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Dark Skies and the VIIRS Radiance Data

Acquisition Light Pollution Dark Site Deep Sky
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Danny Lee avatar
Danny Lee avatar
Hi all, 

I was wondering whether anyone knows much about the Visible Infrared Imaging Radiometer Suite (VIIRS) and the radiance data it provides via https://www.lightpollutionmap.info/ 

How should these radiance numbers be interpreted?

As an example my house sits in a Bortle 5 zone and the latest VIIRS data states a radiance level of 13. My parents house only one Bortle level lower at 4 but the VIIRS radiance level is stated as being significantly lower than at my house with a rating of 2. 

Just wondering whether the VIIRS data could be used to help look for a dark sky location and if/how it compares to the Bortle scale. 

CS
Danny
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andrea tasselli avatar
andrea tasselli avatar
The Bortle scale is a visual scale only hence it does not directly compare with a linear scale such as, I believe, the one provided by radiance data.
Jurij Stare avatar
Jurij Stare avatar
In a nutshell radiance map shows you light sources on the ground, while World Atlas shows you sky brightness (due to light sources on the ground). Radiance map is useful for figuring out sources of light pollution when when looking for a site using the World Atlas layer. It has no direct impact on sky brightness, but is used to calculate the sky brightness like World Atlas overlay. You can actually see individual lights on VIIRS  overlay if they are in the middle of nothing, World Atlas doesn't show you that. But be aware that if there's nothing on the VIIRS map it doesn't necessarily mean there will be no lights at the location when you arrive. It could be that they switch them off before 1:30 am when the satellite passes over the location  Bortle scale is there (on  www.lightpollutionmap.info) just for the sake of large amount of people asking me to put it there. Personally I hate to use it because it is such a subjective assessment of the sky conditions.
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Danny Lee avatar
Danny Lee avatar
Jurij Stare:
In a nutshell radiance map shows you light sources on the ground, while World Atlas shows you sky brightness (due to light sources on the ground). Radiance map is useful for figuring out sources of light pollution when when looking for a site using the World Atlas layer. It has no direct impact on sky brightness, but is used to calculate the sky brightness like World Atlas overlay. You can actually see individual lights on VIIRS  overlay if they are in the middle of nothing, World Atlas doesn't show you that. But be aware that if there's nothing on the VIIRS map it doesn't necessarily mean there will be no lights at the location when you arrive. It could be that they switch them off before 1:30 am when the satellite passes over the location  Bortle scale is there (on  www.lightpollutionmap.info) just for the sake of large amount of people asking me to put it there. Personally I hate to use it because it is such a subjective assessment of the sky conditions.

Thanks for the detailed reply, that's helpful and useful to know. 

I'll use a combination of both to try and hunt down a suitable site near me. I live in England so unfortunately Bortle 1-3 pretty much doesn't exist!
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klm avatar
klm avatar
I live in the flat midwest USA, so horizon is 3 miles (5 km) away.  Suffice to say there are no dark skies here.  Even so, am trying to get the darkest skies possible within a reasonable driving distance, but am wondering if there will be a meaningful difference.  My understanding is that World Atlas view (showing sky brightness) was last calculated in 2015, but VIIRS (showing ground sources) was updated as recently as 2022.   Assuming yes, based on VIIRS 2022, my backyard shows a radiance of 13.6.  I can find patches on the map that are a reasonable distance and have a radiance of 0.0, which sounds great.  However, the patches are only about 1000 m in diameter and surrounded (within a 3 mile radius) by areas with radiance of anywhere from 1-5, depending upon direction. 

My question:  Do I get any practical benefit from centering myself in one of those patches? 
Or, do I get the same sky brightness if I'm in the center of any 3-mile diameter circle with, for example, a radiance of 1, but still surrounded by areas with radiance of anywhere from 1-5, depending upon direction?

