Is there a correlation between Bortle and brightness off of the LP maps?

[Bugdozer]

Theoretically, it would be possible to invent a very simple "Bortle Meter". All you'd need would be some card and some transparent plastic with different gradations of neutral density.

You make something that covers your eyes like a VR goggle out of the card. In this, you have a hole which you can see through to view the sky, through which you can slide the neutral density graded plastic strips through, akin to a filter wheel.

You put this on and let your eyes adapt to the dark. Then, in a process similar to calibrating the brightness of a video game, you slide the strips through until you can no longer see any brightness from the sky and the hole appears to let in no light. The darker the strip needed for this, the higher Bortle number you have.

This has one big advantage over meters which objectively measure the real brightness of the sky, which is that it self-compensates for differences in light sensitivity from person to person. This is significant because the actual description of "what you can see on the Bortle scale" seems to assume that contrast against the overall sky background is the only limiting factor in ability to see objects, as if reducing the glow automatically cranks up the intrinsic brightness of everything. For example, Bortle 1 lists M33 as being a direct vision naked eye object, whereas in reality, even under PERFECT skies, some people's night vision isn't good enough to detect an extended 5.7 magnitude object.

[Fraunhoffer]

On 08/11/2023 at 17:59, vlaiv said:

I was going to try and use an old photo exposure meter, but i dont think it goes low enough. I can read to 1.4 lux and with a bit of faffing about with the settings, maybe 0.35 lux.

Plan B is to cross check my mirror less camera with the exposure meter and point the camera with a 30 deg fov straight up at my grey sky and adjust the camera exposure until the 'average' setting says +-0. This should put a sharp histogram peak in the middle. Then work out extrapolating the setting to get another estimate of background lux. in c/m^2

I thought one of the table values from my exp meter might be useful.

Asa 50, f/2.8 , 30s is listed as 1.4 lux., so im thinking by cranking up the camera iso i can get an estimate of sky background. At, least to compare sites.

 

Edited November 13, 2023 by Fraunhoffer

[Stu]

On 08/11/2023 at 17:59, vlaiv said:

That’s a really great chart Vlad, thanks for posting it up 👍

[vlaiv]

2 hours ago, Fraunhoffer said:

I was going to try and use an old photo exposure meter, but i dont think it goes low enough. I can read to 1.4 lux and with a bit of faffing about with the settings, maybe 0.35 lux.

Plan B is to cross check my mirror less camera with the exposure meter and point the camera with a 30 deg fov straight up at my grey sky and adjust the camera exposure until the 'average' setting says +-0. This should put a sharp histogram peak in the middle. Then work out extrapolating the setting to get another estimate of background lux. in c/m^2

I thought one of the table values from my exp meter might be useful.

Asa 50, f/2.8 , 30s is listed as 1.4 lux., so im thinking by cranking up the camera iso i can get an estimate of sky background. At, least to compare sites.

 

There is a tool called Sky Quality Camera that can measure the whole sky for LP levels.

All you need is particular software, DSLR and fisheye lens.

I have no idea how to get it, or is it available to general public. You can read an article about it here:

https://www.boisestate.edu/physics-cidsrsn/2022/06/27/sky-quality-camera-a-tool-to-track-and-analyze-light-pollution-in-the-cidsr/

There are also some measurements on light pollution info website taken with this method (filter for SQC). Here is measurement from 2019 made just couple of km away from me:

[Mr Spock]

In the days of orange streetlights my sky to the east was orange and I couldn't see anything. Sky to the west was dark, and overhead, I could see the milky way quite clearly. I could get my scopes down to very faint magnitudes in that area.

Now with LED streetlights the sky is a uniform grey. The faintest magnitude I can see is 4.5 on a good day.

[vlaiv]

This will be slightly off topic, but for anyone interested - Sky Quality Camera appears to be commercial software made by Euromix d.o.o in Slovenia - but I'm unable to find any official way of obtaining that software.

I just found a bunch of mentions in academic literature / papers that are written on topic of LP but can't find anything else on that software.

