supposedly around 21.92-21.94, is about an hour and 15 min away, and is remote. On a really dark night from there I can detect faint sky-glow from the central belt of Scotland/Dundee some 60-70 miles away to the south, if I go north, I start picking up skyglow from Aberdeen so it's in a sweet spot.

If you have the time I agree with your approach - go for the darkest site when possible instead of trying to do visual from light polluted areas, I find that is often an exercise in frustration. Fewer nights under darker skies seems to work a lot better for me than frequent ho-hum sorts of nights battling light pollution.

I've family in hospital so don't really have a choice now, viewing is on hold for the time being, not much I can do about that. Hopefully by mid to end of January I'll have another chance and things will improve health-wise on the home front.

Edited December 31, 2019 by Ships and Stars

[jetstream]

9 hours ago, vlaiv said:

What is not to like about it?

I agree! some have reservations about accuracy, but they are deadly accurate. Thats the problem really- they show up very small differences in darkness, which is affected by transparency.

 

9 hours ago, vlaiv said:

I don't know the answer to a simple question - if SQM reading is mag20 and nebula brightness after extinction is mag23 - will contrast be enough to be detectable by human eye (I can tell you in percentage how brighter nebula will be compared to background but that tells me nothing if it will be visible by human eye)?

Have you heard of Mr Clark and Mr Blackwell Vlaiv?

" A low-contrast object is more easily detected if it is larger. For an extended object such as a galaxy viewed in a telescope, magnification does not change the contrast with the background, because both the sky's and the object's surface brightnesses are affected equally "

Understanding extended objects is key to seeing them well.

https://clarkvision.com/articles/visastro/omva1/index.html

[jetstream]

8 hours ago, vlaiv said:

I'm sure that quality varies between nights but on a single night with two or more repeated measurements in same direction - there probably is no much difference in obtained value?

Yes, if transparency is stable-aerosols,dust and whatever else- any change here can change the reading.

Of note and not mentioned much is the effect of the Milky Way-under excellent skies the MW can show up to almost .5 magnitudes brighter, with my average here just over .25 mag. This is huge...

If the SQM picks up the edge of the MW one time and not the next the readings will differ.

The SQM is accurate and points out that sky darkness is not a static quality IMHO. Oh yeah, they need a few readings to heat the sensor up or they read high. I usually take 4 or 5 readings and then use the ones after that- when the readings stabilize.

[PeterW]

Friends have told me that e difference from suburbia to mag21.5 is about the same as from the mag21/5 to perfection (la Palma), tiny SQM differences can mask a lot, Mr Bortles descriptions seem to offer some differentiation, but not having enough nights under good enough skies I can’t tell how useful they are.

 

peter

Ive been under skies with SQM reading, the clouds came over, same SQM ;-)!

[vlaiv]

10 minutes ago, PeterW said:

Ive been under skies with SQM reading, the clouds came over, same SQM ;-)!

That is to be expected - most of LP is in fact in lower parts of atmosphere where atmosphere is denser. Also - almost all source of LP is ground based. SQM can't distinguish if light coming from the sky is ground LP scattered in atmosphere or reflected of clouds, but we can be pretty sure there is same amount of LP originating from the ground.

[vlaiv]

Here is an interesting question related to observing and sky brightness.

When looking at different Bortle scales I noticed something interesting. Here is example from wiki page on Bortle scale, it says:

Bortle 1 sky (let's say that is SQM 22) - by using 32mm (or 12.5") telescope, limiting magnitude is about 17 (on wiki page it says 17.5 but I think that is a stretch, as 12.5" aperture can at best deliver about x2090 more light than naked eye, and NELM is said to be mag7.6 - mag8, while x2090 represents difference of ~8.3 magnitudes - those two combined give 15.9 - 16.3 mag rather than 17.5).

In any case - I selected mag17 as limit for ease of calculation. How come that we can't distinguish such star from background even if difference in brightness is whole 5 magnitudes - sky being mag 22 and star being mag 17?

Maybe mag 17 star does not give off enough photons to be detected? I don't think so as mag 18.5 sky looks bluish/gray in the scope, and we can do calculation on how many photons does it produce. Mag 0 star will do something like 980,000 photons per second per cm^2. Mag 17.5 (again let's use this number as it is easier to calculate) has 10e-7 less photons than that, so it is 0.098p/s/cm^2.

12.5" scope has something like ~800cm^2, so it captures ~78.8 p/s. Now we have figure of 5-7photons being detectable by human eye (in single burst - will be seen as a flash) - having x10 as much every second or that much every 0.1s is going to be detectable by eye.

In any case, we can do the same for stronger LP - Bortle 4 is SQM 20.4-21.3 and limiting mag in 12.5" scope is now 15.5 - again about 5 magnitudes difference (x100 intensity), but plenty of photons this time.

So there is enough contrast - x100 but sky prevents us to see the star?

Maybe issue is with surface vs point brightness. Surface is per arc second, and our eye can only resolve about one arc minute. Maybe it sees integrated brightness of sky over that one arc minute, so it sees mag 8.89 brighter sky than SQM says (in terms of photon count)?

Let's go with that - and case of SQM 21 and limit magnitude of 15.5 and try to see if it "fits". 21 - 8.89 = ~12.1 and star is now 15.5. Sky is brighter 3.4 mags then star that is 4.3% difference in brightness (sky alone vs sky+star). That makes sense.

Converted in magnitude difference (not sky to star, but contrast - sky to sky+star) that would be  -2.5*log(1/1.043) = ~ 0.05 mag difference.

I already posted question in variable /double star section of SQL - what is magnitude difference that human eye can see. I expected answer to be something like 0.1mag or similar - but 0.05 mag is not far off.

 

[vlaiv]

It looks like my above calculation was pretty good

Thanks to @Martin Meredith for pointing out what to look for in the thread I posted about magnitude difference (and that is Weber's Law and JND - just noticeable difference), I was able to find that difference in intensity of light that we can distinguish is something like 1%-3% according to this:

https://www.sciencedirect.com/topics/engineering/just-noticeable-difference

That is fairly close to 4.3% that I got from assuming that our brightness perception needs to be considered in mag/arcmin^2 rather than mag/arcsec^2 when comparing to star brightness (as we can't resolve past 1 arc minute) and from Bortle scale data on limiting stellar magnitude under certain SQM reading.

We now have a tool to do simulation of how the DSO will look like under different circumstances: SQM, alt, elevation, AOD, telescope/eyepiece combination. That is something I wanted to do for a long time - make image of what can be expected to be seen at eyepiece for given conditions - but never really knew how to go about it.

[vlaiv]

For anyone interested in simulation of effect of SQM reading - and other related bits - here is separate thread: