Bortle scales and Millicandela measurements

[bomberbaz]

First of all, hello guys and long time no see.  I have been around just not been commenting as much, hope to get a little more active again going forwards as my circumstances have changed somewhat.

I have grown tired of observing from light polluted skies. A bortle 4 site I visit quite a lot  has a millicandela measurement of 0.360 mcd/m2 but has had all the lights converted to lcd at the nearby town. There is now a horrible sky glow ruining the horizon. So my intention going forward is to spend less time at these now poor sites, fewer trips but travelling further to much darker sites.

Anyway, to the topic title. I realise bortle reading is important, but also that it can vary quite a lot but still remain a same numbered site, in this case 4.

I have a few prospective sites for possible observing to examine near to me, one is a confirmed area, three others look positive but need a proper recce. The confirmed site is also bortle 4 as above but the millicandela reading is 0.240 mcd/m2, the unconfirmed sites are all bortle 3 go down to as little as 0.210 mcd/m2.

So my question is, how big a difference is the millicandela readings. I really don't get it in technical terms even though I have spent time reading up on it. Obviously a difference of 0.360 (as above) to 0.240 mcd/m2 as at the confirmed site is going to be quite significant. But is 0.240 to 0.210 going to make a huge difference.

I know the stock answer is darkest site possible, but the confirmed site is the best option. 

Come on you techy types, let me know your finding please.

regards

Steve.

[vlaiv]

Maybe best type of unit to be used is mag/arcsec^2, or simply magnitude (surface brightness - not to be confused with stellar magnitude although it is the same thing).

Magnitudes are better suited to our visual perception than physical quantities because our eye/brain works on close to logarithmic scale.

Surface brightness of targets is also given in these units (or mag/arcmin^2 and here it is simple conversion as add 8.89 to get mag/arcsec^2 - but these tend to be average brightness) and you can estimate visibility from that.

According to SQM calculator page (found here), to convert from cd/m2 to mag/arcmin2 use:

or rather opposite one in above case.

We have 0.24 mcd/m2 and 0.21 mcd/m2, let's convert that to mag/arcsec2

-2.5*log(0.00024 / 108000)  = 21.633

-2.5*log(0.00021 / 108000) = 21.778

How big a difference is 0.14 magnitudes? Not much - change in transparency from excellent to very good will make about 0.1 mag attenuation of targets. Single air mass has about mag 0.16 attenuation.

[jetstream]

I can say that there is a big difference when the sky reads 21.8 mag from a 21.4/21.5  transparency being equal, however the lower readings are associated with lower transparency levels usually around here.

That being said- I would take a 21.5 mag super transparent sky over a higher SQM reading at lower transparencies. Transparency is a major factor in how well DSO show in the eyepiece.

[jetstream]

8 minutes ago, vlaiv said:

Maybe best type of unit to be used is mag/arcsec^2, or simply magnitude (surface brightness - not to be confused with stellar magnitude although it is the same thing).

Magnitudes are better suited to our visual perception than physical quantities because our eye/brain works on close to logarithmic scale.

Surface brightness of targets is also given in these units (or mag/arcmin^2 and here it is simple conversion as add 8.89 to get mag/arcsec^2 - but these tend to be average brightness) and you can estimate visibility from that.

According to SQM calculator page (found here), to convert from cd/m2 to mag/arcmin2 use:

or rather opposite one in above case.

We have 0.24 mcd/m2 and 0.21 mcd/m2, let's convert that to mag/arcsec2

-2.5*log(0.00024 / 108000)  = 21.633

-2.5*log(0.00021 / 108000) = 21.778

How big a difference is 0.14 magnitudes? Not much - change in transparency from excellent to very good will make about 0.1 mag attenuation of targets. Single air mass has about mag 0.16 attenuation.

Hey Vlaiv- I'm curious- how many airmasses at the horizon compared to around 45 deg up and also zenith? I uses visible horizon stars as part of sky assesment.

[vlaiv]

1 minute ago, jetstream said:

Hey Vlaiv- I'm curious- how many airmasses at the horizon compared to around 45 deg up and also zenith? I uses visible horizon stars as part of sky assesment.

For "normal" angles it is just 1/cos(angle)

You can use short cut cos values for 30, 45 and 60 degrees

and that will give you air mass at each angle as:

0 degrees = 1

30 degrees = 1/(sqrt(3)/2) = 2 / sqrt(3) = ~1.1547

45 degrees = 2/sqrt(2) = ~1.4142

60 degrees = 2

But as you start approaching horizon - above formula will give wrong results. It assumes flat earth scenario, while earth is of course curved.

Different models give different results for horizon but most are about 38 air mass value.

See here for different models:

https://en.wikipedia.org/wiki/Air_mass_(astronomy)

 

[jetstream]

11 minutes ago, vlaiv said:

For "normal" angles it is just 1/cos(angle)

You can use short cut cos values for 30, 45 and 60 degrees

and that will give you air mass at each angle as:

0 degrees = 1

30 degrees = 1/(sqrt(3)/2) = 2 / sqrt(3) = ~1.1547

45 degrees = 2/sqrt(2) = ~1.4142

60 degrees = 2

But as you start approaching horizon - above formula will give wrong results. It assumes flat earth scenario, while earth is of course curved.

Different models give different results for horizon but most are about 38 air mass value.

See here for different models:

https://en.wikipedia.org/wiki/Air_mass_(astronomy)

 

Thanks Vlaiv- when I can see numerous stars just above the trees from my high vantage point observing will be good. I had heard that the horizon was 2 airmasses but figured it had to be more.

Do you have any idea what the effect on a star, say 10 mag at the different angles? I'm wondering about extinction. When the airmass effect is calculated I wonder what transparency they are assuming?

I've thought about this for a while now.

[vlaiv]

1 minute ago, jetstream said:

Thanks Vlaiv- when I can see numerous stars just above the trees from my high vantage point observing will be good. I had heard that the horizon was 2 airmasses but figured it had to be more.

Do you have any idea what the effect on a star, say 10 mag at the different angles? I'm wondering about extinction. When the airmass effect is calculated I wonder what transparency they are assuming?

I've thought about this for a while now.

Here is a good article on that subject:

https://www.skyandtelescope.com/astronomy-resources/transparency-and-atmospheric-extinction/

You need to account your altitude above sea level, position of the star but also AOD - aerosol optical depth which is general measure of atmospheric transparency.

good AOD forecast can be seen on Copernicus

https://atmosphere.copernicus.eu/charts/cams/aerosol-forecasts?facets=undefined&time=2019123000,3,2019123003&projection=classical_global&layer_name=composition_aod550

AOD has impact on light pollution levels as well - but I don't know exactly how to calculate any of it. Relationship is simple - more dust and particles in atmosphere - more light scatter there will be from both target but also from ground sources that make up light pollution.

In perfect transparency we would have absolutely no issues with light pollution.

[bomberbaz]

Thanks gentlemen. I think that a recce with perhaps some simple snaps of some brighter dso's (M45, 42, 33, 15 for example) and a use of pair of bins and making notes of the sites all on the same night might be the best option to see whether it's significant enough to warrant going for the slightly darker skies. 

[jetstream]

2 minutes ago, vlaiv said:

In perfect transparency we would have absolutely no issues with light pollution.

Yes, this coincides with my variable skies- after a good storm and with no snow on the ground I can get repeatable 21.9 mag readings. Under less transparency my readings go down as low as 21.4 with snow on the ground.

Thanks for the excellent links Vlaiv- I love that Copernicus site which I have now saved.