The latter are closer to home smile
many thanks!
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andrea tasselli avatar
andrea tasselli avatar
I live in the flat midwest USA, so horizon is 3 miles (5 km) away.  Suffice to say there are no dark skies here.  Even so, am trying to get the darkest skies possible within a reasonable driving distance, but am wondering if there will be a meaningful difference.  My understanding is that World Atlas view (showing sky brightness) was last calculated in 2015, but VIIRS (showing ground sources) was updated as recently as 2022.   Assuming yes, based on VIIRS 2022, my backyard shows a radiance of 13.6.  I can find patches on the map that are a reasonable distance and have a radiance of 0.0, which sounds great.  However, the patches are only about 1000 m in diameter and surrounded (within a 3 mile radius) by areas with radiance of anywhere from 1-5, depending upon direction. 

My question:  Do I get any practical benefit from centering myself in one of those patches? 
Or, do I get the same sky brightness if I'm in the center of any 3-mile diameter circle with, for example, a radiance of 1, but still surrounded by areas with radiance of anywhere from 1-5, depending upon direction?

The latter are closer to home
many thanks!

How useful depends on your circumstances, such as:

a. where are you pointing at?
b. how clear (free from dust, moisture and other similar dispersing elements) is your local (with a radius of a mile) atmosphere?

If your target is at the zenith or within a circle of 10 degrees around it then you are the least affected by any nearby illumination. Same goes with the "cleanliness" of you local sky. If I were in Arizona at altitude even local patches of bright light won't affect my imaging as very little gets dispersed back to the ground. I once was imaging from a B2 location with a shed some 100 feet away, brightly lit, but that didn't affetc the quality of the sky above (very dry air, light breeze, no back-scattering dust), it was still a B2 sky albeit not to my eyes.
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Wes Schwarz avatar
Wes Schwarz avatar
I live in the flat midwest USA, so horizon is 3 miles (5 km) away.  Suffice to say there are no dark skies here.  Even so, am trying to get the darkest skies possible within a reasonable driving distance, but am wondering if there will be a meaningful difference.  My understanding is that World Atlas view (showing sky brightness) was last calculated in 2015, but VIIRS (showing ground sources) was updated as recently as 2022.   Assuming yes, based on VIIRS 2022, my backyard shows a radiance of 13.6.  I can find patches on the map that are a reasonable distance and have a radiance of 0.0, which sounds great.  However, the patches are only about 1000 m in diameter and surrounded (within a 3 mile radius) by areas with radiance of anywhere from 1-5, depending upon direction. 

My question:  Do I get any practical benefit from centering myself in one of those patches? 
Or, do I get the same sky brightness if I'm in the center of any 3-mile diameter circle with, for example, a radiance of 1, but still surrounded by areas with radiance of anywhere from 1-5, depending upon direction?

The latter are closer to home
many thanks!

Do you measure via sqm-l?
klm avatar
klm avatar
Andrea, thank you for the feedback. Makes sense that all things being equal (dust, moisture, temperature), poorer seeing will be due to total light blasting into the sky from within the area of "my circle" rather than an otherwise dark site with a few hot spots.  A quick visual assessment suggests that the average light in my dark patch site is lower than the options closer to home.  Guess I'll make the drive.

I should also point out that while I avoid the horizon for obvious reasons, I cannot do the zenith.  Due to zoom lens "slippage" I cannot shoot much above 60 degrees or so.

One final comment, my quest is to get respectable shots with relatively inexpensive equipment. I use a Canon T3, Tamron 18-400, iOptron SkyGuider Pro, and a decent tripod.  That's it.  I'll never get the shots I see posted by this community, but I have a good time anyway.
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klm avatar
klm avatar
Wes, I am only using the light pollution map referenced at the beginning of this conversation, so all I do is scan the map for darker areas indicated by low radiance values on the VIIRS 2022 overlay.  Hopefully, I am using the map correctly.
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andrea tasselli avatar
andrea tasselli avatar
should also point out that while I avoid the horizon for obvious reasons, I cannot do the zenith.  Due to zoom lens "slippage" I cannot shoot much above 60 degrees or so.


An elastic band will fix that. I use it with the 200-500.
klm avatar
klm avatar
good idea andrea, i'll try it!
thanks