[inFINNity Deck]

The plate-solving software ASTAP does the same using your deep-sky images. I have written a tool FITSalize that uses ASTAP to produce an ASCII table that can be imported into Excel (it allows for deformation measurements as well). Here is an example of a measurement I did last year:

Care should be taken that the H17 or H18 database with Gaia blue magnitudes (BP) is used as reference.

Nicolàs

[vlaiv]

5 minutes ago, inFINNity Deck said:

The plate-solving software ASTAP does the same using your deep-sky images. I have written a tool FITSalize that uses ASTAP to produce an ASCII table that can be imported into Excel (it allows for deformation measurements as well).

This is quite cool. With a bit more code - it can be turned into "trajectory" in "all sky map" sort of thing (just a projection of measurement points onto circle that represents fish eye lens view of the sky).

[jjohnson3803]

Aside from the LP maps and my eye, I refer to this comparison chart.  Seems quite similar to what I perceive and puts me in a 7+ at home.  I can see Pherkad and Kochab (and Polaris) in UMi, but none of the other stars in the dipper, so my naked eye limit is around mag = +3.5 or so.

Edited November 17, 2023 by jjohnson3803

[Nigella Bryant]

4 hours ago, jjohnson3803 said:

Aside from the LP maps and my eye, I refer to this comparison chart.  Seems quite similar to what I perceive and puts me in a 7+ at home.  I can see Pherkad and Kochab (and Polaris) in UMi, but none of the other stars in the dipper, so my naked eye limit is around mag = +3.5 or so.

Thanks, looking at that chart I'd put my sky at 3 bordering 2. It's supposed to be 4 bordering 3 yet I don't have that kind of background glare that this chart suggests at 4. 

[SteveNickolls]

On 13/11/2023 at 07:03, Fraunhoffer said:

I was going to try and use an old photo exposure meter, but i dont think it goes low enough. I can read to 1.4 lux and with a bit of faffing about with the settings, maybe 0.35 lux.

Plan B is to cross check my mirror less camera with the exposure meter and point the camera with a 30 deg fov straight up at my grey sky and adjust the camera exposure until the 'average' setting says +-0. This should put a sharp histogram peak in the middle. Then work out extrapolating the setting to get another estimate of background lux. in c/m^2

I thought one of the table values from my exp meter might be useful.

Asa 50, f/2.8 , 30s is listed as 1.4 lux., so im thinking by cranking up the camera iso i can get an estimate of sky background. At, least to compare sites.

Here you go.

https://www.pbase.com/image/37608572

Cheers,

Steve

[bwj]

Sorry I'm so late to add to this discussion. Here is the SQM to Bortle conversion the Starry Sky Survey includes in the kit for people to report data with.

https://montanalearning.org/starry-sky-survey/

Edited December 6, 2023 by bwj
correct typo

[ollypenrice]

On 06/12/2023 at 17:52, bwj said:

Sorry I'm so late to add to this discussion. Here is the SQM to Bortle conversion the Starry Sky Survey includes in the kit for people to report data with.

https://montanalearning.org/starry-sky-survey/

That's an interesting conversion which I find more convincing than most attempts to define the Bortle scale. My own site, on nights on which you'd want to go out and do astronomy, varies between SQM 21 and 22, so Bortle 4 to Bortle 1. The problem with the Bortle scale, as usually presented, is that it seems to assume consistency.

Olly

[bwj]

Yeah. And from what I've read, the original purpose was for personal assessments. The Bortle number was based on visual observation, so varies with each individual, depending on their visual ability/ condition. As such it is really a very subjective measurement.  It can also vary from hour to hour or even minute to minute with atmospheric conditions. I think people try to use it for more than it is because most of us have trouble justifying the cost of an SQM and this provides the next best "measurement". I think the table I posted is a good step towards actually quantifying the Bortle numbers.

Clear Skies!

Edited December 22, 2023 by bwj

[Mr Spock]

I hope you don't mind me editing the title. The misspelling of Bortle was grating a little