[jetstream]

1 minute ago, bomberbaz said:

Thanks gentlemen. I think that a recce with perhaps some simple snaps of some brighter dso's (M45, 42, 33, 15 for example) and a use of pair of bins and making notes of the sites all on the same night might be the best option to see whether it's significant enough to warrant going for the slightly darker skies. 

For me I judge sky conditions by the appearance of the Milky Way- the sharper and more jagged it appears the better and is a very easy method to asses with. That Copernicus site is interesting...

[vlaiv]

4 minutes ago, bomberbaz said:

Thanks gentlemen. I think that a recce with perhaps some simple snaps of some brighter dso's (M45, 42, 33, 15 for example) and a use of pair of bins and making notes of the sites all on the same night might be the best option to see whether it's significant enough to warrant going for the slightly darker skies. 

Do you intend to image at these sites?

It is far easier to asses difference between LP levels on imaging than it is for visual. Above difference of 0.14 mags in LP levels translate into difference of ~x0.88 (same as 0.21/0.24), or if we take inverse of that 1.143 or 14.3% in signal level.

Take square root of that and you will see that noise increase due to change in LP levels is only 6.9% - not sure that you will be able to tell the difference (maybe only threshold objects that have intensity of the same order as LP noise levels).

[bomberbaz]

1 minute ago, vlaiv said:

Do you intend to image at these sites?

It is far easier to asses difference between LP levels on imaging than it is for visual. Above difference of 0.14 mags in LP levels translate into difference of ~x0.88 (same as 0.21/0.24), or if we take inverse of that 1.143 or 14.3% in signal level.

Take square root of that and you will see that noise increase due to change in LP levels is only 6.9% - not sure that you will be able to tell the difference (maybe only threshold objects that have intensity of the same order as LP noise levels).

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. 

[vlaiv]

1 minute ago, 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. 

For some reason I related use of the word "snaps" as meaning photography .

I've only observed from Bortle 4 / mag 21.1 skies and was blown away by views (was rather transparent sky that particular evening), so I can only imagine what it looks like in mag 21.7 skies.

 

[jetstream]

17 minutes ago, vlaiv said:

For some reason I related use of the word "snaps" as meaning photography .

I've only observed from Bortle 4 / mag 21.1 skies and was blown away by views (was rather transparent sky that particular evening), so I can only imagine what it looks like in mag 21.7 skies.

 

Vlaiv, it is a sight to behold.

[bomberbaz]

41 minutes ago, vlaiv said:

For some reason I related use of the word "snaps" as meaning photography .

I've only observed from Bortle 4 / mag 21.1 skies and was blown away by views (was rather transparent sky that particular evening), so I can only imagine what it looks like in mag 21.7 skies.

 

you are right in the snaps thing there my friend, but simple snaps with my iPhone using a time lapse app. It takes half decent pics of some brighter dso, milky way etc. Might be able to take some pictures from different sites for comparison purposes.

[bomberbaz]

2 hours ago, vlaiv said:

Maybe best type of unit to be used is mag/arcsec^2, or simply magnitude (surface brightness - not to be confused with stellar magnitude although it is the same thing).

Magnitudes are better suited to our visual perception than physical quantities because our eye/brain works on close to logarithmic scale.

Surface brightness of targets is also given in these units (or mag/arcmin^2 and here it is simple conversion as add 8.89 to get mag/arcsec^2 - but these tend to be average brightness) and you can estimate visibility from that.

According to SQM calculator page (found here), to convert from cd/m2 to mag/arcmin2 use:

or rather opposite one in above case.

We have 0.24 mcd/m2 and 0.21 mcd/m2, let's convert that to mag/arcsec2

-2.5*log(0.00024 / 108000)  = 21.633

-2.5*log(0.00021 / 108000) = 21.778

How big a difference is 0.14 magnitudes? Not much - change in transparency from excellent to very good will make about 0.1 mag attenuation of targets. Single air mass has about mag 0.16 attenuation.

Just realised the web page I am using has the SQM readings as well, sorry mate but thanks as well. Now I better understand what I am looking at a little more. 

