Translation of SQM to Bortle scale

[gorann]

After I have started posting my images of Integrated Flux Nebulosity I have got several questions about what Bortle my sky is. I do not trust my old eyes to subjectively figure this out, but I do take regular measurements with my SQM meter and it usually shows values between 21.4 and 21.6 on good nights. I had  previously seen this translated to Bortle 2 - 3, see the table below and for example:  https://jonrista.com/the-astrophotographers-guide/astrophotography-basics/snr/

I thought that made sence since I am in a rural area with no major light pollution near by and no city lighting up the horizon.

But, when I now check on Wikipedia the translation table is completely different. Accoring to Wikipedia I am more like Bortle 4 and I expect very few people, including @ollypenrice in rural France are in Bortle 2 - 3 skies. The Wikipedia scale seems very compressed at the top. https://en.wikipedia.org/wiki/Bortle_scale

Does anyone know why these scales differ so much, and which one to use?

Traditional table:

[ollypenrice]

I don't know anything about these things, technically, and never use the Bortle scale in my own thinking. I have what I have and won't be moving, so I can't do anything about it! I'm happy to see 21.4 or better on the SQM and we sometimes just squeeze over 22. One of my robotic teams take the SQM automatically on all clear nights and there is certainly a seasonal factor. There, again, I can't do anything about it...

At one time we had a regular guest who also visited the Tivoli facility in Namibia and he told me that there was no systematic difference between the zenith SQM reading taken there and here. The difference went increasingly in Nambia's favour as the readings were taken at lower elevations, which you'd expect.

I use the SQM system simply because I can point the device at the sky, press a button and get a reading!

Olly

[vlaiv]

I know of this comparison - maybe not as easy to read?

http://www.darkskiesawareness.org/img/sky-brightness-nomogram.gif

(sgl no longer allows embedding of http images so I'll post screen shot as well):

However, lightpollutionmap.info is similar to wiki - only 21.99 is shown as class 1

[gorann]

Thanks @ollypenrice and @vlaiv. My problem is that Bortle is mentioned a lot on SGL and many people ask me of my Bortle. Maybe Bortle is especially popular in the UK? I wish that SQM meters were not as rediculously expensive so we all could go over to that less subjective scale. How could 25 years old compare his/her impression of the sky with that of Olly and me?

[vlaiv]

I'm not sure if SQM is that expensive.

For example - it can be done with Arduino (just light sensor, Arduino nano and small 3-4 digit LCD digit display needed).

https://sourceforge.net/projects/arduinomysqmskyqualitymeter/

Sure, original thing is, but that is mostly the cost of labor and not of material.

[wimvb]

This article from Sky and Telescope, written by Jerry Lodriguss, relates integration time to sky brightness.

https://skyandtelescope.org/astronomy-blogs/astrophotography-benefits-dark-skies/

This should be relevant to the discussion in this thread.

Edited April 6, 2021 by wimvb

[ollypenrice]

1 hour ago, gorann said:

Thanks @ollypenrice and @vlaiv. My problem is that Bortle is mentioned a lot on SGL and many people ask me of my Bortle. Maybe Bortle is especially popular in the UK? I wish that SQM meters were not as rediculously expensive so we all could go over to that less subjective scale. How could 25 years old compare his/her impression of the sky with that of Olly and me?

My SQM meter was a present from a very kind guest (and SGL member) so I don't actually know what they cost but I was certainly aware that it was a generous gift. It has been going strong for a good number of years, too.

Olly

[Deadlake]

Does anyone know how accurate 'Dark Sky Meter' is on iPhones at all?

[vlaiv]

26 minutes ago, wimvb said:

This article from Sky and Telescope, written by Jerry Lodriguss, relates integration time to sky brightness.

https://skyandtelescope.org/astronomy-blogs/astrophotography-benefits-dark-skies/

This should be relevant to the discussion in this thread.

It certainly is, but it tends to be true only for faint(er) objects.

When object and sky are equally bright - then their shot noise is comparable, but when object is much brighter or much darker than the sky - then shot noise of either object or sky tend to dominate over other noise source (noises add in quadrature).

This creates two distinct regimes - if object is brighter than the sky - there is no major difference in change in sky brightness.  It is the reason why bright objects like M31 and M42 and some globulars can be imaged from high LP in reasonable amount of time - but try to capture something really faint like IF - you'll need weeks of exposure.

 

[vlaiv]

Just now, Deadlake said:

Does anyone know how accurate 'Dark Sky Meter' is on iPhones at all?

I'll hazard a guess that it is more accurate than eyeball Mark I .

In principle, camera + lens + processor is quite enough for SQM meter, and iPhones tend to be rather uniform as there is very small number of models (unlike for example android phones) - which means that calibration information is easily bundled with the app.

I think it is pretty accurate - at least to one digit - like sqm 21.1 if not two digits.

[pete_l]

2 hours ago, gorann said:

My problem is that Bortle is mentioned a lot on SGL

Yes, more's the pity. It seems to me to be a case of valuing what you can measure, rather than measuring what you value. 😬

The other thing to go by is the descriptions Mr. Bortle gives for his various scale values. None of which correspond with my experience of a wide range of SQM readings. For me, my SQM provides some relative measurements of my sky at different times of the night / year / sunspot cycle as the measurements I get vary significantly with all of those and probably many more factors. I'd say there is no single, constant SQM value for a location, (and therefore no fixed "Bortle" value). Only what you experience at a point in time.

[DaveS]

I think the Bortle scale was developed in the American South West, ie low latitude dry atmosphere conditions, so a lot of his DSO criteria just don't apply this far north.