[DaveS]

I have theoretical 21.66 Bortle 4 (Nearly 3) but likewise transparency makes a big difference. A couple of weeks back when I was stuck indoors with a duff back I saw M31 bright and clear through the double glazing when I went to the loo in the night. There was very little light from Bridport which is usually quite noticeable.

[PeterW]

I was at a mag21.5 site and the Nebula visibility was dramatically different on different nights.. same size reading.
Once you get to around 21.6 or so I’d look at the bortle descriptions in preference as the numeric differences are slight, but the visual differences keep coming (allegedly as 21.5 is the best I’ve come across),

Peter

[jetstream]

6 hours ago, PeterW said:

I was at a mag21.5 site and the Nebula visibility was dramatically different on different nights.. same size reading.
Once you get to around 21.6 or so I’d look at the bortle descriptions in preference as the numeric differences are slight, but the visual differences keep coming (allegedly as 21.5 is the best I’ve come across),

Peter

I think that when you get a truly high SQM reading-over 21.6 IME- the skies are very transparent.  This low level of transparency also minimizes light domes and their extent. Obviously clouds can give false high SQM readings.

[bomberbaz]

1 hour ago, jetstream said:

I think that when you get a truly high SQM reading-over 21.6 IME- the skies are very transparent.  This low level of transparency also minimizes light domes and their extent. Obviously clouds can give false high SQM readings.

Another option for me a little nearer is the ribblehead viaduct, loads of parking with a beautiful clear southern horizon and the sqm is given as 21.71, that is now top of my list to try out as its less than an hour away too.

[vlaiv]

I just noticed that different people quote different SQM to Bortle mapping - very interesting.

Bortle scale nomogram places natural unpolluted sky at about mag 21.75 - 21.8? It also looks that Bortle 1 begins there - here is image of this:

Wiki page on Bortle scale lists these SQM values as Bortle 1, 2 and 3 skies:

Bortle 1 - 21.7–22.0

Bortle 2 - 21.5–21.7

Bortle 3 - 21.3–21.5

This resource - https://www.handprint.com/ASTRO/bortle.html

lists following SQM values:

Bortle 1 22.00–21.99

Bortle 2 21.99–21.89

Bortle 3 21.89–21.69

What should we adapt as being correct scale?

[Miguel1983]

That second scale doesn't leave much tolerance, i think most people would like the top one.

I got a mere 19,68 last night, measured with Dark Sky Meter app.

[jetstream]

1 hour ago, bomberbaz said:

Another option for me a little nearer is the ribblehead viaduct, loads of parking with a beautiful clear southern horizon and the sqm is given as 21.71, that is now top of my list to try out as its less than an hour away too.

Excellent Steve.

A friend from UK has done extensive SQMing (not near you though) and finds that posted SQM readings can be optimistic... this place sounds like a VG starting point for sure. If however that sky listed at 21.7mag doesn't "startle" or "dazzle" you it most likely is not at that level.

A true 21.7mag is extremely good. I'm excited for you in your pursuit of dark skies.

[jetstream]

59 minutes ago, vlaiv said:

What should we adapt as being correct scale?

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.

[bomberbaz]

24 minutes ago, jetstream said:

Excellent Steve.

A friend from UK has done extensive SQMing (not near you though) and finds that posted SQM readings can be optimistic... this place sounds like a VG starting point for sure. If however that sky listed at 21.7mag doesn't "startle" or "dazzle" you it most likely is not at that level.

A true 21.7mag is extremely good. I'm excited for you in your pursuit of dark skies.

Thanks Gerry. As I posted earlier, I am looking for a super dark site which will be my only dark sky viewing place, I am not going to mess about with half rated skies anymore, it's down heartening and wasteful of good viewing time.

The back garden is now relegated to planets and lunar only. ( also for when betelguese goes 😉🤣 )