[jetstream]

I also have an issue with the Bortle scale- how many here can see M33 naked eye in Bortle 4? yeah right.

[jetstream]

3 hours ago, pete_l said:

I'd say there is no single, constant SQM value for a location, (and therefore no fixed "Bortle" value). Only what you experience at a point in time.

Exactly.

NELM of 8?  There must be a bunch of people with better eyes than me in Bortle 1. I get around 7.

[Corncrake]

34 minutes ago, DaveS said:

I think the Bortle scale was developed in the American South West, ie low latitude dry atmosphere conditions, so a lot of his DSO criteria just don't apply this far north.

Exactly this 👍

I've being saying it for years, but the chances now of persuading people surrounded by the cold damp Atlantic to stop using it ? Good luck with that !

Also his notions of what constitutes a suburban location dont fit us either.

 

Edited April 6, 2021 by Corncrake

[gorann]

Interesting replies here. It seems like no one likes the Bortle scale even if so many here on SGL states what Bortle sky they have. For the future I am just going to give my SQM range if anyone asks. By the way I have my two RASAs shooting away at IFN tonight although my SQM reading now was only 21.3. I hope for the best!

And @ollypenrice, I just checked, your guest gave you a 129 pound present: https://www.firstlightoptics.com/unihedron-sky-quality-meters/unihedron-sqm-l-sky-quality-meter.html

Edited April 6, 2021 by gorann

[wimvb]

35 minutes ago, gorann said:

129 pound present

Not exactly bank-breaking expensive. But since it can't darken the sky, it's basically just a nice gadget.

About a year ago, I built a Sky Quality Meter and "calibrated" it using Göran's Unihedron. According to that, I have a sky magnitude of about 20.5 - 20.7 at best. But it seems to me that this year (haven't measured it), the sky hasn't been that dark. Most likely, high clouds are to blame. We just haven't had one of thos crisp, clear skies this year.

[Corncrake]

9 hours ago, vlaiv said:

just

https://sourceforge.net/projects/arduinomysqmskyqualitymeter/

Just ! That sounds like in my student math days He>"It is obvious therefore that , , , " me> butbut !!  

Hmmm, it says  
"A sky quality meter based on TLS237 and Arduino Nano"
Leaps into a search on TLS237, but Google says "Including results for TSL237"

Oh dear, that is not a good start !

I am not sure why I would want a SQM but I have some spare Arduinos.
No 237 on ebay uk  but there is a 235 for £10 which would not break the piggybank,,,
the plot down the rabbit-hole thickens, , ,

I always have trouble, coming at a subject cold, with sourceforge sites. They always need superhuman intelligence to initially understand where they are coming from and the bascs of where we start !

[ollypenrice]

The usefulness of the SQM for me, is twofold. It allows me to give an objective answer when propsective guests ask about our sky quality and it lets me assess the sky immediately after stepping out of the observatory. It now takes me five minutes to give even a rough estimate of sky quality after staring at two PC screens in the warm room. I may suddenly see that I'm getting nothing out of a camera: is it a fault or has the sky fallen over? It ought to be easy to look outside and see but, as I approach seventy, it isn't! 

Olly

[tomato]

The Dark Sky Meter App still works on the latest iphones, there is an interesting thread on this on Cloudy Nights with contributions from the creator discussing calibration and how well it compares to the SQM meter.

https://www.cloudynights.com/topic/597759-sqm-vs-dark-sky-meter-ios-app/

While staring at the laptop screens following the imaging, I find the PHD SNR value is a good guide to changing sky conditions.

It doesn’t help with nightly comparisons of course, unless you kept it on the same star all year round...

[wimvb]

6 hours ago, Corncrake said:

Just ! That sounds like in my student math days He>"It is obvious therefore that , , , " me> butbut !!  

Hmmm, it says  
"A sky quality meter based on TLS237 and Arduino Nano"
Leaps into a search on TLS237, but Google says "Including results for TSL237"

Oh dear, that is not a good start !

I am not sure why I would want a SQM but I have some spare Arduinos.
No 237 on ebay uk  but there is a 235 for £10 which would not break the piggybank,,,
the plot down the rabbit-hole thickens, , ,

I always have trouble, coming at a subject cold, with sourceforge sites. They always need superhuman intelligence to initially understand where they are coming from and the bascs of where we start !

Here's my version

According to some very old thread here on sgl, the sqm with a light-to-frequency converter can run into problems at very low light intensities, because the frequency is so low. If I were to make such a device again, I would add a display. At the time I intended to mount the sqm permanently in my obsy, and I didn't need a display.

[clarkpm4242]

Hi @gorann 

I had some discussion with John Bortle on this many, many years ago.  His fame is as a visual comet observer. 

I've found it does work well in the UK with some modifications.  Use the best part of the sky and don't bother with point objects - stars.

It also helps if you've had experience of visual observations under a darker sky than your home location.

When using a SQM I also aim towards the dark part of the sky.  I take into account the time of night, and particularly if the streetlights are off.

You seem to be in Bortle 3, occasionally 2..?

Cheers

Paul

[gorann]

1 hour ago, clarkpm4242 said:

Hi @gorann 

I had some discussion with John Bortle on this many, many years ago.  His fame is as a visual comet observer. 

I've found it does work well in the UK with some modifications.  Use the best part of the sky and don't bother with point objects - stars.

It also helps if you've had experience of visual observations under a darker sky than your home location.

When using a SQM I also aim towards the dark part of the sky.  I take into account the time of night, and particularly if the streetlights are off.

You seem to be in Bortle 3, occasionally 2..?

Cheers

Paul

Yes Paul, Bortle 2-3 unless I use the "new" Wikipedia scale, then it is 4.....